Abscisic Acid Is Required for Root Elongation Associated With Ca2+ Influx in Response to Water Stress

Abscisic acid (ABA) is a critical hormone for plant survival under water stress. In this study, large-scale mutants of the Arabidopsis ecotype Columbia-0 (Col-0) were generated by ethyl methanesulfonate (EMS)-mutagenesis, and an improved root elongation under water-stress 1 (irew1) mutant showing significantly enhanced root growth was isolated under a water potential gradient assay. Then, irew1 and ABA-related mutants in Arabidopsis or tomato plants were observed under water potential gradient assay or water-deficient conditions. ABA pathway, Ca2+ response, and primary root (PR) elongation rate were monitored in addition to DNA- and RNA-Seq analyses. We found that based on phenotyping and transcriptional analyses, irew1 exhibited enhanced PR growth, ABA, and Ca2+ responses, compared to wild type subjected to water stress. Interestingly, exogenous Ca2+ application enhanced PR growth of irew1, ABA-biosynthesis deficient mutants in Arabidopsis, and tomato plants, in response to water potential gradients or water-deficient conditions. In combination with other ABA-related mutants and pharmacological studies, our results suggest that ABA is required for root elongation associated with Ca2+ influx in response to water stress.