Mechanical stress in tree roots induces the production of reaction wood (RW) and the formation of new branch roots, both functioning to avoid anchorage failure and limb damage. The vascular cambium (VC) is the factor responsible for the onset of these responses as shown by their occurrence when all primary tissues and the root tips are removed. The data presented confirm that the VC is able to evaluate both the direction and magnitude of the mechanical forces experienced before coordinating the most fitting responses along the root axis whenever and wherever these are necessary. The coordination of these responses requires intense crosstalk between meristematic cells of the VC which may be very distant from the place where the mechanical stress is first detected. Signaling could be facilitated through plasmodesmata between meristematic cells. The mechanism of RW production also seems to be well conserved in the stem and this fact suggests that the VC could behave as a single structure spread along the plant body axis as a means to control the relationship between the plant and its environment. The observation that there are numerous morphological and functional similarities between different meristems and that some important regulatory mechanisms of meristem activity, such as homeostasis, are common to several meristems, supports the hypothesis that not only the VC but all apical, primary and secondary meristems present in the plant body behave as a single interconnected structure. We propose to name this structure "meristematic connectome" given the possibility that the sequence of meristems from root apex to shoot apex could represent a pluricellular network that facilitates long-distance signaling in the plant body. The possibility that the "meristematic connectome" could act as a single structure active in adjusting the plant body to its surrounding environment throughout the life of a plant is now proposed.

Background The production of a new lateral root from parental root primary tissues has been investigated extensively, and the most important regulatory mechanisms are now well known. A first regulatory mechanism is based on the synthesis of small peptides which interact ectopically with membrane receptors to elicit a modulation of transcription factor target genes. A second mechanism involves a complex cross-talk between plant hormones. It is known that lateral roots are formed even in parental root portions characterized by the presence of secondary tissues, but there is not yet agreement about the putative tissue source providing the cells competent to become founder cells of a new root primordium. Scope We suggest models of possible regulatory mechanisms for inducing specific root vascular cambium (VC) stem cells to abandon their activity in the production of xylem and phloem elements and to start instead the construction of a new lateral root primordium. Considering the ontogenic nature of the VC, the models which we suggest are the result of a comparative review of mechanisms known to control the activity of stem cells in the root apical meristem, procambium and VC. Stem cells in the root meristems can inherit various competences to play different roles, and their fate could be decided in response to cross-talk between endogenous and exogenous signals. Conclusions We have found a high degree of relatedness among the regulatory mechanisms controlling the various root meristems. This fact suggests that competence to form new lateral roots can be inherited by some stem cells of the VC lineage. This kind of competence could be represented by a sensitivity of specific stem cells to factors such as those presented in our models.

Higher education has been perceived as a value, necessary for the process of societal development, as well as the development of individuals in post-socialist Europe. In this context, the paper aims to focus on Geography students and graduates to analyse their expectations for their futures in the discipline. Geography students are discussed here at Comenius University, Bratislava, Slovakia, but implications can be applied broadly. In particular, gender-based differences were revealed in Geography students' ambitions related to their future careers. Not surprisingly, attitudes of students and Geography teaching staff differ substantially in some aspects, including labour-market orientation of Geography curriculum. Lack of practical and business-related skills in existing curriculum might be a limitation of the graduates' labour prospects.

The traditional value of higher education to train students to be productive and informed citizens who appreciate the arts and who are critical thinkers is now being challenged. The popular media in particular is weighing the cost of higher education against the employability of graduates and asking if it is worth it. In this study we examined the expectations of students in biology at Comenius University. We also examined faculty members to see if they understood the expectations of their students. Questionnaires were administered to BSc and first year MSc students and a sample of faculty members in the program. There were differences between student and faculty member responses. Among them were that 2nd year students and beyond selected ‘learn things that interest me’ as a reason to attend the university, whereas biology faculty selected ‘get a good job’ and ‘prepare for a professional career’ as most important. Another difference was that ‘raising a family’ was the student’s most voted option regarding their futures, while faculty members considered ‘becoming an authority in my area’ as what they thought was most important for students. Students and faculty members, on the other hand, did agree on some of the questionnaire points, for example, both students and faculty indicated that ‘laboratory and field instruction’ are the most important teaching method. Both students and faculty considered ‘soft skills’ as important and both stated that the current curriculum included some of that training, and they both agreed that the most important role for a university teacher was to ‘motivate students to learn’. The long-term goal of the study was to help faculty members better understand student expectations and then to apply any learned insights to updating the curriculum to better meet them.

