Daniel P. Schachtman

The auxin influx carrier LAX3 promotes lateral root emergence

Lateral roots originate deep within the parental root from a small number of founder cells at the periphery of vascular tissues and must emerge through intervening layers of tissues. We describe how the hormone auxin, which originates from the developing lateral root, acts as a local inductive signal which re-programmes adjacent cells. Auxin induces the expression of a previously uncharacterized auxin influx carrier LAX3 in cortical and epidermal cells directly overlaying new primordia. Increased LAX3 activity reinforces the auxin-dependent induction of a selection of cell-wall-remodelling enzymes, which are likely to promote cell separation in advance of developing lateral root primordia.

Avenues for increasing salt tolerance of crops, and the role of physiologically based selection traits

Increased salt tolerance is needed for crops grown in areas at risk of salinisation. This requires new genetic sources of salt tolerance, and more efficient techniques for identifying salt-tolerant germplasm, so that new genes for tolerance can be introduced into crop cultivars. Screening a large number of genotypes for salt tolerance is not easy. Salt tolerance is achieved through the control of salt movement into and through the plant, and salt-specific effects on growth are seen only after long periods of time. Early effects on growth and metabolism are likely due to osmotic effects of the salt, that is to the salt in the soil solution. To avoid the necessity of growing plants for long periods of time to measure biomass or yield, practical selection techniques can be based on physiological traits. We illustrate this with current work on durum wheat, on selection for the trait of sodium exclusion. We have explored a wide range of genetic diversity, identified a new source of sodium exclusion, confirmed that the trait has a high heritability, checked for possible penalties associated with the trait, and are currently developing molecular markers. This illustrates the potential for marker-assisted selection based on sound physiological principles in producing salt-tolerant crop cultivars.

Chemical root to shoot signaling under drought

Chemical signals are important for plant adaptation to water stress. As soils become dry, root-sourced signals are transported via the xylem to leaves and result in reduced water loss and decreased leaf growth. The presence of chemical signals in xylem sap is accepted, but the identity of these signals is controversial. Abscisic acid (ABA), pH, cytokinins, a precursor of ethylene, malate and other unidentified factors have all been implicated in root to shoot signaling under drought. This review describes current knowledge of, and advances in, research on chemical signals that are sent from roots under drought. The contribution of these different potential signals is discussed within the context of their role in stress signaling.

Molecular pieces to the puzzle of the interaction between potassium and sodium uptake in plants

Potassium uptake is vital for plant growth but in saline soils sodium competes with potassium for uptake across the plasma membrane of plant cells. This can result in high Na+:K+ ratios that reduce plant growth and eventually become toxic. Our understanding of the molecular basis underlying the interaction between essential potassium and toxic sodium was limited until the recent cloning and electrophysiological characterization of several genes encoding different types of molecules that are involved in K+ and Na+ transport. These molecules, and their regulation, are important in determining the K+:Na+ homeostasis of plants in saline soils, although it is not yet known which is most critical in determining the K+:Na+ ratios in whole plants.

Expression of KT/KUP Genes in Arabidopsis and the Role of Root Hairs in K+ Uptake

