DROUGHT RHIZOGENESIS IN ARABIDOPSIS-THALIANA - DIFFERENTIAL RESPONSESOF HORMONAL MUTANTS

Drought rhizogenesis is an adaptive strategy that occurs during progre ssive drought stress and is characterized in the Brassicaceae and rela ted families by the formation of short, tuberized, hairless roots. The se roots are capable of withstanding a prolonged drought period and gi ve rise to a new functional root system upon rehydration. The kinetics of drought rhizogenesis during progressive water shortage was analyze d in the Arabidopsis thaliana wild-type ecotypes Landsberg erecta and Columbia. In both genotypes, this response started from a similar thre shold of soil humidity (about 2%). The intensity of drought rhizogenes is was compared in various A. thaliana hormonal mutants. The wild-type lines and most of the mutants achieved a similar drought rhizogenetic index (DRI), defined as the maximum number of short roots produced pe r mg of root biomass, after progressive drought stress. However, this DRI was dramatically reduced in the abscisic acid (ABA)-deficient aba, ABA-insensitive abi1-1, and auxin-resistant axr1-3 mutants. These dat a indicate that endogenous ABA and auxin play a promotive role in drou ght rhizogenesis. The DRI was highly increased in the gibberellin (GA) biosynthetic mutant ga5, suggesting that some GAs might also particip ate in this process. The possible role and identity of the CA species involved is discussed in view of the unaltered DRI values of the ga2, ga3, and ga4 mutants. The present analysis also allowed further discri mination among the various ABA-insensitive (abi1 versus abi2 and abi3) and auxin-resistant (axr1 versus aux1) mutants tested. In particular, drought rhizogenesis is the first physiological response shown to be differentially affected by the abi1-1 and abi2-1 mutations.