ROS are regarded not only as signals but also as byproducts of aerobic pathways generated in different cellular compartments and cause PCD at higher concentrations Gechev and Hille, In the rice mads3 mutant, anthers exhibit oxidative stress-related phenotypes, since tapetal PCD occurs prematurely Hu et al. Elimination of H 2 O 2 through non-enzymatic and enzymatic pathways maintains the level of ROS, while catalase, peroxidase, ascorbate peroxidase, and glutathione reductase are considered to be the main H 2 O 2 scavengers, which can effectively reduce the amount of H 2 O 2 Kapoor et al.
In summary, the role of ROS in the dPCD process strictly depends on its concentration, and its interaction with other signaling molecules also determines cell fate Niu and Liao, Nitric oxide NO is a small gaseous and highly reactive molecule that can take part in a wide range of physiological processes, such as nucellar degeneration and leaf senescence.
Due to NO being highly reactive, a series of NO derivatives could be formed, resulting in a redox-mediated modification in plants.
The effects of NO as an activator or repressor seem to be due to the concentration and timing patterns of NO Mur et al. High levels of NO are associated with the progression of natural senescence, cell death, and DNA fragmentation Carimi et al. Studies have indicated that NO and H 2 O 2 can be induced to synthesize each other and that the signaling crosstalk between H 2 O 2 and NO synergistically regulates leaf cell death and delays senescence Iakimova and Woltering, During nucellar cell degeneration, considerable production of NO and H 2 O 2 caused an induction of caspase-like proteases, leading to nucellar dPCD Lombardi et al.
In addition, rapid and transient increases in ROS and NO were also detected by self-incompatibility in the pollen tube growth of Papaver Wilkins et al. Calcium-mediated signaling between two synergids determined their fate death or survival in the control of sperm delivery Ngo et al.
Current studies have shown that the fate of cells undergoing PCD is controlled by complex signaling and transcriptional regulatory networks. Transcriptional regulation has been revealed to play a key role in specific dPCD events. However, the link between signaling pathways and dPCD-related gene expression regulation is rarely clearly established. Especially in woody plants, the signal transduction and transcriptional regulatory mechanism of dPCD during wood formation are still unclear.
Recently, Zhang et al. This approach provides a new strategy for exploring new regulatory mechanisms in biological processes. Therefore, combining cell biology, biochemistry, and molecular biology methods to detect how signaling pathways control dPCD through gene regulatory networks will become an overall trend in future research.
Specifically, xylogenesis contributes to the largest bioenergy source, in which the components and contents of SCW are the main influencing factors in the improvement of industrial production. In short, the analysis of the molecular regulation mechanism of plant dPCD can lay the foundation for regulating specific developmental processes of plants through genetic engineering methods and further applying their products to industrial production.
JZ conceived the study. CJ and JZ collected and synthesized the data and draft the manuscript. All authors contributed to the article and approved the submitted version. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher. Plant Physiol. Bai, M. Bennett, T. Plant Cell 22, — Bishopp, A. A mutually inhibitory interaction between auxin and cytokinin specifies vascular pattern in roots.
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Increased gibberellin biosynthesis in transgenic trees promotes growth, biomass production and xylem fiber length. Escamez, S. Programmes of cell death and autolysis in tracheary elements: when a suicidal cell arranges its own corpse removal. Fendrych, M. Fu, Z. Plant Cell 26, — Gadjev, I. Transcriptomic footprints disclose specificity of reactive oxygen species signaling in Arabidopsis. Gechev, T. Reactive oxygen species as signals that modulate plant stress responses and programmed cell death.
BioEssays 28, — Hydrogen peroxide as a signal controlling plant programmed cell death. Cell Biol. Anther and pollen development: a conserved developmental pathway. Graaff, E. Transcription analysis of Arabidopsis membrane transporters and hormone pathways during developmental and induced leaf senescence. Groover, A. Tracheary element differentiation uses a novel mechanism coordinating programmed cell death and secondary cell wall synthesis.
