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

Abstract

SummaryGenetic approaches using Arabidopsis thaliana aimed at the identification of mutations affecting events involved in auxin signalling have usually led to the isolation of auxinresistant mutants. From a selection screen specifically developed to isolate auxin-hypersensitive mutants, one mutant line was selected for its increased sensitivity to auxin (⍥ 2 to 3) for the root elongation response. The genetic analysis of sax1 (hypersensitive to abscisic acid and auxin) indicated that the mutant phenotype segregates as a single recessive Mendelian locus, mapping to the lower arm of chromosome 1. Sax1 seedlings grown in vitro showed a short curled primary root and small, round, dark-green cotyledons. In the greenhouse, adult sax1 plants were characterized by a dwarf phenotype, delayed development and reduced fertility. Further physiological characterization of sax1 seedlings revealed that the most striking trait was a large increase (ϫ 40) in ABA-sensitivity of root elongation and, to a lesser extent, of ABA-induced stomatal closure; in other respects, hypocotyl elongation was resistant to gibberellins and ethylene. These alterations in hormone sensitivity in sax1 plants co-segregated with the dwarf phenotype suggesting that processes involved in cell elongation are modified. Treatment of mutant seedlings with an exogenous brassinosteroid partially rescued a wild-type size, suggesting that brassinosteroid biosynthesis might be affected in sax1 plants. Wild-type sensitivities to ABA, auxin and gibberellins were also restored in sax1 plants by exogenous

ArabidopsisLATERAL ORGAN BOUNDARIES negatively regulates brassinosteroid accumulation to limit growth in organ boundaries

“…Furthermore, germination on increasing concentrations of BL resulted in an increase in hypocotyl length in dark-grown 35S:LOB-GR seedlings on DEX, whereas mock-treated seedlings exhibited inhibition of hypocotyl growth (Fig. 2D), similar to that reported for wild type (16). Apical hook formation was not restored by treatment with the phytohormones gibberellic acid (GA), auxin (IAA), or the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) (Fig.…”

Inhibition of Brassinosteroid Biosynthesis by Either a dwarf4 Mutation or a Brassinosteroid Biosynthesis Inhibitor Rescues Defects in Tropic Responses of Hypocotyls in the Arabidopsis Mutant nonphototropic hypocotyl 4

“…Increase in auxin sensitivity has already been reported in dwf4 (Azpiroz et al, 1998) and sax1, a putative BR biosynthesis mutant (Ephritikhine et al, 1999a) with respect to root growth. Sensitivity of hypocotyls to auxin is not examined in either case.…”

The β-Subunit of the Arabidopsis G Protein Negatively Regulates Auxin-Induced Cell Division and Affects Multiple Developmental Processes[W]

“…Considering the recent evidence suggesting that brassinosteroid signaling is coupled by a heterotrimeric G protein in Arabidopsis (Ullah et al, 2002) and the well-known observation that brassinosteroid and auxin signaling interact (Clouse, 1998;Ephritikhine et al, 1999;Yin et al, 2002), we propose that brassinosteroids fulfill this role.…”