The sax1 dwarf mutant of Arabidopsis thaliana shows altered sensitivity ofgrowth responses to abscisic acid, auxin, gibberellins and ethylene and ispartially rescued by exogenous brassinosteroid

Genetic approaches using Arabidopsis thaliana aimed at the identification o f mutations affecting events involved in auxin signalling have usually led to the isolation of auxin-resistant mutants. From a selection screen specif ically developed to isolate auxin-hypersensitive mutants, one mutant line w as selected for its increased sensitivity to auxin (x 2 to 3) for the root elongation response. The genetic analysis of sax1 (hypersensitive to abscis ic acid and auxin) indicated that the mutant phenotype segregates as a sing le recessive Mendelian locus, mapping to the lower arm of chromosome 1. Sax 1 seedlings grown in vitro showed a short curled primary root and small, ro und, dark-green cotyledons. In the greenhouse, adult sari plants were chara cterized by a dwarf phenotype, delayed development and reduced fertility. F urther physiological characterization of sari seedlings revealed that the m ost striking trait was a large increase (x 40) in ABA-sensitivity of root e longation and, to a lesser extent, of ABA-induced stomatal closure; in othe r respects, hypocotyl elongation was resistant to gibberellins and ethylene . These alterations in hormone sensitivity in sari plants co-segregated wit h the dwarf phenotype suggesting that processes involved in cell elongation are modified. Treatment of mutant seedlings with an exogenous brassinoster oid partially rescued a wild-type size, suggesting that brassinosteroid bio synthesis might be affected in sari plants. Wild-type sensitivities to ABA, auxin and gibberellins were also restored in sax1 plants by exogenous appl ication of brassinosteroid, illustrating the pivotal importance of the BR-r elated SAX1 gene.