Gunawardena, A. Programmed cell death and tissue remodelling in plants. Han, S. New Phytol. He, Y. Evidence supporting a role of jasmonic acid in Arabidopsis leaf senescence. Hirano, K. Plant Cell Physiol. Hu, L. Plant Cell 23, — Huysmans, M. The cell system is also well accessible for applications of exogenous agents to study the signalling processes during the stages of xylogenesis Demura and Fukuda ; Watanabe and Fukuda ; Fukuda ; ; , ; Milioni et al.
The protocol for establishment of xylogenic zinnia cell culture, introduced by Fukuda and Komamine has been applied as originally described or with modifications aiming at improving the TE differentiation rate e.
Church and Galston ; Roberts et al. Prerequisites for realization of the xylogenic potential of zinnia cells to yield sufficient amount of differentiated TEs are the age of leaves from which the mesophyll cells are isolated, cell density, viability and health status of the culture, pH, cellular CO and extracellular osmolarity EO , and medium composition, particularly the requirement for the presence of both hormones auxin and CK Fukuda and Komamine ; Turner et al.
These factors can impair the transdifferentiation if not properly considered Supplemental File S2. The basic principles of the procedure for establishment of zinnia cell system have been developed for establishing xylogenic cultures of other species such Arabidopsis and for elaboration of new models for induction of xylogenesis on cultured leaf segments Kubo et al.
The differentiation of xylem tissue is a paradigm of a developmental program in which differentiation, SCWs formation and cell death are tightly coupled. In planta, the process proceeds through a sequence of events, involving differentiation of cambial and procambial cells into TEs.
This includes synthesis and deposition of SCWs material and lignification and is completed through developmentally established commitment to cellular suicide, followed by autolysis, finally resulting in generation of mature dead vessel elements capable to performing their function of water transporting system. During transdifferentiation of in vitro cultured zinnia mesophyll cells into TEs three major partially overlapping consecutive stages Fig.
Stage I includes dedifferentiation of mesophyll cells which is stimulated by the wounding at isolation of the culture and during which the cells lose their ability to photosynthesize and, acquire competence for responding to auxin and cytokinin; in this stage cell division may or may not take place. Stage II is characterized by transdifferentiation, induced by exogenous supply of auxin and CK and proceeds through development of procambial initials-like cells, procambial-like cells, synthesis and deposition of SCW material, formation of immature xylem-like cells and TE precursors.
Stage III is the late process of TE maturation including continuation of SCW formation and cell death execution, the latter accompanied by vacuole expansion, disruption of tonoplast integrity followed by release of endonucleases, proteases and other hydrolytic enzymes from the vacuole, partial lysis of the cellular content and DNA fragmentation. Stages of xylogenesis in zinnia cell culture. TE differentiation in zinnia in vitro proceeds through four stages: stage I: dedifferentiation of mesophyll cells and acquisition of competence to respond to auxin and cytokinin; stage II: transdifferentiation, including development of TE precursors, TE maturation and deposition of SCWs; stage III: cell death execution, continuation of SCW formation; stage IV: post mortem autolysis and lignification resulting in formation of completed TEs.
For more detailed explanation, please refer to the text. It has been assumed that final TE cell death execution and autolysis following the vacuole burst resulting in complete digestion of the protoplast and the nucleus are a common expression of the cell death process occurring in stage III Fukuda ; Greenberg ; Fukuda et al. However, it was also suggested that the final stage of PCD may be split in two consecutive phases—cell death execution and autolysis, the latter of which is responsible for complete protoplast elimination Mittler and Lam ; Jones and Dangl ; Groover et al.
Escamez and Tuominen described the cell death and autolysis of TEs as two separate consecutive phases in stage III, in the first of which TE cells die but autolysis resulting in clearance of organelles remnants to form hollow dead mature TEs occurs post mortem , within few hours after cell death Fig.
When the vacuole collapses, the cell is dead but the released lytic enzymes proceed to degrade the protoplast debris. Moreover, the post - mortem stage we suggest to be determined as stage IV is featured by an active process of SCWs lignification which is non-autonomous and is supported by substances delivered from neighbouring living cells both in zinnia in vitro and in planta , and in other cell cultures and in planta systems such as differentiating xylem in Arabidopsis roots and hypocotyls of Phaseolus vulgaris Smith et al.