To better understand water uptake patterns in root systems of woody perennial crops, we detailed the developmental anatomy and hydraulic physiology along the length of grapevine (Vitis berlandieri × Vitis rupestris) fine roots from the tip to secondary growth zones. Our characterization included the localization of suberized structures and aquaporin gene expression and the determination of hydraulic conductivity (Lp r) and aquaporin protein activity (via chemical inhibition) in different root zones under both osmotic and hydrostatic pressure gradients. Tissue-specific messenger RNA levels of the plasma membrane aquaporin isogenes (VvPIPs) were quantified using laser-capture microdissection and quantitative polymerase chain reaction. Our results highlight dramatic changes in structure and function along the length of grapevine fine roots. Although the root tip lacked suberization altogether, a suberized exodermis and endodermis developed in the maturation zone, which gave way to the secondary growth zone containing a multilayer suberized periderm. Longitudinally, VvPIP isogenes exhibited strong peaks of expression in the root tip that decreased precipitously along the root length in a pattern similar to Arabidopsis (Arabidopsis thaliana) roots. In the radial orientation, expression was always greatest in interior tissues (i.e. stele, endodermis, and/or vascular tissues) for all root zones. High Lp r and aquaporin protein activity were associated with peak VvPIP expression levels in the root tip. This suggests that aquaporins play a limited role in controlling water uptake in secondary growth zones, which contradicts existing theoretical predictions. Despite having significantly lower Lp r, woody roots can constitute the vast majority of the root system surface area in mature vines and thus provide for significant water uptake potential.

This special issue is dedicated to root biologists past and present who have been exploring all aspects of root structure and function with an extensive publication record going over 100 years. The content of the Special Issue on Root Biology covers a wide scale of contributions, spanning interactions of roots with microorganisms in the rhizosphere, the anatomy of root cells and tissues, the subcellular components of root cells, and aspects of metal accumulation and stresses on root function and structure. We have organized the papers into three topic categories: (1) root ecology, interactions with microbes, root architecture and the rhizosphere; (2) experimental root biology, root structure and physiology; and (3) applications of new technology to study root biology. Finally, we will speculate on root research for the future.

BACKGROUND AND AIMS:The bacterium Xylella fastidiosa (Xf), responsible for Pierce's disease (PD) of grapevine, colonizes the xylem conduits of vines, ultimately killing the plant. However, Vitis vinifera grapevine varieties differ in their susceptibility to Xf and numerous other plant species tolerate Xf populations without showing symptoms. The aim of this study was to examine the xylem structure of grapevines with different susceptibilities to Xf infection, as well as the xylem structure of non-grape plant species that support or limit movement of Xf to determine if anatomical differences might explain some of the differences in susceptibility to Xf.METHODS:Air and paint were introduced into leaves and stems to examine the connectivity between stem and leaves and the length distribution of their vessels. Leaf petiole and stem anatomies were studied to determine the basis for the free or restricted movement of Xf into the plant.KEY RESULTS:There were no obvious differences in stem or petiole vascular anatomy among the grape varieties examined, nor among the other plant species that would explain differences in resistance to Xf. Among grape varieties, the more tolerant 'Sylvaner' had smaller stem vessel diameters and 20 % more parenchyma rays than the other three varieties. Alternative hosts supporting Xf movement had slightly longer open xylem conduits within leaves, and more connection between stem and leaves, when compared with alternative hosts that limit Xf movement.CONCLUSIONS:Stem--leaf connectivity via open xylem conduits and vessel length is not responsible for differences in PD tolerance among grape varieties, or for limiting bacterial movement in the tolerant plant species. However, it was found that tolerant host plants had narrower vessels and more parenchyma rays, possibly restricting bacterial movement at the level of the vessels. The implications of xylem structure and connectivity for the means and regulation of bacterial movement are discussed.

