The late embryogenesis abundant (LEA) proteins are plant proteins that are synthesized at the onset of desiccation in maturing seeds and in vegetative organs exposed to water deficit. Here, we show that most LEA proteins are comprised in a more widespread group, which we call “hydrophilins.” The defining characteristics of hydrophilins are high glycine content (>6%) and a high hydrophilicity index (>1.0). By data base searching, we show that this criterion selectively differentiates most known LEA proteins as well as additional proteins from different taxons. We found that within the genomes of Escherichia coli and Saccharomyces cerevisiae, only 5 and 12 proteins, respectively, meet our criterion. Despite their deceivingly loose definition, hydrophilins usually represent <0.2% of the proteins of a genome. Additionally, we demonstrate that the criterion that defines hydrophilins seems to be an excellent predictor of responsiveness to hyperosmosis since most of the genes encoding these proteins in E. coli and S. cerevisiae are induced by osmotic stress. Evidence for the participation of one of the E. coli hydrophilins in the adaptive response to hyperosmotic conditions is presented. Apparently, hydrophilins represent analogous adaptations to a common problem in such diverse taxons as prokaryotes and eukaryotes.

Citrus is the main fruit tree crop in the world and therefore has a tremendous economical, social and cultural impact in our society. In recent years, our knowledge on plant reproductive biology has increased considerably mostly because of the work developed in model plants. However, the information generated in these species cannot always be applied to citrus, predominantly because citrus is a perennial tree crop that exhibits a very peculiar and unusual reproductive biology. Regulation of fruit growth and development in citrus is an intricate phenomenon depending upon many internal and external factors that may operate both sequentially and simultaneously. The elements and mechanisms whereby endogenous and environmental stimuli affect fruit growth are being interpreted and this knowledge may help to provide tools that allow optimizing production and fruit with enhanced nutritional value, the ultimate goal of the Citrus Industry. This article will review the progress that has taken place in the physiology of citrus fruiting during recent years and present the current status of major research topics in this area.

Summary: Chloride (Cl⁻) is an essential nutrient and one of the most abundant inorganic anions in plant tissues. We have cloned an Arabidopsis thaliana cDNA encoding for a member of the cation–Cl⁻ cotransporter (CCC) family. Deduced plant CCC proteins are highly conserved, and phylogenetic analyses revealed their relationships to the sub‐family of animal K⁺:Cl⁻ cotransporters. In Xenopus laevis oocytes, the A. thaliana CCC protein (At CCC) catalysed the co‐ordinated symport of K⁺, Na⁺ and Cl⁻, and this transport activity was inhibited by the ‘loop’ diuretic bumetanide, a specific inhibitor of vertebrate Na⁺:K⁺:Cl⁻ cotransporters, indicating that At CCC encodes for a bona fide Na⁺:K⁺:Cl⁻ cotransporter. Analysis of At CCC promoter‐β‐glucuronidase transgenic Arabidopsis plants revealed preferential expression in the root and shoot vasculature at the xylem/symplast boundary, root tips, trichomes, leaf hydathodes, leaf stipules and anthers. Plants homozygous for two independent T‐DNA insertions in the CCC gene exhibited shorter organs such as inflorescence stems, roots, leaves and siliques. The elongation zone of the inflorescence stem of ccc plants often necrosed during bolt emergence, while seed production was strongly impaired. In addition, ccc plants exhibited defective Cl⁻ homeostasis under high salinity, as they accumulated higher and lower Cl⁻ amounts in shoots and roots, respectively, than the treated wild type, suggesting At CCC involvement in long‐distance Cl⁻ transport. Compelling evidence is provided on the occurrence of cation–chloride cotransporters in the plant kingdom and their significant role in major plant developmental processes and Cl⁻ homeostasis.

Whole‐genome duplication, or polyploidy, is common in many plant species and often leads to better adaptation to adverse environmental condition. However, little is known about the physiological and molecular determinants underlying adaptation. We examined the drought tolerance in diploid (2x) and autotetraploid (4x) clones of Rangpur lime (Citrus limonia) rootstocks grafted with 2x Valencia Delta sweet orange (Citrus sinensis) scions, named V/2xRL and V/4xRL, respectively. Physiological experiments to study root–shoot communication associated with gene expression studies in roots and leaves were performed. V/4xRL was much more tolerant to water deficit than V/2xRL. Gene expression analysis in leaves and roots showed that more genes related to the response to water stress were differentially expressed in V/2xRL than in V/4xRL. Prior to the stress, when comparing V/4xRL to V/2xRL, V/4xRL leaves had lower stomatal conductance and greater abscisic acid (ABA) content. In roots, ABA content was higher in V/4xRL and was associated to a greater expression of drought responsive genes, including CsNCED1, a pivotal regulatory gene of ABA biosynthesis. We conclude that tetraploidy modifies the expression of genes in Rangpur lime citrus roots to regulate long‐distance ABA signalling and adaptation to stress.

Chloride (Cl^–) is a micronutrient that accumulates to macronutrient levels since it is normally available in nature and actively taken up by higher plants. Besides a role as an unspecific cell osmoticum, no clear biological roles have been explicitly associated with Cl^– when accumulated to macronutrient concentrations. To address this question, the glycophyte tobacco (Nicotiana tabacum L. var. Habana) has been treated with a basal nutrient solution supplemented with one of three salt combinations containing the same cationic balance: Cl^–-based (CL), nitrate-based (N), and sulphate+phosphate-based (SP) treatments. Under non-saline conditions (up to 5mM Cl^–) and no water limitation, Cl^– specifically stimulated higher leaf cell size and led to a moderate increase of plant fresh and dry biomass mainly due to higher shoot expansion. When applied in the 1–5mM range, Cl^– played specific roles in regulating leaf osmotic potential and turgor, allowing plants to improve leaf water balance parameters. In addition, Cl^– also altered water relations at the whole-plant level through reduction of plant transpiration. This was a consequence of a lower stomatal conductance, which resulted in lower water loss and greater photosynthetic and integrated water-use efficiency. In contrast to Cl^–, these effects were not observed for essential anionic macronutrients such as nitrate, sulphate, and phosphate. We propose that the abundant uptake and accumulation of Cl^– responds to adaptive functions improving water homeostasis in higher plants.

Chloride (Cl⁻) has traditionally been considered a micronutrient largely excluded by plants due to its ubiquity and abundance in nature, its antagonism with nitrate (NO₃⁻), and its toxicity when accumulated at high concentrations. In recent years, there has been a paradigm shift in this regard since Cl⁻ has gone from being considered a harmful ion, accidentally absorbed through NO₃⁻ transporters, to being considered a beneficial macronutrient whose transport is finely regulated by plants. As a beneficial macronutrient, Cl⁻ determines increased fresh and dry biomass, greater leaf expansion, increased elongation of leaf and root cells, improved water relations, higher mesophyll diffusion to CO₂, and better water- and nitrogen-use efficiency. While optimal growth of plants requires the synchronic supply of both Cl⁻ and NO₃⁻ molecules, the NO₃⁻/Cl⁻ plant selectivity varies between species and varieties, and in the same plant it can be modified by environmental cues such as water deficit or salinity. Recently, new genes encoding transporters mediating Cl⁻ influx (ZmNPF6.4 and ZmNPF6.6), Cl⁻ efflux (AtSLAH3 and AtSLAH1), and Cl⁻ compartmentalization (AtDTX33, AtDTX35, AtALMT4, and GsCLC2) have been identified and characterized. These transporters have proven to be highly relevant for nutrition, long-distance transport and compartmentalization of Cl⁻, as well as for cell turgor regulation and stress tolerance in plants.