Various studies have demonstrated that the zinnia cell system is well appropriate for assessment of the morphological appearance and cell death progression in the consecutive stages of TE development by means of high resolution microscopy such as light, fluorescent and confocal laser scanning microscopy CLSM , transmission electron microscopy TEM , atomic force microscopy AFM , synchrotron radiation-based SR -FTIR spectromicroscopy and other techniques Supplemental File S3.
Our own experience supported the suitability of some of the labelling methods for identification of cell death features in in vitro differentiating zinnia TEs Fig. The labelling techniques used for the studies with zinnia are applicable also for similar purposes in other xylogenic systems. Zinnia culture isolation and induction and cellular morphology during transdifferentiation and cell death progression of TEs in xylogenic zinnia cell system.
Envy, as described in Tuwmasi et al. Three different laser wavelengths , and nm were employed for excitation, three emission channels for fluorescence imaging and one separate channel for non-confocal transmission imaging. Overlays and orthogonal projections were made using the Leica Confocal software. Cell wall cw , lysed cellular content lcc , nucleus nu , protoplast p , plasma membrane pm , secondary cell wall scw , vacuole v.
The use of chemical agents interacting with various pathways known as pharmacological analysis is well established approach for investigating the cellular signalling in vitro and in planta. Xylogenesis in zinnia cell system proceeds through a well-coordinated program in which a number of regulatory pathways are integrated.
A network of signalling interactions, metabolic pathways and gene and transcriptional factors involved in zinnia differentiation and PCD in vitro has been described also during xylem genesis in other cell and in planta model systems such as Arabidopsis thaliana , Populus, Pinus, Phyllostachys bamboo, Musa banana and others Fukuda and Komamine ; Iwasaki et al.
In plant tissues the wounding induces a cascade of signalling events culminating in various defence responses and PCD. The latter in turn may amplify the wound signal through enhancing prosystemin gene expression and the expression of other genes contributing to differentiation of xylem cells to build new xylem routes for bypassing the injured leaf area.
Transdifferentiation of in vitro cultured zinnia mesophyll cells is stimulated by wounding during the isolation of the cells Kuriyama and Fukuda ; Fukuda , Matsubayashi et al.
This indicated that wounded cells may produce and release PSK into the medium thus promoting the transdifferentiation.
Through the use of specific inhibitors and gene expression analysis, a role of PSK in the wound response has been confirmed. This hormone accumulates in the early stages after culture initiation and subsequently in the last stage of TE development.
It was established that in response to wounding PSK precursor gene ZePSK1 transcripts transiently accumulate in 24 h cultures and again at the entry into the final differentiation stage, whereby ZePSK1 expression was dependent on brassinosteroids BRs Yamamoto et al. Such interaction was supported by findings showing that uniconazole and brassinazole, which inhibit the synthesis of BRs can repress transdifferentiation in the early and late stages Iwasaki and Shibaoka ; Yamamoto et al.
Phytosulfokine performs multiple regulatory functions in plants and is suggested to integrate the growth and defence signals Sauter Microarray analysis revealed that in the xylogenic zinnia cell culture, in the presence of PSK, a number of stress-induced genes encoding for e. This suggests that PSK-associated signalling might be involved in the suppression of stress response. It remains to be elucidated whether PSK directly stimulates gene expression responsible for TE differentiation or acts through enhancing metabolic, transcriptional and translational activities that are commonly responsible for TE differentiation and for increasing the cell density in the culture by promoting the cell division Matsubayashi et al.
In isolated zinnia mesophyll cells during dedifferentiation stage I these genes are upregulated and during transdifferentiation stage II are downregulated Fukuda , Such findings suggest that the wound signal may play a dual role: to potentiate a defence reaction by preventing the proteolysis in mesophyll cells and further, by self-amplification to promote gene expression or post-translational activation of proteases involved in the later process of TE cell death Kuriyama and Fukuda In xylem tissue JA or methyl jasmonate MeJa may amplify wound signals by inducing the expression of genes involved in development of the vessel elements Kuriyama and Fukuda In planta JA has been suggested to trigger cambium cell division, which in turn might be related to an effect on xylem formation Sehr et al.