BACKGROUND:The structure of roots has been studied for many years, but despite their importance to the growth and well-being of plants, most researchers tend to ignore them. This is unfortunate, because their simple body plan makes it possible to study complex developmental pathways without the complications sometimes found in the shoot. In this illustrated essay, my objective is to describe the body plan of the root and the root apical meristem (RAM) and point out the control points where differentiation and cell cycle decisions are made. Hopefully this outline will assist plant biologists in identifying the structural context for their observations.SCOPE AND CONCLUSIONS:This short paper outlines the types of RAM, i.e. basic-open, intermediate-open and closed, shows how they are similar and different, and makes the point that the structure and shape of the RAM are not static, but changes in shape, size and organization occur depending on root growth rate and development stage. RAMs with a closed organization lose their outer root cap layers in sheets of dead cells, while those with an open organization release living border cells from the outer surfaces of the root cap. This observation suggests a possible difference in the mechanisms whereby roots with different RAM types communicate with soil-borne micro-organisms. The root body is organized in cylinders, sectors (xylem and phloem in the vascular cylinder), cell files, packets and modules, and individual cells. The differentiation in these root development units is regulated at control points where genetic regulation is needed, and the location of these tissue-specific control points can be modulated as a function of root growth rate. In Arabidopsis thaliana the epidermis and peripheral root cap develop through a highly regulated series of steps starting with a periclinal division of an initial cell, the root cap/protoderm (RCP) initial. The derivative cells from the RCP initial divide into two cells, the inner cell divides again to renew the RCP and the other cell divides through four cycles to form 16 epidermal cells in a packet; the outer cell divides through four cycles to form the 16 cells making up the peripheral root cap packet. Together, the epidermal packet and the peripheral root cap packet make up a module of cells which are clonally related.

In grapevines, scion-rootstock grafting is a common practice to impart pathogen resistance and to manipulate aspects of grapevine physiology, including vigor, yield, and fruit composition. Successful grafting requires the integration of the scion and rootstock vascular networks. The nature and extent of this integration was evaluated to determine the impacts of the graft union on the movement of vascular pathogens between scion and rootstock.. Using both xylem-mobile dyes and the xylem-limited bacterial pathogen Xylella fastidiosa, we demonstrate that the graft union contains open xylem conduits providing for passive pathogen movement. These open conduits may facilitate bacteria overwintering in below-ground vine tissues of grafted vines and systemic infection in subsequent growing seasons.

Pea (Pisum sativum L. cv. Onmard) and cotton (Gossypium hirsutum L. cv. Campo) seedlings were treated with two concentrations (200 and 500mg/l) of sodium tungstate (Na2WO4) and the developmental effects were investigated. Tungstate retarded seedling growth rate and stopped root elongation in both species. Seedling growth recovered when tungstate was removed, but primary roots continued to be stunted, while lateral root initiation and growth were stimulated. Tungstate induced premature vacuolation in cells of the root apical meristem, with vacuoles having an unusual semi-circular or cap-like shape around the nucleus. In control roots, the nuclei were spherical with prominent nucleoli bearing several randomly distributed fibrillar centres. In the tungstate-treated cells nuclei contained spherical nucleoli with a big nucleolar vacuole. Occasionally, cytoplasmic components, such as mitochondria, were entrapped in the nucleoplasm of interphasic cells of the treated roots. In these roots, most cell plates were fused to only one lateral parental wall suggesting a non-uniform centrifugal extension. The vesicles in these cell plates were dark and fused to each other at a much lower rate than in the dividing cells of the untreated seedlings. Phragmoplast and cortical microtubules were abundant in the untreated cells, but scarcely detected in the treated ones. All these observations are consistent with the view that tungstate causes considerable toxic effects to pea and cotton seedlings.

