The irregular xylem 2 mutant is an allele of korrigan that affects the secondary cell wall of Arabidopsis thaliana

The irregular xylem 2 (irx2) mutant of Arabidopsis thaliana exhibits a cellulose deficiency in the secondary cell wall, which is brought about by a point mutation in the KORRIGAN (KOR) beta,1-4 endoglucanase (beta,1-4 EGase) gene. Measurement of the total crystalline cellulose in the inflorescence stem indicates that the irx2 mutant contains approximately 30% of the level present in the wild type (WT). Fourier-Transform Infra Red (FTIR) analysis, however, indicates that there is no decrease in cellulose in primary cell walls of the cortical and epidermal cells of the stem. KOR expression is correlated with cellulose synthesis and is highly expressed in cells synthesising a secondary cell wall. Co-precipitation experiments, using either an epitope-tagged form of KOR or IRX3 (AtCesA7), suggest that KOR is not an integral part of the cellulose synthase complex. These data are supported by immunolocalisation of KOR that suggests that KOR does not localise to sites of secondary cell wall deposition in the developing xylem. The defect in irx2 plant is consistent with a role for KOR in the later stages of secondary cell wall formation, suggesting a role in processing of the growing microfibrils or release of the cellulose synthase complex.     

Effect of auxin transport inhibitors and ethylene on the wood anatomy of poplar

The influence of the auxin transport inhibitors naphthylphthalamic acid (NPA) and methyl-2-chloro-9-hydroxyflurene-9-carboxylate (CF), as well as the gaseous hormone ethylene on cambial differentiation of poplar was determined. NPA treatment induced clustering of vessels and increased vessel length. CF caused a synchronized differentiation of cambial cells into either vessel elements or fibres. The vessels in CF-treated wood were significantly smaller and fibre area was increased compared with controls. Under the influence of ethylene, the cambium produced more parenchyma, shorter fibres and shorter vessels than in controls. Since poplar is the model tree for molecular biology of wood formation, the modulation of the cambial differentiation of poplar towards specific cell types opens an avenue to study genes important for the development of vessels or fibres.     

Unravelling cell wall formation in the woody dicot stem

Populus is presented as a model system for the study of wood formation (xylogenesis). The formation of wood (secondary xylem) is an ordered developmental process involving cell division, cell expansion, secondary wall deposition, lignification and programmed cell death. Because wood is formed in a variable environment and subject to developmental control, xylem cells are produced that differ in size, shape, cell wall structure, texture and composition. Hormones mediate some of the variability observed and control the process of xylogenesis. High-resolution analysis of auxin distribution across cambial region tissues, combined with the analysis of transgenic plants with modified auxin distribution, suggests that auxin provides positional information for the exit of cells from the meristem and probably also for the duration of cell expansion. Poplar sequencing projects have provided access to genes involved in cell wall formation. Genes involved in the biosynthesis of the carbohydrate skeleton of the cell wall are briefly reviewed. Most progress has been made in characterizing pectin methyl esterases that modify pectins in the cambial region. Specific expression patterns have also been found for expansins, xyloglucan endotransglycosylases and cellulose synthases, pointing to their role in wood cell wall formation and modification. Finally, by studying transgenic plants modified in various steps of the monolignol biosynthetic pathway and by localizing the expression of various enzymes, new insight into the lignin biosynthesis in planta has been gained.     

Water deficits and hydraulic limits to leaf water supply

Many aspects of plant water use -- particularly in response to soil drought -- may have as their basis the alteration of hydraulic conductance from soil to canopy. The regulation of plant water potential (Psi) by stomatal control and leaf area adjustment may be necessary to maximize water uptake on the one hand, while avoiding loss of hydraulic contact with the soil water on the other. Modelling the changes in hydraulic conductance with pressure gradients in the continuum allows the prediction of water use as a function of soil environment and plant architectural and xylem traits. Large differences in water use between species can be attributed in part to differences in their 'hydraulic equipment' that is presumably optimized for drawing water from a particular temporal and spatial niche in the soil environment. A number of studies have identified hydraulic limits as the cause of partial or complete foliar dieback in response to drought. The interactions between root:shoot ratio, rooting depth, xylem properties, and soil properties in influencing the limits to canopy water supply can be used to predict which combinations should optimize water use in a given circumstance. The hydraulic approach can improve our understanding of the coupling of canopy processes to soil environment, and the adaptive significance of stomatal behaviour.     

Changes in Stem and Leaf Hydraulics Preceding Leaf Shedding in Castanea Sativa L.