Potassium (K+) is the most abundant cation in plants and is required for plant growth. To ensure an adequate supply of K+, plants have multiple mechanisms for uptake and translocation. However, relatively little is known about the physiological role of proteins encoded by a family of 13 genes, named AtKT/KUP, that are involved in K+ transport and translocation. To begin to understand where and under what conditions these transporters function, we used reverse transcription-PCR to determine the spatial and temporal expression patterns of each AtKT/KUP gene across a range of organs and tested whether selected AtKT/KUP cDNAs function as K+ transporters in Escherichia coli. Many AtKT/KUPs were expressed in roots, leaves, siliques, and flowers of plants grown under K+-sufficient conditions (1.75 mm KCl) in hydroponic culture. AtHAK5 was the only gene in this family that was up-regulated upon K+ deprivation and rapidly down-regulated with resupply of K+. Ten AtKT/KUPs were expressed in root hairs, but only five were expressed in root tip cells. This suggests an important role for root hairs in K+ uptake. The growth and rubidium (Rb+) uptake of two root hair mutants, trh1-1 (tiny root hairs) and rhd6 (root hair defective), were studied to determine the contribution of root hairs to whole-plant K+ status. Whole-plant biomass decreased in the root hair mutants only when K+ concentrations were low; Rb+ (used as a tracer for K+) uptake rates were lower in the mutants at all Rb+ concentrations. Seven genes encoding AtKUP transporters were expressed in E. coli (AtKT3/KUP4, AtKT/KUP5, AtKT/KUP6, AtKT/KUP7, AtKT/KUP10, AtKT/KUP11, and AtHAK5), and their K+ transport function was demonstrated.

Ethylene Mediates Response and Tolerance to Potassium Deprivation in Arabidopsis

Potassium deprivation leads to large reductions in plant growth and yields. How plants sense and transduce the stress signals initiated by potassium deprivation is poorly understood. Both ethylene production and the transcription of genes involved in ethylene biosynthesis increase when plants are deprived of potassium. To elucidate the role of ethylene in low potassium signaling pathways, we used both genetic and chemical approaches. Our results showed that ethylene is important in tolerance to low potassium and for changes in both root hair and primary root growth in Arabidopsis thaliana. We show that ethylene acts upstream of reactive oxygen species in response to potassium deprivation. The expression of High-Affinity K+ Transporter5 was used as a marker of potassium deprivation and was found to be dependent on ethylene signaling. In the ethylene insensitive2-1 (ein2-1) mutant, the ethylene-mediated low potassium responses were not completely eliminated, suggesting that some potassium deprivation–induced responses are either ethylene independent or EIN2 independent. Ethylene signaling is a component of the plants response to low potassium that stimulates the production of reactive oxygen species and is important for changes in root morphology and whole plant tolerance to low potassium conditions.

Powdery Mildew Induces Defense-Oriented Reprogramming of the Transcriptome in a Susceptible But Not in a Resistant Grapevine

Grapevines exhibit a wide spectrum of resistance to the powdery mildew fungus (PM), Erysiphe necator (Schw.) Burr., but little is known about the transcriptional basis of the defense to PM. Our microscopic observations showed that PM produced less hyphal growth and induced more brown-colored epidermal cells on leaves of PM-resistant Vitis aestivalis ‘Norton’ than on leaves of PM-susceptible Vitis vinifera ‘Cabernet sauvignon’. We found that endogenous salicylic acid levels were higher in V. aestivalis than in V. vinifera in the absence of the fungus and that salicylic acid levels increased in V. vinifera at 120 h postinoculation with PM. To test the hypothesis that gene expression differences would be apparent when V. aestivalis and V. vinifera were mounting a response to PM, we conducted a comprehensive Vitis GeneChip analysis. We examined the transcriptome at 0, 4, 8, 12, 24, and 48 h postinoculation with PM. We found only three PM-responsive transcripts in V. aestivalis and 625 in V. vinifera. There was a significant increase in the abundance of transcripts encoding ENHANCED DISEASE SUSCEPTIBILITY1, mitogen-activated protein kinase kinase, WRKY, PATHOGENESIS-RELATED1, PATHOGENESIS-RELATED10, and stilbene synthase in PM-infected V. vinifera, suggesting an induction of the basal defense response. The overall changes in the PM-responsive V. vinifera transcriptome also indicated a possible reprogramming of metabolism toward the increased synthesis of the secondary metabolites. These results suggested that resistance to PM in V. aestivalis was not associated with overall reprogramming of the transcriptome. However, PM induced defense-oriented transcriptional changes in V. vinifera.