It has been hypothesized that the substantial reductions in xylemic water flow occurring at veraison are due to physical disruption (breaking) of the xylem as a result of renewed berry growth. In a companion paper, evidence was presented that the vast majority of xylem tracheary elements remained intact despite the growth of the berry, and it was proposed that existing tracheary elements stretch to accommodate growth and that additional elements may also differentiate after veraison. Measurements of the intergyre distance of tracheary elements in macerated tissue were used to test for stretching, and the numbers of tracheary elements per vascular bundle and of branch points of the peripheral xylem network were analysed to test for continued differentiation from 18 to 120 d after anthesis in Chardonnay berries. The distance between the epidermis and the vasculature increased substantially from pre- to post-veraison, potentially increasing the amount of skin available for analysis of compounds important for winemaking. Tracheary elements continued to differentiate within the existing vascular bundles throughout berry development. Additional vascular bundles also appeared until after veraison, thereby increasing the complexity of the peripheral vascular network. The results also confirmed that tracheary elements stretched by similar to 20%, but this was not as much as that predicted based on the growth of the vascular diameter (40%). These results complete a comprehensive evaluation of grape berry peripheral xylem during its development and show that tracheary development continues further into berry maturation than previously thought.

Vascular occlusion in xylem conduits is a common response to environmental stresses, and plant species are recognized as primarily tylose-forming or gel-forming. These stresses occur throughout the year, but there is little information on the wound responses throughout the year and in growing and dormant tissues. Wound-induced vascular occlusions were evaluated by type (tylose or gel), temporal progress, and spatial distribution for grape stems pruned in four seasons through an entire year. Tyloses were formed predominantly in summer and gels in winter. Cytohistological analyses indicated that wound-induced gels were pectin-rich. Both gel formation and tylose development were complete within 7 d and 10 mm from the cut regardless of the season of the wounding. Most vessels were affected by wounding, but a higher fraction of vessels developed occlusions in summer and autumn (over 80%) than in winter and spring (about 60%). The study is the first to show a single species is capable of producing primarily either tyloses or gels and that the type of wound-induced occlusion is dependent upon the season in which wounding occurs. Winter conditions limit the wound response to reversible gel formation that may contribute to refilling of embolized vessels in the spring.

During the development of many fleshy fruits, water flow becomes progressively more phloemic and less xylemic. In grape (Vitis vinifera L.), the current hypothesis to explain this change is that the tracheary elements of the peripheral xylem break as a result of berry growth, rendering the xylem structurally discontinuous and hence non-functional. Recent work, however, has shown via apoplastic dye movement through the xylem of post-veraison berries that the xylem should remain structurally intact throughout berry development. To corroborate this, peripheral xylem structure in developing Chardonnay berries was investigated via maceration and plastic sectioning. Macerations revealed that, contrary to current belief, the xylem was comprised mostly of vessels with few tracheids. In cross-section, the tracheary elements of the vascular bundles formed almost parallel radial files, with later formed elements toward the epidermis and earlier formed elements toward the centre of the berry. Most tracheary elements remained intact throughout berry maturation, consistent with recent reports of vascular dye movement in post-veraison berries.

