Ferns and their relatives are vascular plants, meaning they have xylem and phloem tissues. Because of the presence of vascular tissue, the leaves of ferns and their relatives are better organized than the mosses and liverworts. Bryophytes nonvascular plants are a plant group characterized by lacking vascular tissues. They include the mosses, the liverworts, and the hornworts.
These groups of plants require external water, usually in the form of dew or rain. Some of them grow exclusively in dark, damp environments in order to provide moisture.
Find out more about them here Leaves are the major photosynthetic organ of a plant. Apart from that, they are also crucial to water movement. In this tutorial, various plant processes are considered in more detail.
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Sign In or Create an Account. Sign In. Advanced Search. Search Menu. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents Abstract. Materials and methods. Vascular tissue in the stem and roots of woody plants can conduct light.
Qiang Sun , Qiang Sun. Oxford Academic. Google Scholar. Kiyotsugu Yoda. Mitsuo Suzuki. Hitoshi Suzuki. Select Format Select format. Permissions Icon Permissions. Abstract The role of vascular tissue in conducting light was analysed in 21 species of woody plants. Open in new tab Download slide. Plant, Cell and Environment. Netherlands: Kluwer Academic Publishers, The Plant Cell.
Amazingly, in the present Arabidopsis model, impressive variety of tracheary elements is detected, not previously documented in analyzed hypocotyls [ 37 , 38 ] or adult stems of Arabidopsis [ 39 — 41 ].
Secondary xylem in the weight stimulated stems of Arabidopsis. A Secondary xylem elements, like vessels and fibers, are produced from cambial derivatives after numerous periclinal divisions of fusiform cambial cells. Cortex parenchyma is visible outside the secondary vascular tissues. B Schematic visualization of the tissue arrangement in stem and localization of the tissues showed in A is indicated by the square.
C Vessel strand developed parallel to longitudinal axis of stem. Patterning of vascular tissue and variety of tracheary elements developed as a dynamically operating water-conducting system and was extensively studied in the woody plants [ 13 , 14 ]. However, mechanism regulating xylogenesis at cellular and molecular levels remains unclear, and many questions are unanswered. For example, differentiation of tracheids as a type of tracheary elements commonly found in trees, but for the first time detected in mechanically stimulated Arabidopsis , led to important conclusions about the involvement of the artificial weight in wood formation.
Following stages of xylogenesis involving formation of the variety of tracheary elements, such as recognized tracheids, will be helpful in future analysis. In , Sachs postulated canalization hypothesis according to which vasculature patterning is based on the positive feedback loop between auxin flow and cellular polarity.
Consequently, in the primary uniform tissue, cellular auxin transporters emerge as the so-called auxin channels that transport the hormone through the tissue in the polar direction. Emergence of auxin channels is correlated with establishment of cellular polarity inside these specific auxin transport routs. Finally, new vessels develop directly along the auxin channels. Canalization hypothesis is strongly supported by many classical experiments with the incised plants, i. It is well documented that initially broadly elevated auxin response in wounded tissues is gradually restricted to narrow auxin channels, in which auxin level is still very high [ 4 ].
The obtained results showed that patterning of vascular tissue, explicitly visible during regeneration and new vasculature development, is dependent on new ways of canalized auxin flow. Well-functioning vascular cambium plays the most important role for the secondary growth in the woody plants, both secondary xylem formation and stem thickness [ 14 , 21 , 22 , 60 ].
Many results revealed an important role for this meristematic tissue during vasculature regeneration process. For decades analysis of vascular patterning and incised vascular cambium regeneration was restricted mainly to trees [ 61 — 63 ] because these woody plants undergo secondary growth with enlarged amount of secondary xylem wood and active cylinder of vascular cambium [ 64 ]. Studies were based mainly on the histological analysis, thus limited only to the final effects of regeneration.
Thus, it was impossible to analyze vasculature regeneration, including vascular cambium, on the cellular and molecular levels. Some experimental studies on trees showed that in the wounded areas, the cambium and vascular tissue regenerate very fast both in vivo [ 19 , 20 ] and in vitro [ 25 , 65 , 66 ]. Regeneration is accompanied by numerous anticlinal divisions of cambial cells and their dynamic intrusive growth [ 19 , 20 , 64 ], which finally leads to the reconstruction of vasculature and new vessel patterning in the incised regions [ 25 , 65 ].
In some instances, when the auxin flow is locally reversed, the so-called circular vessels develop [ 32 , 67 , 68 ]. In the nondisturbed woody plants, circular vessels are often found in branch junctions, above the axillary buds [ 68 ], whereas in incised plants, after transversal cuts and exogenous auxin application to stem segments, in wounded regions [ 32 , 67 ].
Accordingly, circular vessels occur in the form of rings and are presumably induced as a consequence of the circular auxin flow and the establishment of the circular polarity of individual cells that dedifferentiated into this type of vessels [ 67 ].
Thus, according to Sachs and Cohen [ 67 ], circular vessels develop as a response of individual cells to the auxin flux rather than to the high local auxin concentration. In nonwoody dicotyledonous plants characterized by primary tissue architecture, such as Phaseolus vulgaris , Pisum sativum , or Coleus sp. New vessels are arranged either around the wound according to the presumable new auxin flow [ 69 ] or form the so-called bypass strands directly through the wound [ 3 ] or bridges between the neighboring vascular bundles [ 70 ].