By expression profiling of hormone-related gene homologues in xylogenic zinnia cell culture Yoshida et al. Cis -abscisic acid ABA is suggested to play signalling role in association with the wound response in xylem Kuriyama and Fukuda An expression of ABA-inducible homeobox gene has been detected in Arabidopsis vascular bundles Vicient et al. The contribution of both JA and ABA to wound signalling has led to suggestion that they might be involved in the transduction of wound signals during zinnia xylogenesis in vitro Fukuda and Komamine ; Kuriyama and Fukuda And indeed, in zinnia culture Yoshida et al.
In similarity to JA, these genes have been expressed in stage I of xylogenesis. It was suggested that JA and ABA might indirectly contribute to xylem differentiation and that the induction of the culture with auxin, which stimulates transdifferentiation, might interrupt the progression of stage I. Motose et al. In PSK treated zinnia cell culture in the absence of auxin and CK, they found that several stress-responsive genes such as those encoding enzymes in phenylpropanoid pathway, chitinases, receptor-like protein kinases, ACS and other defense-associated proteins were downregulated.
The suppression of genes from ethylene biosynthesis and PAL pathway suggested that this may result in suppression of SA and ethylene production, thus affecting the mediation of wound-induced signalling in which SA and ethylene are supposed to play a role. The results point to a role of PSK in mitigation of the wound response in the early stages of TE differentiation.
This assumption was supported by additional experiments involving the application of stress-inducible hormones in conjunction with PSK. Jasmonic acid and MeJa suppressed the PSK-induced TE formation, whereas in the presence of SA, acetyl salicylic acid, ethylene precursor 1-aminocyclopropanecarboxylic acid ACC and ethylene releasing compound 2-chloroethylphosphonic acid ethephon , the percentage of formed TEs was almost unaffected Motose et al.
Cytokinin and auxin are compulsory required for induction of transdifferentiation of zinnia mesophyll cells. In planta the polar auxin flow ensures the continuous formation of vascular tissue Sachs The acropetal auxin transport drives the hormone from apical meristem, where it is synthesized, toward procambial cells resulting in their differentiation to form mature vessel strands.
In the case of wounding, auxin transport is interrupted leading to disturbed mode of xylem development Kuriyama and Fukuda ; Fukuda ; Mattsson et al. The molecular components of auxin perception in transdifferentiating zinnia cells are still poorly understood. The number of R-like TEs was not affected.
Auxin was suggested to repress the wound response thereby promoting the early stages of TE differentiation. This was supported by microarray analysis of in vitro transdifferentiating zinnia cells, reported by Yoshida et al. The authors identified cDNAs corresponding to proteins involved in auxin biosynthesis, metabolism, transport, and cDNAs acting as transcription factors homologues to Arabidopsis. Early auxin response genes were identified 0. Additionally, in the same set of experiments genes homologues to auxin transporter proteins, the influx carrier AUX1 and the efflux carrier proteins of PIN family were upregulated 4 h after addition of NAA.
These data substantiated the role of auxin in the early stages of TE development Yoshida et al. However, the expression of ethylene-related genes was almost unaffected when the zinnia cell suspension was supplemented with auxin Yoshida et al.
Cytokinins are responsible for vascular development through promoting cambium and procambium cell proliferation and acting in crosstalk with auxin Fukuda and Komamine ; Church and Galston ; Aloni ; Church ; Fukuda , , ; Kuriyama and Fukuda ; Pesquet et al. Bishopp et al. In turn, these factors might function as transcriptional activators of procambium genes including the genes of their repressors, the type-A ARRs, finally resulting in CK responses.
In the zinnia cell system, cytokinin is suggested to promote dedifferentiation of mesophyll cells prior to transdifferentiation into TEs Turner et al. The interplay of auxin and CK in the early stages of transdifferentiation of zinnia cultured cells was supported by the finding that 4 h after the administration of NAA the expression of cytokinin oxidase homologue was enhanced which indicated that auxin might act through activating this enzyme and in this way reducing the CK level Yoshida et al.