This progress report shows that there is a difference in the structure of xylem pathways available for Xylella fastidiosa (Xf) movement in host plants known to support systemic bacterial movement and those that don’t. In addition to a reduced interorgan connectivity, non-systemic species also show a shorter path available for bacterial movement in the leaves. However, systemic and non-systemic species show similar rates of tylose formation, signifying that tyloses don’t seem to be responsible for the lack of Xf movement in the non-systemic plants. To be more conclusive, more xylem characteristics from the different hosts are being examined INTRODUCTION Xylella fastidiosa (Xf) capacity to move in plants differs greatly among species (Purcell, 2004), ranging from moving everywhere in the stem and leaves to only a few centimeters from the infection point. Our lab showed the presence of long xylem conduits from stem to leaves in grape cultivars chardonnay and cowart (Thorne et al., 2006; Chatelet et al, in press) and we recently reported that these conduits seemed to be shorter in alternate hosts in which bacterial movement is limited. A higher number of tracheids, shorter and narrower vessels, spatial organization of the vessel and of the paratracheal parenchyma cells could be a passive strategy to limit bacterial movement. Another strategy of the non-systemic species could be to confine the bacteria to a limited area by a more timely production of tyloses or, in the case of asymptomatic species showing systemic bacterial movement, to limit the population size under a harmless threshold. The objectives of this study are to carefully study the comparative anatomy of different species of plants which support a range of Xf population sizes and movement characteristics. Our hope is to understand how the xylem network might control bacterial movement in susceptible plants. OBJECTIVES 1. Conduct an anatomical comparison of plant species that support high, medium and low population sizes of Xf. 2. Conduct an anatomical comparison of plant species that show systemic movement of Xf vs. those that do not. RESULTS A range of species was examined: with a high infection rate, high bacterial population and showing systemic movement: Vitis vinifera cv. Chardonnay and Vitis vinifera cv. Cabernet sauvignon; one species with a high infection rate, medium bacterial population and showing systemic movement: Ipomoea purpurea (morning glory), Vinca major (periwinkle), Citrus sinensis (Orange), Prunus anygdalus (Almond), and species showing non-systemic movement: Alnus rhombifolia (white alder), Umbellularia californica (california laurel), Artemisia douglasiana (mugwort) and Chenopodium quinoa (quinoa), Datura wrightii (datura), Eucalyptus globules (eucalyptus). Stem-petiole-leaf lamina connectivity Grape shoots have open xylem conduits that allow the passive movement of GFP-Xf from the stem to 50-60% of the leaf length through the primary xylem (Rost et al., PD symposium report 2005; Chatelet et al., in press). The xylem of several different plant species harboring Xf was examined using air and paint injection to determine if similar xylem conduits exist. When loaded at the base of the petiole, air and paint traveled to various extents into the leaf blade of all examined species (Figure 1).

