Role of hormones in controlling vascular differentiation and the mechanism of lateral root initiation
The vascular system in plants is induced and controlled by streams of inductive hormonal signals. Auxin produced in young leaves is the primary controlling signal in vascular differentiation. Its polar and non-polar transport pathways and major controlling mechanisms are clarified. Ethylene produced in differentiating protoxylem vessels is the signal that triggers lateral root initiation, while tumor-induced ethylene is a limiting and controlling factor of crown gall development and its vascular differentiation. Gibberellin produced in mature leaves moves non-polarly and promotes elongation, regulates cambium activity and induces long fibers. Cytokinin from the root cap moves upward to promote cambial activity and stimulate shoot growth and branching, while strigolactone from the root inhibits branching. Furthermore, the role of the hormonal signals in controlling the type of differentiating vascular elements and gradients of conduit size and density, and how they regulate plant adaptation and have shaped wood evolution are elucidated.
KeywordsAuxin-transport pathwaysEthylene signalingLateral root initiationMobile gibberellin signalCambium activityVessel diameterXylem adaptation
Lateral root initiation
The aim of this review is to integrate recent molecular with organismal findings in a holistic approach for elucidating the hormonal mechanisms that control vascular differentiation in the whole plant and their adaptation with evolutionary aspects. I apologize to colleagues whose work could not be included only because of space restrictions.
Research on vascular differentiation has expanded at an impressive rate focusing on molecular genetic aspects of xylem cell and tissue differentiation, and the topic has been intensively reviewed (e.g., Scarpella and Helariutta 2010; Caño-Delgado et al. 2010; Oda and Fukuda 2012; Bollhöner et al. 2012; Aloni 2013; Lucas et al. 2013; Milhinhos and Miguel 2013; Miyashima et al. 2013; Sorce et al. 2013).
Here, I provide evidence on auxin-transport pathways, discuss the importance of ethylene production in maturing vessels (Pesquet and Tuominen 2011) for understanding the mechanism inducing lateral root initiation (LRi), elucidate the nature of the mobile gibberellin signal (Dayan et al. 2012), how the duration of cell differentiation influences the gradient in tracheid size along the stem (Anfodillo et al. 2012), how soluble carbohydrates regulate auxin biosynthesis which improves environmental adaptation (Lilley et al. 2012; Sairanen et al. 2012), the role of strigolactone in controlling shoot architecture (Agusti et al. 2011; Shinohara et al. 2013) and evidence for non-cell-autonomous postmortem lignification of tracheary elements (Pesquet et al. 2013).
This review appears after 40 productive years of publications in Planta (Aloni and Sachs 1973), during which I have tried to uncover control mechanisms of vascular differentiation in plants. Throughout these years, I have suggested hypotheses which stem from intuition, before obtaining their supporting evidence, aiming to stimulate students to discover how plants operate. One of them is our lateral root initiation hypothesis (Aloni et al. 2006), for which today we have the needed evidence that will be presented below. Although numerous excellent molecular and genetic results have been published, the mechanism of LRi has remained in the dark. This situation justifies suggesting hypotheses to improve understanding.
Auxin is the young leaf signal
The auxin hormone, namely, indole-3-acetic acid (IAA) is the most common naturally occurring auxin. IAA is the major shoot signal which regulates all aspects of vascular differentiation in plants (Aloni 2001).
The pioneering study of Jacobs (1952) demonstrated that the IAA produced in young leaves is the limiting and controlling signal of xylem regeneration around a wound. The polar transport of IAA from young shoot organs (Aloni 2010) downward via the procambium and cambium to the root tips (Aloni et al. 2006) induces and controls vascular differentiation.
The continuity of the vascular tissues along the plant axis is a result of the steady polar flow of IAA from leaves to roots (Sachs 1981; Aloni 1987; Berleth et al. 2000; Scarpella and Helariutta 2010).
