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1.  Plant root research: the past, the present and the future 
Annals of Botany  2012;110(2):201-204.
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.
doi:10.1093/aob/mcs156
PMCID: PMC3394661  PMID: 22966495
2.  Xylem structure of four grape varieties and 12 alternative hosts to the xylem-limited bacterium Xylella fastidious 
Annals of Botany  2011;108(1):73-85.
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.
doi:10.1093/aob/mcr106
PMCID: PMC3119617  PMID: 21546428
Grape; grapevine; Vitis vinifera; host; leaf; stem; xylem; Pierce's disease; Xylella fastidiosa
3.  The organization of roots of dicotyledonous plants and the positions of control points 
Annals of Botany  2010;107(7):1213-1222.
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.
doi:10.1093/aob/mcq229
PMCID: PMC3091796  PMID: 21118839
Root apical meristem; RAM; cell cycle; differentiation; peripheral root cap; closed RAM organization; open RAM organization; epidermis; module; determination; levels of organization; plasmodesmata; T-division; root cap/protoderm initial; columella initial
4.  The peripheral xylem of grapevine (Vitis vinifera) berries. 2. Anatomy and development 
Journal of Experimental Botany  2008;59(8):1997-2007.
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 ∼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.
doi:10.1093/jxb/ern061
PMCID: PMC2413279  PMID: 18440930
Tracheary element; vasculature; vessel; water movement
5.  The peripheral xylem of grapevine (Vitis vinifera). 1. Structural integrity in post-veraison berries 
Journal of Experimental Botany  2008;59(8):1987-1996.
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.
doi:10.1093/jxb/ern060
PMCID: PMC2413285  PMID: 18440931
Tracheary element; vasculature; vessel; water movement
6.  Xylem Structure and Connectivity in Grapevine (Vitis vinifera) Shoots Provides a Passive Mechanism for the Spread of Bacteria in Grape Plants 
Annals of Botany  2006;98(3):483-494.
• Background and Aims Bacterial leaf scorch occurring in a number of economically important plants is caused by the xylem-limited bacterium Xylella fastidiosa (Xf). In grapevine, Xf systemic infection causes Pierce's disease and is lethal. Traditional dogma is that Xf movement between vessels requires the digestion of inter-vessel pit membranes. However, Yersinia enterocolitica (Ye) (a bacterium found in animals) and fluorescent beads moved rapidly within grapevine xylem from stem into leaf lamina, suggesting open conduits consisting of long, branched xylem vessels for passive movement. This study builds on and expands previous observations on the nature of these conduits and how they affect Xf movement.
• Methods Air, latex paint and green fluorescence protein (GFP)-Xf were loaded into leaves and followed to confirm and identify these conduits. Leaf xylem anatomy was studied to determine the basis for the free and sometimes restricted movement of Ye, beads, air, paint and GFP-Xf into the lamina.
• Key Results Reverse loading experiments demonstrated that long, branched xylem vessels occurred exclusively in primary xylem. They were observed in the stem for three internodes before diverging into mature leaves. However, this stem—leaf connection was an age-dependent character and was absent for the first 10–12 leaves basal to the apical meristem. Free movement in leaf blade xylem was cell-type specific with vessels facilitating movement in the body of the blade and tracheids near the leaf margin. Air, latex paint and GFP-Xf all moved about 50–60 % of the leaf length. GFP-Xf was never observed close to the leaf margin.
• Conclusions The open vessels of the primary xylem offered unimpeded long distance pathways bridging stem to leaves, possibly facilitating the spread of bacterial pathogens in planta. GFP-Xf never reached the leaf margins where scorching appeared, suggesting a signal targeting specific cells or a toxic build-up at hydathodes.
doi:10.1093/aob/mcl124
PMCID: PMC2803575  PMID: 16790469
Xylella fastidiosa; pathogens; xylem; stem—leaf connection; grapevine; passive movement; systemic
7.  The Production and Release of Living Root Cap Border Cells is a Function of Root Apical Meristem Type in Dicotyledonous Angiosperm Plants 
Annals of Botany  2006;97(5):917-923.
• Background and Aims The root apical meristems (RAM) of flowering plant roots are organized into recognizable pattern types. At present, there are no known ecological or physiological benefits to having one RAM organization type over another. Although there are phylogenetic distribution patterns in plant groups, the possible evolutionary advantages of different RAM organization patterns are not understood. Root caps of many flowering plant roots are known to release living border cells into the rhizosphere, where the cells are believed to have the capacity to alter conditions in the soil and to interact with soil micro-organisms. Consequently, high rates of border cell production may have the potential to benefit plant growth and development greatly, and to provide a selective advantage in certain soil environments. This study reports the use of several approaches to elucidate the anatomical and developmental relationships between RAM organization and border cell production.
• Methods RAM types from many species were compared with numbers of border cells released in those species. In addition, other species were grown, fixed and sectioned to verify their organization type and capacity to produce border cells. Root tips were examined microscopically to characterize their pattern and some were stained to determine the viability of root cap cells.
• Key Results The first report of a correlation between RAM organization type and the production and release of border cells is provided: species exhibiting open RAM organization produce significantly more border cells than species exhibiting closed apical organization. Roots with closed apical organization release peripheral root cap cells in sheets or large groups of dead cells, whereas root caps with open organization release individual living border cells.
• Conclusions This study, the first to document a relationship between RAM organization, root cap behaviour and a possible ecological benefit to the plant, may yield a framework to examine the evolutionary causes for the diversification of RAM organization types across taxa.
doi:10.1093/aob/mcj602
PMCID: PMC2803423  PMID: 16488922
Border cells; root caps; root apical organization; root meristem

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