This progress report shows that open pathways likely exist for Xylella fastidiosa (Xf) movement across grafts in grape stems via xylem pathways. Studies thus far have been conducted on grafted and non-grafted three-year-old Vitis vinifera cv. Chardonnay plants. The movement of air was used to determine if open xylem conduits were present through grafts into canes, and the length of these pathways was measured. It was determined that connections via xylem vessels are generally about twice as long in non-grafted plants (450 mm) compared with grafted plants (225 mm). Current investigations are underway with dilute use latex paint and tagged Xf to understand the pathways for bacterial movement across grafts. INTRODUCTION Grapes are one of the important crop plants in which the shoots of one variety are grafted to root stocks of another to generate plants with the desired characteristics of both. Reports have clearly shown the presence of long, open xylem conduits that connect stems to leaves in chardonnay (Thorne et al., 2006; Chatelet et al, in press). Anatomical studies have also indicated that Xylella fastidiosa (Xf) appears to be primarily restricted to xylem vessels in canes, however little is known about the vessels, and subsequently the movement of Xf across grafts. The capacity for Xf to move in plants differs among species ranging from generally unrestricted throughout the major organs, to only a few centimeters from the original inoculation point. The objectives of this study are to examine the connection of vessels from canes into stems through grafts, and determine if it is possible for Xf movement to occur freely across these grafts. To meet these objectives a strategy of air and latex paint are being implemented to study open anatomical systems, and most importantly the use of Xf to examine movement across grafts. OBJECTIVES 1. Conduct a study of connections in grafted Vitis vinifera cv. Chardonnay, and determine if open vessel systems allow movement of Xf across grafts via air pressure. 2. Conduct an anatomical study of connections in grafted Vitis vinifera cv. Chardonnay, and determine if open vessel systems allow movement of Xf across grafts with latex paint. 3. Use PCR to determine the presence of Xf across graft unions after inoculation at known positions relative to the graft. RESULTS Following inoculation in grapevine, Xf moves in the nutrient poor xylem vessels and eventually causes disease symptoms that result plant death by unknown mechanisms. Previously, reports from our labs have indicated that bacteria can move freely in canes and from petioles into leaves during a systemic infection process. It is of interest to determine the movement across grafts to clarify movement into stems and possibly into root systems. Our preliminary results indicate that the graft unions of Vitis vinifera cv. Chardonnay do indeed contain continuous vessels; however, the open system length into canes is about 1⁄2 of that when compared with non-grafted plants of the same cultivar (Figure 1A). Measurements collected of cane length and associated open conduits appeared significantly different between grafted and non-grafted plants (Figure 1B). Differences were not found to be significant in stem length between grafted and non-grafted plants (Figure 1C). These results indicate that graft unions would not be an impediment to bacterial movement, and that Xf would be able to move further distances across in non-grafted areas of the plant because of the presence of continuous vessels. This study of the xylem structure will be further evaluated with current studies to determine the connective pathway of air movement by latex paint, and confirm that Xf can be moved through the vessels in the presumed transpiration stream. CONCLUSIONS From our preliminary results, graft unions in stems do not appear to restrict the movement of Xf in Vitis vinifera cv. Chardonnay. Although the length of open vessels is reduced by about 1⁄2, open vessels cross the graft union as determined by air movement. However, the numbers of vessels that cross the graft are less than 10% in distribution when compared with non-grafted plants (data not shown). In order for Xf to move from a cane or leaf across a graft it would need to be inoculated into a vessel that happens to extend through the graft union, or the bacteria would need to degrade membranes to move into adjacent vessels through bordered pits. Current studies with paint and PCR detection of Xf will confirm these results.