The orderly pattern of vascular tissues from leaves to roots was explained by the canalization hypothesis (Sachs 1981). According to this hypothesis, IAA flow, which starts by diffusion, induces a polar IAA transport system that promotes IAA movement and leads to canalization of the IAA flow along a narrow file of cells. The continuous polar transport of IAA through these cells induces a further complex sequence of events which terminates in the formation of a vessel (Sachs 1981). Molecular evidence supports the canalization hypothesis demonstrating that rearrangement of polar IAA flow changes tissue polarity through modification of the site of a PIN-FORMED (PIN) protein (an essential component involved in IAA efflux) on the plasma membrane (Sauer et al. 2006; Teale et al. 2006; Kleine-Vehn et al. 2011).
Repeating some of Sachs’ (1981) experiments and analyzing them with molecular tools reveal that treatments like wounding lead to rearrangements in the subcellular polar localization of the PIN auxin transport components. This auxin effect on PIN polarity is cell specific, does not depend on PIN transcription and involves the IAA response factor (Aux/IAA-ARF) signaling pathway. The experiments show that IAA acts as polarizing signal, which links individual cell polarity with tissue and organ polarity through the control of PIN polar targeting (Sauer et al. 2006).
IAA transport pathways
All living cells in the plant body are capable of transporting IAA, but only those through which IAA is canalized become specialized to transport the hormone rapidly, resulting in canalized files of cells (Sachs 1981). During plant development, initial IAA flows are canalized into three main routes of IAA transport (Fig. 2a). These flow patterns can be visualized by different methods showing polar IAA flow along the epidermis–phellogen, in the procambium–cambium and the non-polar transport via the sieve tubes (Figs. 1, 2).
The non-polar IAA flow originating in mature leaves courses in the phloem conduits, where IAA moves rapidly (Morris et al. 1973; Goldsmith et al. 1974) up and down via the sieve tubes (white arrows in Fig. 1a). This fast non-polar IAA flow is considered a housekeeping signal that reduces callose levels in the sieve tubes (Aloni 2010). The non-polar IAA flow can also remove the dormancy callose and promote the resumption of phloem activity in spring, whereas a non-polar cytokinin flow in the sieve tubes (Kudo et al. 2010) increases callose levels on the sieve plates and can plug the sieve tubes for winter dormancy (Aloni 2010 and references therein).
By combining cross-sectional analysis of both GUS expression and auxin immunolocalization (Figs. 1, 2b), we show the anatomy of each specific transporting cell and demonstrate the three major IAA pathways in the secondary tissues: (1) cambium, (2) sieve tubes and (3) phellogen (Fig. 2) through which IAA moves continuously. However, it should be noted that during the short process of vessel differentiation (which is induced by IAA), the IAA moves polarly through differentiating vessel elements and can be visualized by GUS expression (Fig. 1b, c).
Venation pattern formation in leaves
Vascular differentiation in a leaf is limited to early stages of primordium development. The gradual pattern of IAA production during leaf development was explained by the leaf venation hypothesis (Aloni 2001). This hypothesis was confirmed experimentally (Aloni et al. 2003) showing that the primary sites of IAA production are the developing hydathodes at the leaf margin. During leaf-primordium development, there are gradual shifts in the sites and concentrations of IAA production, progressing from the hydathode of the elongating tip, continuing downward along the expanding blade margins and ending at the central regions of the lamina (Aloni et al. 2003).
In leaves of Arabidopsis plants treated with auxin transport inhibitors, the vascular tissues became progressively confined toward the leaf margin. When the concentration of auxin transport inhibitor was increased, the vascular elements were more restricted to the margin, indicating that the leaf vascular system depended on inductive signals from the leaf margin (Mattsson et al. 1999).
Scarpella et al. (2006) elegantly demonstrated in Arabidopsis leaf primordia that the auxin efflux-associated protein, PIN1, is polarly expressed in the cell membranes prior to pre-procambial formation, demonstrating the IAA flow directions and pathways in the primordium prior to procambium formation. Integrated polarities in all emerging veins indicate IAA drainage toward pre-existing veins, but veins could display divergent polarities until they become connected at both ends (Scarpella et al. 2006).
