The nucleotidase/phosphatase SAL1 is a negative regulator of drought tolerance in Arabidopsis

An Arabidopsis thaliana drought-tolerant mutant, altered expression of APX2 ( alx8 ), has constitutively increased abscisic acid (ABA) content, increased expression of genes responsive to high light stress and is reported to be drought tolerant. We have identified alx8 as a mutation in SAL1, an enzyme that can dephosphorylate dinucleotide phosphates or inositol phosphates. Previously identified mutations in SAL1, including fiery ( fry1-1 ), were reported as being more sensitive to drought imposed by detachment of rosettes. Here we demonstrate that alx8 , fry1-1 and a T-DNA insertional knockout allele all have markedly increased resistance to drought when water is withheld from soil-grown intact plants. Microarray analysis revealed constitutively altered expression of more than 1800 genes in both alx8 and fry1-1. The up-regulated genes included some characterized stress response genes, but few are inducible by ABA. Metabolomic analysis revealed that both mutants exhibit a similar, dramatic reprogramming of metabolism, including increased levels of the polyamine putrescine implicated in stress tolerance, and the accumulation of a number of unknown, potential osmoprotectant carbohydrate derivatives. Under well-watered conditions, there was no substantial difference between alx8 and Col-0 in biomass at maturity; plant water use efficiency (WUE) as measured by carbon isotope discrimination; or stomatal index, morphology or aperture. Thus, SAL1 acts as a negative regulator of predominantly ABA-independent and also ABA-dependent stress response pathways, such that its inactivation results in altered osmoprotectants, higher leaf relative water content and maintenance of viable tissues during prolonged water stress.