Hydraulic Traits Emerge as Relevant Determinants of Growth Patterns in Wild Olive Genotypes Under Water Stress

The hydraulic traits of plants, or the efficiency of water transport throughout the plant hydraulic system, could help to anticipate the impact of climate change and improve crop productivity. However, the mechanisms explaining the role of hydraulic traits on plant photosynthesis and thus, plant growth and yield, are just beginning to emerge. We conducted an experiment to identify differences in growth patterns at leaf, root and whole plant level among four wild olive genotypes and to determine whether hydraulic traits may help to explain such differences through their effect on photosynthesis. We estimated the relative growth rate (RGR), and its components, leaf gas exchange and hydraulic traits both at the leaf and whole-plant level in the olive genotypes over a full year. Photosynthetic capacity parameters were also measured. We observed different responses to water stress in the RGRs of the genotypes studied being best explained by changes in the net CO2 assimilation rate (NAR). Further, net photosynthesis, closely related to NAR, was mainly determined by hydraulic traits, both at leaf and whole-plant levels. This was mediated through the effects of hydraulic traits on stomatal conductance. We observed a decrease in leaf area: sapwood area and leaf area: root area ratios in water-stressed plants, which was more evident in the olive genotype Olea europaea subsp. guanchica (GUA8), whose RGR was less affected by water deficit than the other olive genotypes. In addition, at the leaf level, GUA8 water-stressed plants presented a better photosynthetic capacity due to a higher mesophyll conductance to CO2 and a higher foliar N. We conclude that hydraulic allometry adjustments of whole plant and leaf physiological response were well coordinated, buffering the water stress experienced by GUA8 plants. In turn, this explained their higher relative growth rates compared to the rest of the genotypes under water-stress conditions

Physiological and gene expression responses of sunflower (Helianthus annuus L.) plants differ according to irrigation placement

To investigate effects of soil moisture heterogeneity on plant physiology and gene expression in roots and leaves, three treatments were implemented in sunflower plants growing with roots split between two compartments: a control (C) treatment supplying 100% of plant evapotranspiration, and two treatments receiving 50% of plant evapotranspiration, either evenly distributed to both compartments (deficit irrigation - DI) or unevenly distributed to ensure distinct wet and dry compartments (partial rootzone drying - PRD). Plants receiving the same amount of water responded differently under the two irrigation systems. After 3 days, evapotranspiration was similar in C and DI, but 20% less in PRD, concomitant with decreased leaf water potential (Psi(leaf)) and increased leaf xylem ABA concentration. Six water-stress responsive genes were highly induced in roots growing in the drying soil compartment of PRD plants, and their expression was best correlated with local soil water content. On the other hand, foliar gene expression differed significantly from that of the root and correlated better with xylem ABA concentration and Psi(leaf). While the PRD irrigation strategy triggered stronger physiological and molecular responses, suggesting a more intense and systemic stress reaction due to local dehydration of the dry compartment of PRD plants, the DI strategy resulted in similar water savings without strongly inducing these responses. Correlating physiological and molecular responses in PRD/DI plants may provide insights into the severity and location of water deficits and may enable a better understanding of long-distance signalling mechanisms. (C) 2014 Elsevier Ireland Ltd. All rights reserved

Hydrophilins from distant organisms can protect enzymatic activities from water limitation effects in vitro. Plant Cell Environ

ABSTRACT . To gain insight into the function of hydrophilins, an in vitro assay was developed in which the enzymes malate dehydrogenase (MDH) or lactate dehydrogenase (LDH) are subjected to controlled partial water removal. Subtle changes in conformation during partial water removal were detected using 1-anilinonaphtalene-8-sulphonate (ANS), a fluorescent probe, whose emission at 460 nm increases when bound to hydrophobic groups. The results show that water limitation conditions imposed in this in vitro assay induce changes in MDH or LDH protein structures, which correlate with enzyme inactivation. It is also shown that plant, fungal and bacterial hydrophilins are able to protect enzymatic activities from water-loss effects in this in vitro system, in a wide range of water potentials. In addition, the data in this work indicate that the presence of hydrophilins also avoids the MDH and LDH conformational modifications caused during the assay. These results show that hydrophilins are able to protect enzymatic activities from inactivation due to in vitro partial water limitation and thus suggest a function for these proteins in vivo

Shared and novel molecular responses of mandarin to drought

Drought is the most important stress experienced by citrus crops. A citrus cDNA microarray of about 6.000 genes has been utilized to identify transcriptomic responses of mandarin to water stress. As observed in other plant species challenged with drought stress, key genes for lysine catabolism, proline and raffinose synthesis, hydrogen peroxide reduction, vacuolar malate transport, RCI2 proteolipids and defence proteins such as osmotin, dehydrins and heat-shock proteins are induced in mandarin. Also, some aquaporin genes are repressed. The osmolyte raffinose could be detected in stressed roots while the dehydrin COR15 protein only accumulated in stressed leaves but not in roots. Novel drought responses in mandarin include the induction of genes encoding a new miraculin isoform, chloroplast beta-carotene hydroxylase, oleoyl desaturase, ribosomal protein RPS13A and protein kinase CTR1. These results suggest that drought tolerance in citrus may benefit from inhibition of proteolysis, activation of zeaxanthin and linolenoyl synthesis, reinforcement of ribosomal structure and down-regulation of the ethylene response