It was suggested that BRs contribute to the early transdifferentiation processes in zinnia in vitro Yamamoto et al. This was confirmed by results showing that in xylogenic zinnia cell culture, during stage II, transcripts of genes involved in BR synthesis accumulate in procambium-like cells that differentiate into xylem precursor cells Yamamoto et al. The presence of auxin and CK in inductive zinnia medium is considered sufficient to evoke de novo synthesis of the endogenous BR castersteron which is secreted out of the cells and may function as intercellular signal in the early stage of transdifferentiation and in the terminal stage of cell death Yamamoto et al.
Yoshida et al. They demonstrated that NAA promoted the synthesis of brassinolide intermediates, but suppressed its biosynthesis and stimulated enzymes that inactivate this BR. The authors suggested that the low levels of active BRs may be a mechanism for suppressing the immediate transdifferentiation of mesophyll cells into TEs. In Arabidopsis , these genes are suggested to encode for enzymes in BRs synthesis pathway Mathur et al.
Interaction of auxin, CK and BRs has been suggested to influemce the activity of basic peroxidase izoenzyme ZePrx that is involved in lignin biosynthesis in differentiating xylem in zinnia seedlings.
Treatment of the seedlings with auxin and CK induced ZePrx and metaxylem differentiation during seedling secondary growth whereas the exogenous application of BRs exerted an opposite effect.
Gibberellin effects are generally linked to cell elongation where it cooperates with auxin. It is thought that endogenous GA3 contributes to lignification Tokunaga et al. In conditioned control medium Tokunaga et al. They suggested that GA3 may act through activating the polymerization of lignin precursors. Addition of GA3 before hormonal induction of the culture caused a delay of TE differentiation suggesting that GA3 might exert an inhibitory effect during the early stage of transdifferentiation.
A lignification-associated interaction of auxin with GA3 signalling has been assumed based on the findings that in zinnia cell cultures supplemented with NAA, GA3 synthesis genes were upregulated Yoshida et al. Tokunaga et al. In planta , GA3 might, in cooperation with auxin and ethylene, be able to modulate the establishment of TE cell polarity in order to ensuring proper TE morphogenesis in vascular tissue Aloni ; Kalev and Aloni a , b.
This ubiquitous gaseous molecule is involved in the mediation of diverse physiological processes, abiotic and biotic stress responses and PCD. It can interact with cysteine-thiol groups and inactivate proteins through S-nitrosylation or through inactivating enzyme co-factors such as ferrous ion.
In cooperation with ROS, NO and reactive nitrogen species may exert antioxidant and pro-oxidant as well as cell death-protecting or death-promoting effects Delledonne et al. Recently, in differentiating xylem of Populus roots, in planta , Bagniewska-Zadworna et al. However, to verify the NO effects more profound molecular analysis and gene transcriptional profiling are necessary. The effect of PAs on cell death has been attributed to their ability to protect membrane stability by blocking the ion leakage from vacuoles and preventing the changes in mitochondrial membrane permeability.
The PAs spermine and thermospermine may possibly function as limiting factors that might regulate the levels of endogenous auxin or transcription factors responsible for auxin synthesis and auxin-dependent differentiation response and by such mechanism may control the timing of differentiation. In acl5 mutants the hypocotyls did not develop xylem tissue. The expression of ACL5 in zinnia cultured cells occurred before the onset of transdifferentiation and corresponded to the activation of the same gene established in protoxylem cells in Arabidopsis hypocotyls.
The studies have indicated that ethylene is involved in the signalling of zinnia TE differentiation Pesquet and Tuominen ; Pesquet et al. Ethylene and PA synthesis is intersected at the level of their common precursor s-adenosylmethionine, but to which extent the metabolic pathways of these hormones might crosstalk during zinnia TE differentiation is still not well understood.
This gaseous molecule is also presumed to link PAs and ethylene signalling with cell death Milhinhos and Miguel and references therein. The findings on hormone interactions soundly demonstrate the complexicity of the processes of transdifferentiation and PCD in zinnia in vitro and point out that for better elucidation of these regulatory mechanisms further studies are necessary, especially concerning the molecular targets of the hormonal signals. In the differentiation of xylem tissue, H 2 O 2 is required for lignification Novo-Uzal et al.