Recently, Sawchuk et al. (2013) defined a new control level of vein network formation in Arabidopsis leaves by a concerted action of two spatially separate auxin-transport pathways. In addition to the well-known polar IAA transport from cell to cell by the plasma membrane-localized PIN1, the other pathway occurs inside the cell involving auxin transport mediated by the evolutionarily older endoplasmic reticulum-localized auxin transporters PIN6, PIN8 and PIN5, which are expressed at sites of vein formation. Both PIN6 and PIN8 promote vein patterning and positively regulate PIN1 expression, while PIN5 antagonizes their function. Sawchuk et al.’s (2013) findings suggest an ancestral auxin transport-dependent mechanism of endoplasmic reticulum-localized PIN proteins for vascular canalization of cell files, which predates the evolution of the plasma membrane-localized PIN1.
The lateral root initiation hypothesis
In spite of the increasing physiological and molecular information on lateral root formation, the hormonal mechanism that controls LRi and orientation remains poorly understood (Negi et al. 2008; Dubrovsky et al. 2011; De Smet 2012; Marhavý et al. 2013; Muraro et al. 2013). Aloni et al. (2006) proposed the lateral root initiation hypothesis suggesting that ethylene produced in differentiating protoxylem vessels near the root tip is the signal promoting the earliest stage of LRi at the xylem pole. When we proposed this concept, it was mostly intuition, with no supporting evidence. Today, there is new published experimental evidence from diverse laboratories that strongly support our hypothesis, although no one has related the evidence to our concept. Recently, Pesquet and Tuominen (2011) found that ethylene is produced in maturing vessel elements, indicating that a differentiating protoxylem vessel at the root tip can produce a low amount of ethylene. Furthermore, Ivanchenko et al. (2008) have shown that the application of low concentration of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) promotes LRi. Moreover, the PUCHI gene, which encodes a putative APETALA2/ethylene-responsive element, is expressed in the pericycle at the site where the founder cells of an emerging lateral root will be produced (Hirota et al. 2007, e.g., Fig. 5A, B), indicating the accumulation of ethylene in the pericycle prior to LRi and showing where a future lateral root will be produced (Hirota et al. 2007). Additionally, the plant cytosolic enzyme 1-aminocyclopropane-1-carboxylate synthase (ACS) which catalyzes the rate-limiting step in the ethylene biosynthetic pathway is expressed near the root tip around differentiating protoxylem vessels in both the cell division zone (see: ACS2, ACS8, in Tsuchisaka and Theologis 2004, fig. 11) and the cell expansion zone (ACS5, ACS6, ACS7, also in fig. 11) marking ethylene production by differentiating protoxylem vessels (Tsuchisaka and Theologis 2004). All these experimental results support the idea that low ethylene emission from differentiating protoxylem vessels is positively involved in LRi. Conversely, application of high ethylene concentrations can inhibit LRi.
One should realize that the phenomenon of LRi is controlled from inside the vascular cylinder by a stimulating signal arriving from the differentiating protoxylem vessels. This is the reason why in dicotyledonous roots the LRi occurs at the xylem pole, but never develops at the phloem pole (Dubrovsky et al. 2000). If the stimulation would have arrived from the cortex side, we would expect LRi also in the phloem pole. It has been shown experimentally that sieve element differentiation is induced by very low IAA concentrations and that there is need for high IAA concentration to induce a vessel (Aloni 1980, 2001). Accordingly, the high IAA concentration streams which induce vessels can induce ethylene emission, while the very low IAA stream inducing sieve elements is too low to stimulate ethylene synthesis. Additionally, the innermost layer of the cortex is the endodermis, which is compactly arranged and lacks air spaces. Consequently, the endodermis slows down the transport of ethylene emission from the differentiating protoxylem cells outward to the cortex, thus locally boosting ethylene concentration in the pericycle (which is located just inside the endodermis).
Low-concentration ethylene, which is synthesized in differentiating protoxylem vessel elements, is the trigger that determines the site of LRi at the xylem pole (Fig. 3) by locally concentrating the IAA in the pericycle. The earliest stage of LRi is induced by this local low-concentration ethylene production in the differentiating protoxylem vessel elements. The ethylene (C2H4) is released from the differentiating protoxylem vessel elements and diffuses to the neighboring tissue. In the centrifugal direction, the C2H4 is locally accumulated in the pericycle (as its further centrifugal movement is slowed down by the densely packed endodermis). The local accumulation of ethylene in the pericycle inhibits the polar low-concentration IAA movement in the pericycle adjacent to the differentiating protoxylem vessel. Therefore, immediately at and above this IAA inhibition site, newly arriving IAA from young leaves is locally accumulated in the pericycle; this fast IAA buildup (detected by DR5::GUS expression: Benková et al. 2003, Fig. 1A0) stimulates cell divisions in the pericycle, inducing the founder cells of a new lateral root (Fig. 3).