Lack of the vascular cambium in the studied nonwoody plants restricted a detailed analysis of regeneration of this meristematic tissue and cellular events accompanying this process. Therefore in the used models, the most intriguing questions are still remained of answer: 1 what is the role of vascular cambium in vascular tissue regeneration? Full verification of the postulated canalization hypothesis and identification of the molecular mechanisms accompanied vascular tissue regeneration are still limited.
Because of the difficulties in using woody plants as a convenient model system [ 52 ], mechanisms of cambium regeneration are still poorly understood. Thus, in control conditions, i. Otherwise, in incised stems i.
As a consequence, new vessel strand arrangement is changed, because the new vasculature likely developed according to new directions of auxin cell-to-cell transport Figure 5. In wounded Arabidopsis stems, threads of new vessel strands develop above or around a wound Figure 5A and B , respectively.
Interestingly, vessels above a wound regenerated faster, in the first days after wounding DAW 2 and 3 days , whereas vessel around a wound differentiated in the next few days, beginning the day 4 and circumventing the incised areas.
They developed from cells after their numerous, uneven divisions, what is commonly observed in the wounded tissue. The AtHB8 is positively regulated by auxin, and its extensive activity in wounded regions during vascular tissue regeneration suggested that AtHB8 might play a crucial role in the vasculature development [ 71 , 72 ].
The last observed way of vasculature regeneration is correlated with callus differentiation Figure 5C. Namely, in wounded areas vessels develop from previously proliferated callus tissue cells. Paths of vessel regeneration in wounded Arabidopsis stems. A Threads of short vessel members developed above a wound. B Vessel strands regenerated around a wound.
Arrows indicate regenerated vessel strands. Regeneration of vascular tissue in wounded Arabidopsis stems is accompanied by temporal and spatial changes following new vessel development. New vessel strands regenerated in the incised regions around a wound develop as a consequence of cambial cell regeneration. Longitudinal continuum of vascular cambium is disturbed after the transversal cut.
In such experimental system, rapid auxin response is found as a primary signal of the regeneration. Merely at the first day after incision, elevated auxin concentration is observed above a wound and in the next few days also around a wound [ 8 ].
Vasculature regeneration is strictly correlated with tissue repolarization and establishment of new polarity in neighborhood of the wound. Tissue repolarization always preceded emergence of PIN1-positive auxin channels Figure 6.
As a consequence, layer of new vessels develops around a wound, and the regenerated vasculature becomes enlarged in the days following the incision. Analysis of regeneration process in incised Arabidopsis stems strongly supported canalization hypothesis.
Emergence of new vasculature is correlated here with elevated auxin response and changed polarity in auxin channels, from which new vessel strands develop in the wounded areas. Auxin is regarded as a multifunction plant hormone, which plays a fundamental role in developmental processes during organo- and morphogenesis.
Auxin is a primary signal in regulation of many cellular processes, which control oriented divisions, cell elongation, or differentiation. At last, auxin is a key hormonal factor inducing vascularization—vascular tissue development, patterning, and regeneration.
Polar auxin transport PAT manifested as physiological, basipetal direction of auxin flow represents a unique mechanism specific to plants. The cellular and molecular action of this process, explained in the chemiosmotic model, is based on auxin influx and efflux carriers, namely, AUX and PIN proteins, which actively participate in the cell-to-cell hormone transport [ 73 — 75 ].
The local auxin accumulation, its minima and maxima, or the so-called gradients in tissues are precisely controlled by this process. The role of auxin as a primary signaling cue in vascularization has been widely discussed for decades. Experiments with radioactively labeled auxin show its maximum concentration in the meristematic tissues such as cambium [ 22 , 57 ] and in adjacent cambial derivatives, differentiating into xylem [ 76 ].
Periodic fluctuation of auxin concentration in cambium influences the frequency of cambial cell divisions, production of cambial derivatives, and secondary vascular tissues. Disturbance of these correlations leads to many defects in cambium functioning and xylem formation.
Using transgenic lines of Arabidopsis , elevated auxin response is easily found just in the cambial cells of both types of cambia Figure 7.
Auxin concentration is very high in the fascicular cambium bands, primary meristematic tissue in the vascular bundles Figure 7 , as well as in the interfascicular vascular cambium on the stem circumference Figure 7. Elevated auxin concentration in cambium of non-incised Arabidopsis control stems. From the experimental studies on the vascularization in vitro , it appears that parenchyma callus tissue is the most convenient for the analysis.
Previously uniform callus can form vascular tissue bands or groups of vessels differentiation. However, the process can be realized only in the sufficiently thick callus tissue. It is shown that differentiated xylem in surrounded by cambium-like cells, which additionally are able to produce phloem elements in the inner callus regions.
Auxin-dependent vascularization is also shown in the studies with young Syringa sp. Combination of auxin and sucrose decides about the induction of vascularization in the axillary buds in vitro. Moreover, dependent on the hormone and sucrose concentration, varied vascular tissues develop.
Several reports discussed auxin as a specific morphogenetic signal triggering cell fates during vascular tissue development and its maturation [ 78 ]. Locally created centers characterized by elevated auxin response become more competent for auxin flow through primarily uniform tissues.
Auxin waves created in plant organs as a specific system of hormonal information that decide about realization of many developmental programs in plants, among them cambial activity and differential cambial responses [ 79 , 80 ].
Thus analogically, gradual emergence of auxin channels and gradually narrowing auxin flow finally results in vascular strand differentiation. The formation of the vascular system is a well-organized plant developmental process, but it is also flexible in response to environmental changes.
Provascular cells arise after asymmetric cell division in early embryos and differentiate into various vascular cells, including procambial cells, which function as vascular stem cells.
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