Chapter 3

AIR1A, AIR1B and AIR3 are three auxin-responsive genes from Arabidopsis that are expressed in the outer cell layers of the root at sites of lateral root emergence. The expression of these genes is not directly regulated by auxin but mediated by a secondary messenger. In order to identify this secondary messenger we investigated the effects of known signaling factors/molecules on the expression of AIR::GUS reporter genes. Using two ethylene-insensitive mutant backgrounds, we found that the expression of AIR1 and AIR3 genes is ethylene-independent. Unexpectedly, methyl jasmonate (MeJA) led to increased AIR1::GUS expression with an expression pattern and intensity indistinguishable from that observed after auxin induction. The expression of AIR1::GUS and AIR3::GUS genes in mutants defective in auxin and jasmonate signaling was also studied. In alf4-1, a mutant impaired in lateral root formation, MeJA only poorly induced the expression of the AIR1::GUS gene. In the slr-1 mutant however there was normal induction of the gene by MeJA. In the MeJAinsensitive mutant coi1-1 the normal expression pattern of AIR1 was observed but the enhanced expression of AIR1 after auxin or MeJA stimulation was blocked. We propose that the auxin-induced expression of AIR1 is mediated by MeJA, which activates the SCF machinery for the degradation of proteins repressing the expression of the AIR1 gene. Unexpectedly, the AIR3 gene turned out not to be responsive to MeJA. Thus the secondary signal triggering the auxin-induced expression of the AIR3 gene remains unknown.