The cytokinin (CK), which inhibits LRi, originates in the root cap and moves upward through the root vascular cylinder (Aloni et al. 2005). The distance of LRi from the root cap is regulated by CK concentration. The high CK concentrations at the root cap antagonize IAA and inhibit LRi in the vicinity of the cap, which is crucial for enabling uninterrupted elongation of the root tip. Therefore, lateral roots initiate further away from the CK-synthesizing cap, occurring above the elongation zone, thus ensuring the elongation of a smooth primary root tip free from lateral roots. Above the elongation zone, where concentrations of CK decrease, lateral roots can initiate.
Vascular differentiation in plant tumors
Ethylene is also an important regulator of plant tumor development and vascular differentiation in both the tumor and the adjacent host tissues. The most studied tumorous plant tissues are the crown galls induced by Agrobacterium tumefaciens on many plant species (Aloni and Ullrich 2007). High ethylene production in these galls is likely a result of the high IAA and high CK production by the A. tumefaciens-transformed plant cells (Aloni et al. 1998).
Plant tumors induced by A. tumefaciens were considered unorganized or partly organized masses (Sachs 1991 and references therein). However, a three-dimensional pattern analysis of the phloem and xylem in the A. tumefaciens-induced crown galls unveiled a sophisticated vascular network of continuous vascular strands extending from the host plant up to the tumor surface. The development of these strands indicates synthesis of IAA by the A. tumefaciens-transformed plant cells located immediately beneath the surface of the fast-growing tumor (Aloni et al. 1995).
Analysis of vascular tissues in crown galls and their adjacent tissues have triggered us to propose the gall-constriction hypothesis (Aloni et al. 1995), which explains the mechanism that gives priority to water supply to the growing gall over the host shoot. The hypothesis proposes that a growing gall retards the development of its host shoot by decreasing vessel diameter in the shoot tissues adjacent to the tumor, which substantially reduces water supply to the upper parts of the shoot. It was further postulated that the controlling signal that induces the narrow vessels in the host is the hormone ethylene (Aloni et al. 1995), which is known to reduce vessel diameter (Yamamoto and Kozlowski 1987). The gall-constriction hypothesis was experimentally confirmed by showing that tumor-induced ethylene was a limiting and controlling factor of crown gall morphogenesis; very high ethylene concentrations were produced continuously by a growing crown gall during a few weeks, up to 140 times more ethylene than in wounded, but not infected control stems, reaching a maximum at 5 weeks after infection (Aloni et al. 1998; Wächter et al. 1999). Tumor-induced ethylene diminished vessel diameter in the host stem and enlarged the surface (through which high transpiration occurs) of the tumor (Aloni et al. 1998), thus giving priority to water supply to the growing gall over the host shoot. Comparison between the development of plant and animal tumors has shown an analogous requirement for neovascularization in both, suggesting possible strategies for prevention and therapy (Ullrich and Aloni 2000). The discovery that plant tumors produce ethylene required for gall development (Aloni et al. 1998; Wächter et al. 1999) has promoted the development of ethylene-insensitive fruit trees, which are tumor free.
Xylem cell differentiation and lignification
In the xylem, the conducting cells are the tracheary elements (TE). They function in long-distance water transport (Lucas et al. 2013), as nonliving cells after autolysis of their cytoplasm. TEs are characterized by thick lignified secondary cell wall (Oda and Fukuda 2012), which enable them to retain their shape when dead, despite the pressure of the surrounding cells. Lignin biosynthesis is regulated by both auxin and gibberellin (Aloni et al. 1990; Tokunaga et al. 2006). Moreover, lignification of TEs in Zinnia elegans cell culture can occur after TE programmed cell death (i.e., postmortem). Similarly, non-cell-autonomous postmortem lignification of vessel elements has been shown in Planta (Pesquet et al. 2013). This means that in a growing plant, the living parenchyma cells contribute lignin precursors to vessel elements after their death. However, the mechanism by which the monolignols are transported from living parenchyma cells to the secondary walls of dead vessel elements is unknown.
