The ABSCISIC ACID INSENSITIVE 3 (ABI3) gene is modulated by farnesylation and is involved in auxin signaling and lateral root development in Arabidopsis

Genetic screens have identified a number of genes that regulate abscisic acid (ABA) responsiveness in Arabidopsis . Using a combination of suppressor screens and double mutant analysis, we have determined a genetic relationship for a number of these ABA response loci. Based on germination in the presence of exogenous ABA, the ABI1 and ABI2 phosphatases act at or upstream of the ERA1 farnesyl transferase and the ABI3 and ABI5 transcription factors act at or downstream of ERA1 . In contrast with ABI3 and ABI5 , the ABI4 transcription factor appears to act at or upstream of ERA1 . Based on reporter gene constructs, the upstream regulation of ABI3 by ERA1 occurs at least partially at the level of transcription, suggesting that this lipid modification is required to attenuate ABI3 expression. Similar experiments also indicate that ABI3 is auxin inducible in lateral root primordia. Related to this, loss-of-function abi3 alleles show reduced lateral root responsiveness in the presence of auxin and an auxin transport inhibitor, and era1 mutants have increased numbers of lateral roots. These results suggest the possibility that genes identified through ABA responsive germination screens such as ERA1 and ABI3 have functions in auxin action in Arabidopsis .