Genetic and chemical analyses of the action mechanisms of sirtinol in Arabidopsis.

The synthetic molecule sirtinol was shown previously to activate the auxin signal transduction pathway. Here we present a combination of genetic and chemical approaches to elucidate the action mechanisms of sirtinol in Arabidopsis. Analysis of sirtinol derivatives indicated that the "active moiety" of sirtinol is 2-hydroxy-1-naphthaldehyde (HNA), suggesting that sirtinol undergoes a series of transformations in Arabidopsis to generate HNA, which then is converted to 2-hydroxy-1-naphthoic acid (HNC), which activates auxin signaling. A key step in the activation of sirtinol is the conversion of HNA to HNC, which is likely catalyzed by an aldehyde oxidase. Mutations in any of the genes that are responsible for synthesizing the molybdopterin cofactor, an essential cofactor for aldehyde oxidases, led to resistance to sirtinol, probably caused by the compromised capacity of the mutants to convert HNA to HNC. We also showed that sirtinol and HNA could bypass the auxin polar transport system and that they were transported efficiently to aerial parts of seedlings, whereas HNC and 1-naphthoic acid were essentially not absorbed by Arabidopsis seedlings, suggesting that sirtinol and HNA are useful tools for auxin studies.