The two fundamental types of xylem conduits are: the tracheid (typical to gymnosperms) and the vessel (of angiosperms) which is built of vessel elements. Among the vessels are the fibers, which are the supporting cells (Evert 2006).
Tracheids appeared in ancient land plants about 430 million years ago (Raven et al. 2005). A tracheid is a non-perforated long cell with bordered pits. Tracheids are both the conducting and supporting cells that build the ‘softwood’ of gymnosperms. IAA movement through the cambium of pine trees induces the differentiation of tracheids from cambium initials (Uggla et al. 1996). In young pine seedlings, tracheids can also redifferentiate from parenchyma cells by application of both auxin and gibberellin (GA) (Kalev and Aloni 1998). When only IAA is applied, it induces very short tracheids, while GA, in the presence of IAA, promotes tracheid elongation by stimulating intrusive growth of both the upper and lower ends of the differentiating tracheids. High auxin concentration can induce perforation plates in tracheids (Aloni 2013).
Fibers are long and narrow cells with thick secondary walls that are usually heavily lignified. Differentiation of fibers in the ‘hardwood’ of angiosperms and in the phloem is induced by gibberellin in the presence of auxin, and the GA which induces fibers originates in mature leaves (Hess and Sachs 1972; Aloni 1979, 1987; Dayan et al. 2012).
Over-expression of GA 20-oxidase, a gene encoding the enzyme responsible for the rate-limiting step involved in GA synthesis, enhances fiber yield (Eriksson et al. 2000). The transgenic tobacco plants and poplar trees showed higher levels of GAs in their shoots and an increase in fiber length (Eriksson et al. 2000; Eriksson and Moritz 2002). This genetic manipulation also increased the GA inactivation, due to GA 2-oxidase catalysis. We have therefore used another approach to elevate GA concentrations by silencing the GA 2-oxidase (i.e., preventing deactivation of the bioactive gibberellin), which elevated the bioactive GA concentrations in tobacco, promoted rapid shoot elongation, increased fiber production, enlarged fiber size and decreased fiber lignification (Dayan et al. 2010). Low lignification can reduce the cost of paper and other fibrous materials. Endogenous bioactive gibberellin concentrations could be boosted up by inducing both the over-expression of GA 20-oxidas and silencing the GA 2-oxidas genes (Dayan et al. 2010), which could result in synergistic effects. These molecular manipulations could also modify lignin metabolism and change lignin structure and content.
The GA signal which triggers enhanced secondary xylem differentiation in Arabidopsis is graft transmissible, suggesting that the GA is a mobile signal (Ragni et al. 2011). Interestingly, the precursor of the gibberellin hormone (GA1), namely, GA20, produced in mature leaves of tobacco can flow non-polarly via the phloem, from the mature leaves to sink organs, namely, to both the stem and root [GA19 (the precursor of GA20) is not mobile]. When the mature leaf-induced GA20 precursor arrives to the cambium, it is converted, by local cambial activity of the GA20-oxidase, to the bioactive gibberellin form (GA1), which activates the cambium. The bioactive GA produced in mature leaves is also mobile and moves in the phloem. Therefore, the removal of mature leaves substantially depletes the endogenous GA concentrations in the stem, which impairs cambial activity, fiber differentiation and shoot elongation (Dayan et al. 2012). Stems of transgenic plants with elevated GA concentrations grow rapidly and produce longer fibers (Eriksson et al. 2000; Biemelt et al. 2004; Dayan et al. 2010) and enhanced wood production (Dayan et al. 2010), which should be applied in agriculture and forestry together with selected genes for longer fibers and desired lignin content (Capron et al. 2013).
