Title : Jasmonic acid perception by COI1 involves inositol polyphosphates in Arabidopsis thaliana.
Abstract : Plant responses to wounding are part of defence responses against insects and tightly regulated. The isoleucin conjugate of jasmonic acid (JA-Ile) is a major regulatory molecule. We have previously shown that inositol polyphosphate signals are required for defence responses in Arabidopsis. However, so far it has remained unclear how inositol polyphosphates contribute to plant responses to wounding. Arabidopsis F-box proteins involved in JA-Ile perception (COI1) and auxin perception (TIR1) are structurally similar. Because TIR1 has recently been shown to contain inositol hexakisphosphate (InsP(6) ) as a cofactor of unknown function, here we explored the possibility that InsP(6) or another inositol polyphosphate is required for COI1 function. In support of this hypothesis, COI1 variants with changes in putative inositol polyphosphate coordinating residues exhibited reduced interaction with the COI1 target, JAZ9, in yeast two-hybrid tests. The equivalent COI1 variants displayed reduced capability to rescue jasmonate-mediated root growth inhibition or silique development in Arabidopsis coi1 mutants. Yeast two-hybrid tests using wild type COI1 in an ipk1Δ yeast strain exhibiting increased levels of inositol pentakisphosphate (InsP(5) ) and reduced levels of InsP(6) indicate an enhanced COI1/JAZ9 interaction. Consistent with these findings, Arabidopsis ipk1-1 mutants, also with increased InsP(5) and reduced InsP(6) levels, showed increased defensive capabilities via COI1-mediated processes, including wound-induced gene expression, defence against caterpillars, or root growth inhibition by jasmonate. The combined data from experiments using mutated COI1 variants as well as yeast and Arabidopsis backgrounds altered in inositol polyphosphate metabolism indicate that an inositol polyphosphate and likely InsP(5) contributes to COI1 function