It is involved in peroxidase-mediated oxidative polymerization of cinnamyl alcohols to lignins and in the reinforcement of the cell wall through participating in cross-linking of cell wall proteins Ogawa et al.
The observations indicated that in the cell culture and in zinnia stems the living non-differentiating cells produce ROS before and at the beginning of lignification of SCWs. The early H 2 O 2 synthesis in the vital cells was suggested necessary for lignification in the earlier and later, including post mortem , stages of SCWs formation. A cationic peroxidase was purified from differentiating TEs Sato et al. The authors reported gene expression of basic peroxidase at the time of SCW lignification both in vitro and in planta.
They proposed peroxidase as a marker of TE lignification in zinnia in vitro and in lignifying xylem in planta. A second xylem H 2 O 2 producing pathway was suggested in a study based on addition of peroxidase inhibitor salicylhydroxamic acid which resulted in suppressed TE development Karlsson et al. The function of a NADPH oxidase-like enzyme in lignifying zinnia xylem cells was supported by pharmacological studies involving administration of a range of NADPH oxidase inhibitors such as pyridine, imidazole, quinacrine and diphenylene iodonium.
A non-enzymatic factor implicated in the regulation of cellular redox homeostasis is the peptide glutathione GSH. During isolation, the zinnia mesophyll cells are exposed to wound-induced oxidative stress which stimulates the dedifferentiation. In experiments of Henmi et al. The authors reported that exogenous application of GSH suppressed TE differentiation whereas the addition of GSSG increased the number of differentiated TEs if applied at early stage of cell culturing.
Enzymes such as serine and cysteine proteases and nucleases Fig. Most of these activities are considered as markers of xylogenesis Fukuda , , , ; Groover et al.
Suggested contribution of lytic enzymes to cell death execution of in vitro generated zinnia tracheary elements. In stage III of xylogenesis in in vitro cultured zinnia cells the tonoplast ruptures and various lytic enzymes are released from the vacuole resulting in autolytic digestion of the protoplast and DNA fragmentation.
The SCWs are partially completed, nucleus is condensed. Background image—a dead tracheary element in stage III of transdifferentiation; incomplete and completed SCWs are distinguished following Calcofluor White labelling. It is suggested that the enzyme is released from the vacuole after tonoplast rupture and contributes to autolysis of the cellular content of the TEs. The amino acid sequence of this enzyme was found very similar to barley endonuclease BEN1 which participates in the breakage of nuclear DNA during the cell death of the endosperm in barley seeds Aoyagi et al.
The same types of ZRNase genes were detected in differentiating xylem and in response to wound stress in zinnia plants Ye and Droste These results demonstrated that endonucleases are implicated in the process of xylem differentiation. Other proteolytic enzymes were also reported to accumulate in the vacuole of transdifferentiating zinnia cells and to be released after vacuolar collapse Obara et al.
Among them is thrombin-like protease TLP with pH optimum 5. It was suggested that it participates in the collapse of the tonoplast or in the autolysis of cellular content Yu et al.
In TE-inductive zinnia culture, proteases expressing an activity against several peptidyl 4-methylcoumarylamido MCA substrates have been found. The amount of hydrolyzed carbobenzoxy-Phe-Arg-MCA Z-Phe-Arg-MCA , a specific substrate for cathepsin L enzyme in animal systems, was stable in freshly isolated mesophyll cells but increased in differentiation-related manner following the addition of auxin and CK.
A protein with 30 kDa molecular mass, located in the vacuole, was established to be responsible for this activity and identified as cysteine endopeptidase with a pH optimum at pH 5. Several investigations have shown that similar genes are upregulated and transcripts coding for different proteins related to SCW formation and cell death begin to accumulate at the same time suggesting that common signals may induce both PCD and SCW deposition Fukuda , , ; Demura et al.
A serine protease with molecular mass of 60 kDa was identified during the progression of TE differentiation in zinnia cell culture and was suggested to contribute to cellular autolysis Ye and Varner Groover and Jones detected a kD serine protease which is secreted during SCWs synthesis and is released from the collapsed vacuole after SCWs are visually completed Fukuda This protein was suggested as a possible coordinating factor between SCW synthesis and cell death at the end of PCD process.