From tracheids to vessels and fibers
Fibers, like vessels, have originated from tracheids of more primitive plants. During plant evolution, the ancient inducing mechanism for typically elongated tracheids (a combination of both IAA and GA) in primitive plants has become more specific in higher plants. Each vascular element in higher plants is mainly induced and regulated by one specific hormone: IAA by itself induces short vessel elements (Jacobs 1952; Sachs 1981; Aloni 2010; Scarpella and Helariutta 2010), whereas GA, in the presence of IAA, has become the specific signal which induces long fibers (Aloni 1979, 1987; Dayan et al. 2012). This means that the well-known evolutionary transition from tracheids to fibers and to vessel elements reflects the hormonal specialization which has occurred during plant evolution (Fig. 4).
Adaptations to light and soil nutrition
It has been recently shown that sugars from photosynthesis act as both an energy source and as signaling molecules promoting IAA synthesis (Lilley et al. 2012; Sairanen et al. 2012). This regulation of IAA synthesis by the availability of free sugar means that under favorable light conditions, more IAA can be produced promoting more growth and vascular differentiation in response to upgraded environmental conditions.
Plants can regulate which meristems are active according to their environmental conditions. Inhibition of lateral bud development and consequently shoot branching occur in response to the root hormone strigolactone (SL) (Shinohara et al. 2013). SL is produced in the root and moves upward via the xylem (Kohlen et al. 2011) to the stem, where it inhibits lateral bud development. SL can inhibit shoot branching by triggering rapid depletion of the auxin efflux protein PIN1 from the plasma membrane of the stem’s xylem parenchyma cells (Shinohara et al. 2013). SL positively regulates cambial activity and vascular differentiation (Agusti et al. 2011). The SL signaling in the vascular cambium itself is sufficient for cambium stimulation and it interacts with the auxin signaling pathway to promote xylem differentiation (Agusti et al. 2011).
SL concentration increases as a response to inorganic phosphate deficiency (Kohlen et al. 2011). Accordingly, in response to phosphate stress SL promotes main stem elongation at the expense of branch development, thus giving the plant an advantage in its competition for light against neighboring plants. As SL promotes vascular differentiation in the main stem (Agusti et al. 2011), it also improves nutrient supply to the actively growing apical bud.
Conversely, the cytokinin produced in the root cap (Aloni et al. 2005, 2006) is a general promoting signal for buds and leaf development; its concentration increases following an increase in nitrate (NO3−) supply (Takei et al. 2004; Ruffel et al. 2011), whereas SL promotes only main stem elongation by inhibiting lateral bud development. Thus, root-specific signaling shapes shoot developmental architecture and vascular differentiation in response to phosphate and nitrate concentrations in the soil.
Cambial activity reflects the social status of a forest tree
A study on cambium dynamics and wood formation in a 40-year-old Abies alba plantation near Nancy, France, has shown that the timings, duration and rate of tracheid production change according to the social status (relative size and vitality) of a tree in the forest (Rathgeber et al. 2011). The study demonstrates clear gradients of cambial activity related to the crown area and the height of the trees. Cambial activity started earlier, stopped later and therefore lasted longer in dominant trees than in intermediate and suppressed ones. Cambial activity was more intense in dominant trees than in the smaller trees. It was estimated that about 75 % of tree-ring width variability was attributable to the rate of cell production and only 25 % to extend cambial duration. Interestingly, growth duration was correlated to tree height, while growth rate was correlated to crown area (Rathgeber et al. 2011).
Vigorous crowns likely produce more IAA in their young leaves and more GA in their mature leaves. The synergistic effects of theses two hormones upgrade cambial activity and enhance tracheid production. Together with the expected elevated hormonal production, a larger crown also provides higher sugar contents, as was found in the outer wood of the most productive poplar clones (Deslauriers et al. 2009). Higher soluble carbohydrates promote IAA biosynthesis (Lilley et al. 2012; Sairanen et al. 2012), which positively regulate growth and wood formation. These results show that gradients in cambial activity and of wood formation are strongly related to tree size and vigor.
Regulation of vessel size and density along the plant axis
The six-point hypothesis (Aloni and Zimmermann 1983) suggests that IAA descending from young leaves to root tips acts as a morphogenetic signal which forms polar concentration gradients along the plant axis. Such longitudinal IAA gradients in the vascular cambium provide directional and location information to differentiating cells along the morphogenetic fields. The decreasing gradient of IAA concentrations along a tree axis from leaves to roots results in a general and gradual increase in vessel diameter (also tracheid size), which is associated with a decrease in vessel density, with increasing distance from the young leaves (Aloni and Zimmermann 1983; Aloni 1987; Leitch 2001; Sorce et al. 2013). The high IAA level near the young leaves induce narrow vessels because of their rapid differentiation, allowing only limited time for cell growth. Conversely, low IAA concentrations further down result in slow differentiation, which permits more time for cell expansion before secondary wall deposition and therefore results in wide vessels or wide tracheids.