Grapevine xylem is composed of vessels connected by intervessel bordered pits with pit membranes that prevent the passive movement of particles, especially at the stem-leaf junction where most vessels end. The traditional view of Xylella fastidiosa (Xf) movement within the xylem requires the digestion of the intervessel pit membrane to move from one vessel to another. However, bacteria such as Yersinia enterocolitica (Ye) and fluorescent beads have been observed moving rapidly within the grapevine xylem, suggesting a pathway for passive movement. In this report, we used air and latex paint to confirm the existence a xylem vessel pathway from stems into the leaf lamina. Anatomical investigation of the leaf xylem revealed a switch from vessels to tracheids at about 50-60% the length of the leaf lamina. In addition, inoculations of gfp-Xf showed that bacteria never reached the leaf margin where the symptoms appear, suggesting that tracheids inhibited the free movement of Xf. INTRODUCTION Particle movement is limited by the frequency of vessel endings, especially at the stem-leaf junction, where most vessels have been thought to end, with a few exceptions (Andre, 2002; Larson and Isebrands, 1978; Tyree and Zimmermann, 2002). Indeed a bacterium such as Xf (0.1-0.5 x 1-5 μm, Nyland et al., 1973) is too big to move through the intervessel pit membrane pores (<0.2 μm, Siau, 1984) with water flow. The colonization of a plant by Xf requires that the bacteria move within the xylem between vessels across pit membranes and from one organ to another (Stevenson et al., 2004). A favored hypothesis to explain how bacteria become systemic is that the bacteria digest the pit membrane cell wall (Roper et al., 2002; Stevenson et al., 2004). Another more recent twist in the mechanism is that bacteria might also move through torn or remnant pit pore membranes (Carlquist and Schneider, 2004; Stevenson et al., 2004). This propagation by digestion could be rather slow if vessels are short and if numerous membranes have to be crossed. Bacterial movement in grape stem can be relatively easy because vessels can be very long, up to 1m (Sperry et al., 1987). However, there is still the problem of bacterial passage into leaves if most of the vessels end at the stem-petiole and petiole-lamina junctions. Recent experiments on the passive movement of Ye and fluorescent beads showed the existence of open, continuous xylem conduits (one or more xylem vessels allowing free movement of particles of at least one micrometer in size) from the stem to the leaf lamina of grapevine (Thorne et al., personal communication). They found that Ye and beads were moving freely with the transpiration stream from the stem into primary and secondary veins of the leaf blade in three leaves above the loading point. In addition, they showed that Ye and beads traveled to about 50-60% of the length of the leaf lamina. This shows an open xylem conduit all the way from the stem through the petiole and into the leaf blade without the need to digest a pit membrane, but also suggests that a feature of xylem structure precluded the movement of bacteria all the way to the leaf blade periphery. In this study, we verified the presence of the open xylem conduits by studying the movement of air and latex paint. We also looked at the xylem anatomy of the leaf blade to identify the change in the vascular structure causing the halt of Ye, beads, air and paint within 50-60% of the leaf blade length. Leaves were also inoculated with Xf engineered by the addition of the green fluorescent protein (gfp-Xf) to check its movement within the leaf at different times after inoculation. The question was whether Xf would be affected by the change in xylem structure. Since Xf possess the ability to digest cell walls, we would theoretically expect the bacteria to be able to move farther than 50-60% of the leaf blade length and eventually be found at the leaf margin.

BACKGROUND:The difference between indeterminate and determinate growth in plants consists of the presence or absence of an active meristem in the fully developed organ. Determinate root growth implies that the root apical meristem (RAM) becomes exhausted. As a consequence, all cells in the root tip differentiate. This type of growth is widely found in roots of many angiosperm taxa and might have evolved as a developmental adaptation to water deficit (in desert Cactaceae), or low mineral content in the soil (proteoid roots in various taxa).SCOPE AND CONCLUSIONS:This review considers the mechanisms of determinate root growth to better understand how the RAM is maintained, how it functions, and the cellular and genetic bases of these processes. The role of the quiescent centre in RAM maintenance and exhaustion will be analysed. During root ageing, the RAM becomes smaller and its organization changes; however, it remains unknown whether every root is truly determinate in the sense that its RAM becomes exhausted before senescence. We define two types of determinate growth: constitutive where determinacy is a natural part of root development; and non-constitutive where determinacy is induced usually by an environmental factor. Determinate root growth is proposed to include two phases: the indeterminate growth phase, when the RAM continuously produces new cells; and the termination growth phase, when cell production gradually decreases and eventually ceases. Finally, new concepts regarding stem cells and a stem cell niche are discussed to help comprehend how the meristem is maintained in a broad taxonomic context.

The pruning of actively growing grapevines (Vitis vinifera) resulted in xylem vessel embolisms and a stimulation of tylose formation in the vessels below the pruning wound. Pruning was also followed by a 10-fold increase in the concentration of ethylene at the cut surface. When the pruning cut was made under water and maintained in water, embolisms were prevented, but there was no reduction in the formation of tyloses or the accumulation of ethylene. Treatment of the stems with inhibitors of ethylene biosynthesis (aminoethoxyvinylglycine) and/or action (silver thiosulfate) delayed and greatly reduced the formation of tyloses in xylem tissue and the size and number of those that formed in individual vessels. Our data are consistent with the hypotheses that wound ethylene production is the cause of tylose formation and that embolisms in vessels are not directly required for wound-induced tylosis in pruned grapevines. The possible role of ethylene in the formation of tyloses in response to other stresses and during development, maturation, and senescence is discussed.