This paper describes changes in leaf water status and in stem, petiole and leaf blade hydraulics preceding leaf senescence and shedding in Castanea sativa L. (chestnut). Measurements of maximum diurnal leaf conductance to water vapour (g_L), minimum water potential (_L), hydraulic conductance per unit leaf surface area of stems (K_SL), petioles (K_PL) and leaf blades (K_LL) and number of functional conduits and inside diameter distribution were performed in June, September and October 1999. In September, still green leaves had undergone some dehydration as indicated by decreased g_L (by 75 %) and _L with respect to June. In the same time, K_SL decreased by 88 %, while K_PL and K_LL decreased by 50 % and 20 % of the conduits of stems and 10 % of the petioles (all belonging to the widest diameter range) were no longer functioning, causing a decrease in the theoretical flow by 82 % in stems and 27 % in petioles. Stem xylem blockage was apparently due to tyloses growing into conduits. We advance the hypothesis that the entire process of leaf shedding and winter rest may be initiated by extensive stem embolism occurring during the summer.     

On the progressive enrichment of the oxygen isotopic composition of water along a leaf

A model has been derived for the enrichment of heavy isotopes of water in leaves, including progressive enrichment along the leaf. In the model, lighter water is preferentially transpired leaving heavier water to diffuse back into the xylem and be carried further along the leaf. For this pattern to be pronounced, the ratio of advection to diffusion (Péclet number) has to be large in the longitudinal direction, and small in the radial direction. The progressive enrichment along the xylem is less than that occurring at the sites of evaporation in the mesophyll, depending on the isolation afforded by the radial Péclet number. There is an upper bound on enrichment, and effects of ground tissue associated with major veins are included. When transpiration rate is spatially nonuniform, averaging of enrichment occurs more naturally with transpiration weighting than with area-based weighting. This gives zero average enrichment of transpired water, the modified Craig-Gordon equation for average enrichment at the sites of evaporation and the Farquhar and Lloyd (In Stable Isotopes and Plant Carbon-Water Relations, pp. 47-70. Academic Press, New York, USA, 1993) prediction for mesophyll water. Earlier results on the isotopic composition of evolved oxygen and of retro-diffused carbon dioxide are preserved if these processes vary in parallel with transpiration rate. Parallel variation should be indicated approximately by uniform carbon isotope discrimination across the leaf.     

Effects of branch solid Fe sulphate implants on xylem sap composition in field-grown peach and pear: changes in Fe, organic anions and pH

The effects of placing solid implants containing Fe sulfate in branches of Fe-deficient pear and peach trees on the composition of the xylem sap have been studied. Iron sulfate implants are commercially used in northeastern Spain to control iron chlorosis in fruit trees. Implants increased Fe concentrations and decreased organic acid concentrations in the xylem sap, whereas xylem sap pH was only moderately changed. The citrate to Fe ratios decreased markedly after implants, therefore improving the possibility that Fe could be reduced by the leaf plasma membrane enzyme reductase, known to be inhibited by high citrate/Fe ratios. In peach, the effects of the implants could be observed many months post treatment. In pear, some effects were still observed one year after the implants had taken place. Results obtained indicate that solid Fe sulfate implants were capable of significantly changing the chemical composition of the xylem sap in fruit trees.     

Xylem feeding by spittlebug nymphs: some observations by optical and cryo-scanning electron microscopy

The feeding of spittlebug nymphs (Philaenus spumarius) from mature xylem vessels was studied by optical and cryo-analytical scanning electron microscopy. Feeding did not produce xylem embolisms and vessels remained liquid-filled during the day. Saliva secreted by the insect forms a hardened lining (salivary sheath) between the stylet bundle and the plant tissues. This sheath is continuous through the hole made by the stylets as they enter a vessel, and it extends into the vessel and along its periphery beyond the breach. The sheath is heterogeneous, with a thin outer layer adjoining the plant tissues and a thicker layer that contacts the stylet bundle. Both layers give positive histochemical reactions for proteins and, in fresh tissues, contain a red, strongly autofluorescent pigment, possibly condensed tannin derived from the plant (which is lost during tissue preparation), and other phenyl propanoid compounds, which are retained and which may produce the intense reaction of the periodic-acid-Schiff's-positive inner layer. It is concluded that the salivary sheath allows the insects to feed from functioning vessels without embolizing them or losing xylem fluid to the surrounding tissues. These findings and others in the entomological literature indicate low daytime tensions in the xylem conduits of the host plants.