Pharmacological studies revealed the participation of more proteolytic factors in the different stages of differentiation Supplemental File S4. Among them are various specific and broad range cysteine and serine proteases and the proteasome Minami and Fukuda ; Ye and Varner ; Woffenden et al. Fukuda reported changes in tubulin synthesis during cell division of isolated zinnia mesophyll cells and TE differentiation.
These genes encode for the protein tubulin which controls the orientation of newly deposited cellulose microfibrils related to the positioning of SCWs. In the seedlings ZeTubB transcripts were detected in the ground meristem and in procambium. Together, these findings suggested preferential expression of tubulin genes in procambium stem cells and in differentiating xylem cells Yoshimura et al.
Among them are the findings of Twumasi et al. However, because in the experiments of these researchers TE formation was partially but not entirely inhibited by tetrapeptide caspase inhibitors, it was proposed that the CL enzymes might be activated in early stages of transdifferentiation, upstream of cell wall deposition or at least before visual appearance of cell wall thickenings while the cell is still alive. In support to this suggestion was our observation that the caspase inhibitors did have an effect on TE differentiation only if applied simultaneously with hormonal induction and not earlier or later Iakimova and Woltering unpublished data.
Han et al. Together these findings show that common CLPs pathways might contribute to the early and late stages of zinnia TE development in vitro and might be engaged even during stage I when the cells acquire a competence to respond to auxin and CK. Although the knowledge about the contribution of CLPs to TE differentiation in zinnia cell culture is still in its infancy, in other plant systems their involvement in xylogenesis has been proven Petzold et al. By immunohistochemical methods and immunoelectron microscopy, Hao et al.
During development of secondary xylem in Populus tomentosa , by using liquid chromatography-tandem mass spectrometry, Han et al. They found that in the presence of Ac-DEVD-CHO xylem formation in Arabidopsis cotyledons was suppressed, which additionally pointed to a role of caspaselike protease in xylem cell death. The CL enzyme VPE a plant protease that expresses caspaselke activity has been shown to play a role in posttranslational modification of a variety of vacuolar proteins Hatsugai et al.
Further, microarray analysis reported by Courtois-Moreau et al. As VPE is implicated in almost all known forms of vacuolar PCD and is considered as one of the hallmarks of this cell death type van Doorn and Woltering , ; van Doorn et al. Metacaspases are a class of cell death-associated proteases that are structurally related to animal caspases but have different substrate specificity Uren et al.
During the late stage of TE differentiation in Arabidopsis and during xylem maturation in Populus microarray analysis revealed upregulation of a homolog of Arabidopsis metacaspase 9 AtMC9 Turner et al. These proteases were implicated in micro-autolysis of cellular structures before tonoplast rupture and in mega-autolysis of the entire protoplast following tonoplast breakage in differentiating TEs in Arabidopsis cell culture Zhao et al.
The authors suggested that AtMC9 may potentially affect other papain-like proteases participating in post - mortem protoplast clearance. This presumption was substantiated by observations indicating that a cysteine protease Tr-cp14 that is closely related to XCP1 and XCP2 accumulated in the ER and Golgi vesicles, from where it appeared to spread throughout the cell during the collapse of the central vacuole of in planta differentiating TEs of Trifolium repens Mulisch et al.
No reports about metacaspase identification in transdifferentiating cultured zinnia cells are yet available. In zinnia cell culture the major portion of mesophyll cells transdifferentiate into TEs without prior cell division Church and Galston ; Church Initially Fukuda and Komamine a and earlier Basile et al.
The TE differentiation in the xylogenic cell cultures and in planta is non-autonomous process dependent on substances supplied by the living non-differentiated cells and from immature TEs. The factors involved in cell-to-cell signalling operate in complicated but well synchronised manner Fig. The studies on the role of intercellular signalling during TE development in zinnia cell system, zinnia stems and Arabidopsis have revealed a messenger role of arabinogalactan ARG -like proteins, BRs and PSK in the control over initiation of differentiation program.
Mono- and dilignols, and H 2 O 2 produced in the living cells and transferred through extracellular space to differentiating TEs are responsible for lignification of the SCWs in the immature, maturating and mature TEs, including the process post - mortem.
