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.

Chloride (Cl − ) has traditionally been considered harmful to agriculture because of its toxic effects in saline soils and its antagonistic interaction with nitrate (NO 3 − ), which impairs NO 3 − nutrition. It has been largely believed that Cl − antagonizes NO 3 − uptake and accumulation in higher plants, reducing crop yield. However, we have recently uncovered that Cl − has new beneficial macronutrient functions that improve plant growth, tissue water balance, plant water relations, photosynthetic performance, and water-use efficiency. The increased plant biomass indicates in turn that Cl − may also improve nitrogen use efficiency (NUE). Considering that N availability is a bottleneck for the growth of land plants excessive NO 3 − fertilization frequently used in agriculture becomes a major environmental concern worldwide, causing excessive leaf NO 3 − accumulation in crops such as vegetables, which poses a potential risk to human health. New farming practices aimed to enhance plant NUE by reducing NO 3 − fertilization should promote a healthier and more sustainable agriculture. Given the strong interaction between Cl − and NO 3 − homeostasis in plants, we have verified if indeed Cl − affects NO 3 − accumulation and NUE in plants. For the first time to our knowledge, we provide a direct demonstration which shows that Cl − , contrary to impairing NO 3 − nutrition, facilitates NO 3 − utilization and improves NUE in plants. This is largely due to Cl − improvement of the N-NO 3 − utilization efficiency (NU T E), having little or moderate effect on N-NO 3 − uptake efficiency (NU P E) when NO 3 − is used as the sole N source. Clear positive correlations between leaf Cl − content vs. NUE/NU T E or plant growth have been established at both intra-and interspecies levels. Optimal NO 3 − vs. Cl − ratios become a useful tool to increase crop yield and quality, agricultural sustainability and to reduce the negative ecological impact of NO 3 − on the environment and on human health.

Wild subspecies of Olea europaea constitute a source of genetic variability with huge potential for olive breeding to face global changes in Mediterranean-climate regions. We intend to identify wild olive genotypes with optimal adaptability to different environmental conditions to serve as a source of rootstocks and resistance genes for olive breeding. The SILVOLIVE collection includes 146 wild genotypes representative of the six O. europaea subspecies and early-generations hybrids. These genotypes came either from olive germplasm collections or from direct prospection in Spain, continental Africa and the Macaronesian archipelago. The collection was genotyped with plastid and nuclear markers, confirming the origin of the genotypes and their high genetic variability. Morphological and architectural parameters were quantified in 103 genotypes allowing the identification of three major groups of correlative traits including vigor, branching habits and the belowground-to-aboveground ratio. The occurrence of strong phenotypic variability in these traits within the germplasm collection has been shown. Furthermore, wild olive relatives are of great significance to be used as rootstocks for olive cultivation. Thus, as a proof of concept, different wild genotypes used as rootstocks were shown to regulate vigor parameters of the grafted cultivar "Picual" scion, which could improve the productivity of high-density hedgerow orchards.