Micronutrients

The functions of iron, manganese, copper, zinc, nickel, molybdenum, boron, and chlorine in plants are discussed. Iron (Fe) plays a crucial role in redox systems in cells and in various enzymes. The strategies of dicotyledonous and graminaceous plants to acquire Fe are described. Manganese (Mn) and copper (Cu) are important for redox systems and as activators of various enzymes involved in photosynthesis, detoxification of superoxide radicals, and the synthesis of lignin. Zinc (Zn) plays a key role in the structural and functional integrity of cell membranes, biosynthesis of proteins and detoxification of superoxide radicals. Nickel (Ni) is involved in nitrogen (N) metabolism as metal component of the enzyme urease. Molybdenum (Mo) is important for N metabolism as metal component of nitrogenase (N2 fixation) and nitrate reductase. Boron (B) is crucial for the stability and function of cell wall and membranes, whereas chlorine (Cl) is essential for the proper functioning of photosystem II (PS II) and cell osmotic regulation. For each micronutrient, the effects of deficiency and toxicity are described

Acta Horticulturae

Elucidation of genomic sequences is an essential and necessary first step to identify genetic variants associated with agronomic traits of interest. The Citruseq Consortium, a joint effort developed by public institutions and private companies has sequenced the genomes of 150 citrus varieties with the Illumina platform at high average coverage. Paired end genomic DNA libraries of 100 (2x) bp reads and with an average insert size of 500 bp were constructed. The haploid genome of Citrus clementine (www.phytozome.com) was used as reference genome. More than 150 Citrus genomes including rootstocks and species of mandarins, oranges, lemons, grapefruits and limes were sequenced. The consortium also generated a web portal that allows searching and sequence comparisons among approximately 8 million of gene variants and 45 billion of base pairs. Initial analyses suggest that the stored information can help unravel fundamental aspects of biological and evolutionary interest such as the origin of citrus. From the commercial standpoint this information may help to authenticate varieties, to improve marker-assisted breeding and to identify genes of major agronomic interest

SEQUENCING OF 150 CITRUS VARIETIES: LINKING GENOTYPES TO PHENOTYPES

Elucidation of genomic sequences is an essential and necessary first step to identify genetic variants associated with agronomic traits of interest. The Citruseq Consortium, a joint effort developed by public institutions and private companies has sequenced the genomes of 150 citrus varieties with the Illumina platform at high average coverage. Paired end genomic DNA libraries of 100 (2x) bp reads and with an average insert size of 500 bp were constructed. The haploid genome of Citrus clementine (www.phytozome.com) was used as reference genome. More than 150 Citrus genomes including rootstocks and species of mandarins, oranges, lemons, grapefruits and limes were sequenced. The consortium also generated a web portal that allows searching and sequence comparisons among approximately 8 million of gene variants and 45 billion of base pairs. Initial analyses suggest that the stored information can help unravel fundamental aspects of biological and evolutionary interest such as the origin of citrus. From the commercial standpoint this information may help to authenticate varieties, to improve marker-assisted breeding and to identify genes of major agronomic interest

Silent S-Type Anion Channel Subunit SLAH1 Gates SLAH3 Open for Chloride Root-to-Shoot Translocation

Higher plants take up nutrients via the roots and load them into xylem vessels for translocation to the shoot. After uptake, anions have to be channeled toward the root xylem vessels. Thereby, xylem parenchyma and pericycle cells control the anion composition of the root-shoot xylem sap [1-6]. The fact that salt-tolerant genotypes possess lower xylem-sap Cl- contents compared to salt-sensitive genotypes [7-10] indicates that membrane transport proteins at the sites of xylem loading contribute to plant salinity tolerance via selective chloride exclusion. However, the molecular mechanism of xylem loading that lies behind the balance between NO3- and Cl- loading remains largely unknown. Here we identify two root anion channels in Arabidopsis, SLAH1 and SLAH3, that control the shoot NO3-/Cl- ratio. The AtSLAH1 gene is expressed in the root xylem-pole pericycle, where it co-localizes with AtSLAH3. Under high soil salinity, AtSLAH1 expression markedly declined and the chloride content of the xylem sap in AtSLAH1 loss-of-function mutants was half of the wild-type level only. SLAH3 anion channels are not active per se but require extracellular nitrate and phosphorylation by calcium-dependent kinases (CPKs) [11-13]. When co-expressed in Xenopus oocytes, however, the electrically silent SLAH1 subunit gates SLAH3 open even in the absence of nitrate- and calcium-dependent kinases. Apparently, SLAH1/SLAH3 heteromerization facilitates SLAH3-mediated chloride efflux from pericycle cells into the root xylem vessels. Our results indicate that under salt stress, plants adjust the distribution of NO3- and Cl- between root and shoot via differential expression and assembly of SLAH1/SLAH3 anion channel subunits