Intercellular signalling in xylogenic zinnia cell cultures. Some of the coniferous genera division Coniferophyta are the most important timber trees in the world. Since these species have several cotyledons inside their seeds, they are conveniently referred to as polycots. In dicot stems, the cambium layer gives rise to phloem cells on the outside and xylem cells on the inside.
All the tissue from the cambium layer outward is considered bark, while all the tissue inside the cambium layer to the center of the tree is wood. Xylem tissue conducts water and mineral nutrients from the soil upward in plant roots and stems. It is composed of elongate cells with pointed ends called tracheids, and shorter, wider cells called vessel elements. The walls of these cells are heavily lignified, with openings in the walls called pits. Tracheids and vessels become hollow, water-conducting pipelines after the cells are dead and their contents protoplasm has disintegrated.
The xylem of flowering plants also contains numerous fibers, elongate cells with tapering ends and very thick walls. Dense masses of fiber cells is one of the primary reasons why angiosperms have harder and heavier wood than gymnosperms.
This is especially true of the "ironwoods" with wood that actually sinks in water. Holbrook, M. Zwieniecki and P. Melcher suggests that xylem cells may be more than inert tubes. They appear to be a very sophisticated system for regulating and conducting water to specific areas of the plant that need water the most.
This preferential water conduction involves the direction and redirection of water molecules through openings pores in adjacent cell walls called pits. The pits are lined with a pit membrane composed of cellulose and pectins. According to the researchers, this control of water movement may involve pectin hydrogels which serve to glue adjacent cell walls together.
One of the properties of polysaccharide hydrogels is to swell or shrink due to imbibition. But when pectins shrink, the pores can open wide, and water flushes across the xylem membrane toward thirsty leaves above. Magnified horizontal view x of an inner perianth segment of a Brodiaea species in San Marcos showing a primary vascular bundle composed of several strands of vessels.
The strands consist of vessels with spirally thickened walls that appear like minute coiled springs. Although this species has been called B. This species contains at least 3 strands of vessels per bundle, while B. T he water-conducting xylem tissue in plant stems is actually composed of dead cells. In fact, wood is essentially dead xylem cells that have dried out. The dead tissue is hard and dense because of lignin in the thickened secondary cell walls.
Lignin is a complex phenolic polymer that produces the hardness, density and brown color of wood. Cactus stems are composed of soft, water-storage parenchyma tissue that decomposes when the plant dies. The woody lignified vascular tissue provides support and is often visible in dead cactus stems. Left: Giant saguaro Carnegiea gigantea in northern Sonora, Mexico.
The weight of this large cactus is largely due to water storage tissue in the stems. Right: A dead saguaro showing the woody lignified vascular strands that provide support for the massive stems. It is composed of sieve tubes sieve tube elements and companion cells. The perforated end wall of a sieve tube is called a sieve plate. Thick-walled fiber cells are also associated with phloem tissue.
I n dicot roots, the xylem tissue appears like a 3-pronged or 4-pronged star. The tissue between the prongs of the star is phloem. The central xylem and phloem is surrounded by an endodermis, and the entire central structure is called a stele. Microscopic view of the root of a buttercup Ranunculus showing the central stele and 4-pronged xylem. The large, water-conducting cells in the xylem are vessels.
Phloem tissue is produced on the outside of the cambium. The phloem of some stems also contains thick-walled, elongate fiber cells which are called bast fibers. Bast fibers in stems of the flax plant Linum usitatissimum are the source of linen textile fibers. Gymnosperms generally do not have vessels, so the wood is composed essentially of tracheids.
The notable exception to this are members of the gymnosperm division Gnetophyta which do have vessels. See Article About Welwitschia P ine stems also contain bands of cells called rays and scattered resin ducts. Rays and resin ducts are also present in flowering plants. In fact, the insidious poison oak allergen called urushiol is produced inside resin ducts. Wood rays extend outwardly in a stem cross section like the spokes of a wheel.
The rays are composed of thin-walled parenchyma cells which disintegrate after the wood dries. This is why wood with prominent rays often splits along the rays.
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