Recently, an important experimental support to the six-point hypothesis (Aloni and Zimmermann 1983) was documented by studying the duration of cell differentiation along the stem of a Picea abies tree (Anfodillo et al. 2012). This study demonstrates that the duration of the expansion phase is positively correlated with tracheid width, and that tracheid diameter from the top of the tree to its base is linearly dependent on the time during which the differentiating tracheids remain in the expansion phase (Anfodillo et al. 2012). Their results showed that at the top of the tree’s trunk (9 m from the ground), the tracheid expansion time was 7 days, at 6 m above ground the cells expanded for 14 days and at 3 m for 19 days (Anfodillo et al. 2012). Therefore, the tracheids at the base of the tree that have the longest period of cell expansion before secondary wall deposition become the widest conduits (Sorce et al. 2013).
Vascular plants grow in different environments, ranging from deserts to rain forests and from Arctic regions to the tropics. Comparative anatomical studies (Olson and Rosell 2013 and references therein) reveal similarities in structure of the vascular system in plants grown in extreme habitats versus the ones grown in favorable environments. Desert, Arctic, and alpine shrubs show a high density of very narrow vessels. Such vascular systems are typical of extreme habitats and are deemed adaptive safety mechanisms against drought and freezing (Tyree and Zimmermann 2002; Lucas et al. 2013). Conversely, forest trees and lianas, which are typical in the tropics and rain forests, have low-density vessels of very wide diameter, which affords maximal efficiency of water conduction and is considered to be an adaptation to mesic conditions (Tyree and Zimmermann 2002).
The vascular adaptation hypothesis (Aloni 1987) suggests that the environment controls the plant’s vascular system through its control of the plant’s development, height and shape. The hypothesis proposes that the ecological conditions control the width and frequency of vessels in plants by regulating plant size and shape. Limiting conditions limit the final size of the plant and result in small and suppressed shoots, whereas favorable conditions allow the plant to attain its maximal height. Likewise, the duration of the growth period affects the annual rate of plant growth. Consequently, the height of the plant and the degree of its branching determine the gradients of IAA along the plant’s axis. In small shrubs, which are typical of extreme habitats, the distances from the young leaves to the roots are very short and no substantial gradient of IAA can be formed. Therefore, the levels of IAA along these small plants are relatively high and result in the differentiation of numerous very narrow vessels in the greatest densities, as predicted by the six-point hypothesis (Aloni and Zimmermann 1983). Conversely, in large trees and long lianas, the very great distances from the young leaves to the roots enable a substantial decrease in IAA concentrations in the lower parts of the stem and in the roots, resulting in the differentiation of very wide vessels in low density (Aloni 1987).
The quantitative results of Olson and Rosell (2013) obtained from 237 species of over 40 angiosperm orders across a wide range of habits and habitats support the vascular adaptation hypothesis (Aloni 1987) showing that vessels are proportional to both stem length and stem diameter. Accordingly, plant size is related to climate, leading to the vessel–climate relationship: vessels are narrower in drier communities because dryland plants are on average smaller than the wider vessels found in large forest trees and long lianas grown under favorable conditions (Olson and Rosell 2013).
A concluding remark
To understand plant development, we need to investigate their vascular tissues, where the inducing and regulating signals are transported. We should combine innovative molecular tools with organismal methods when studying plant vascular differentiation for better whole plant understanding. The mechanisms discussed above provide concepts for improving plant development and vascular differentiation by modifying plant hormonal regulation. To improve fiber and wood production in industrial plants and forest trees their endogenous hormonal concentrations and sensitivity to hormones can be modified. Likewise, fruit plants can be screened for reduced sensitivity to ethylene. In the selected plants the damage from tumorous crown galls will be prevented, resulting in healthy plant growth and high crop yield.