Sodium Accumulation in Leaves of Triticum Species That Differ in Salt Tolerance

The ability to limit the accumulation of Na+ in leaves may be an important mechanism in salt tolerance because the excessive accumulation of Na+ causes the premature senescence of leaves. The objectives of this study were to test the importance of Na+ accumulation rates in determining salt tolerance and to determine whether observed genotypic differences in leaf Na+ accumulation are linked to rates of leaf expansion. Six salt-sensitive and salt-tolerant genotypes of Triticum tauschii (Coss.) Schmal., T. aestivum L. and T. turgidum L. were grown in 150 mol m-3 NaCl and harvested at regular intervals over approximately 3 weeks. Na+ concentrations and leaf growth were measured in individual leaf blades over this time. The salt-tolerant accessions all had lower rates of Na+ accumulation than the salt-sensitive. This was not due to genotypic differences in growth rates: it was independent of the overall growth rate (vigour) of the genotypes, and the growth phase of individual leaves. In the growing leaf, the rate of Na+ accumulation was lower in salt-tolerant genotypes both during and after the phase of expansion. Leaf longevity was greater in the salt-tolerant genotypes. In one salt-sensitive genotype, the maximum Na+ concentration was much lower than that of all the other genotypes. Two mechanisms of salt tolerance appear to be operating in Triticum genotypes. One is a lower rate of Na+ accumulation which is independent of the growth of individual leaves and therefore probably regulated by some root process. The second is ion compartmentation within leaves, which enhances the ability to tolerate high concentrations of Na+ in leaves.

Cell Wall Proteome in the Maize Primary Root Elongation Zone. II. Region-Specific Changes in Water Soluble and Lightly Ionically Bound Proteins under Water Deficit

Previous work on the adaptation of maize (Zea mays) primary roots to water deficit showed that cell elongation is maintained preferentially toward the apex, and that this response involves modification of cell wall extension properties. To gain a comprehensive understanding of how cell wall protein (CWP) composition changes in association with the differential growth responses to water deficit in different regions of the elongation zone, a proteomics approach was used to examine water soluble and loosely ionically bound CWPs. The results revealed major and predominantly region-specific changes in protein profiles between well-watered and water-stressed roots. In total, 152 water deficit-responsive proteins were identified and categorized into five groups based on their potential function in the cell wall: reactive oxygen species (ROS) metabolism, defense and detoxification, hydrolases, carbohydrate metabolism, and other/unknown. The results indicate that stress-induced changes in CWPs involve multiple processes that are likely to regulate the response of cell elongation. In particular, the changes in protein abundance related to ROS metabolism predicted an increase in apoplastic ROS production in the apical region of the elongation zone of water-stressed roots. This was verified by quantification of hydrogen peroxide content in extracted apoplastic fluid and by in situ imaging of apoplastic ROS levels. This response could contribute directly to the enhancement of wall loosening in this region. This large-scale proteomic analysis provides novel insights into the complexity of mechanisms that regulate root growth under water deficit conditions and highlights the spatial differences in CWP composition in the root elongation zone.

Differential Metal Selectivity and Gene Expression of Two Zinc Transporters from Rice

Zinc is an essential mineral for a wide variety of physiological and biochemical processes. To understand zinc transport in cereals, we identified putative zinc transporters in gene databases. Three full-length cDNAs were identified and characterized from rice (Oryza sativa). Two of the cDNAs partially complemented a yeast (Saccharomyces cerevisiae) mutant deficient in zinc uptake at low concentrations. The two transporters showed many similarities in function but differed in ionic selectivity and pH optimum of activity. Expression patterns also differed between the two genes. One gene was broadly expressed under all conditions, and the other gene was mainly induced by zinc deficiency to higher levels in roots than in leaves. Although the timing of expression differed between the two genes, localization of expression overlapped in roots. Comparisons of the protein sequences, ionic selectivity, and gene expression patterns of the two transporters suggest that they may play different roles in the physiology of the whole plant.