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The stress hormone abscisic acid (ABA) induces expression of defence genes in many organs, modulates ion homeostasis and metabolism in guard cells, and inhibits germination and seedling growth. Concerning the latter effect, several mutants of Arabidopsis thaliana with improved capability for H(+) efflux (wat1-1D, overexpression of AKT1 and ost2-1D) are less sensitive to inhibition by ABA than the wild type. This suggested that ABA could inhibit H(+) efflux (H(+)-ATPase) and induce cytosolic acidification as a mechanism of growth inhibition. Measurements to test this hypothesis could not be done in germinating seeds and we used roots as the most convenient system. ABA inhibited the root plasma-membrane H(+)-ATPase measured in vitro (ATP hydrolysis by isolated vesicles) and in vivo (H(+) efflux from seedling roots). This inhibition involved the core ABA signalling elements: PYR/PYL/RCAR ABA receptors, ABA-inhibited protein phosphatases (HAB1), and ABA-activated protein kinases (SnRK2.2 and SnRK2.3). Electrophysiological measurements in root epidermal cells indicated that ABA, acting through the PYR/PYL/RCAR receptors, induced membrane hyperpolarization (due to K(+) efflux through the GORK channel) and cytosolic acidification. This acidification was not observed in the wat1-1D mutant. The mechanism of inhibition of the H(+)-ATPase by ABA and its effects on cytosolic pH and membrane potential in roots were different from those in guard cells. ABA did not affect the in vivo phosphorylation level of the known activating site (penultimate threonine) of H(+)-ATPase in roots, and SnRK2.2 phosphorylated in vitro the C-terminal regulatory domain of H(+)-ATPase while the guard-cell kinase SnRK2.6/OST1 did not., (© The Author 2014. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

Seed longevity is important to preserve crops and wild plants and it is limited by progressive cellular damage (aging) during storage. The induction of cellular stress defenses and the formation of the seed coat are crucial protecting events during seed development, a process mediated in Arabidopsis thaliana by the transcription factors LEC1, LEC2, FUS3 and the abscisic acid-activated ABI3. In order to identify novel determinants of seed longevity we have screened an activation-tagging mutant collection of Arabidopsis and isolated a dominant mutant with increased seed longevity under both natural and accelerated aging conditions. Molecular characterization indicates that the mutant phenotype is caused by over-expression of the At2g26130 gene encoding a RING-type zinc finger putative ubiquitin ligase. Loss of function of this gene in a T-DNA insertion mutant resulted in decreased seed longevity. We named this important gene for seed longevity RSL1 (from Ring finger of Seed Longevity1) and we could demonstrate ubiquitin ligase activity with the recombinant protein. Morphological alterations in shoot tissues of the RSL1 over-expressing plants and analysis of gibberellins levels suggest that RSL1 may increase gibberellins responses by some unknown mechanism. These results validate the forward genetic approach to seed longevity and anticipate the identification of many novel determinants of this important trait., (Copyright © 2013 Elsevier Ireland Ltd. All rights reserved.)