Understanding the Role of Farnesylation During the Cross-talk Between Brassinosteroids and Abscisic Acid Responses

Abstract

Protein farnesylation is a post-translational modification involving the addition of a 15-carbon farnesyl isoprenoid to the carboxy terminus of select proteins. Although the roles of this lipid modification are clear in both fungal and animal signaling, many of the mechanistic functions of farnesylation in plant signaling are still unknown. In this study, I show that CYP85A2, the cytochrome P450 enzyme that performs the last step in brassinosteroid (BR) biosynthesis (conversion of castasterone to brassinolide (BL)), must be farnesylated to function in Arabidopsis. Loss of either CYP85A2 or CYP85A2 farnesylation results in reduced BL accumulation and increased plant responsiveness to the hormone abscisic acid (ABA) and overall drought tolerance. Further characterization of cyp85a2-2 mutants, suggested that lack of BL resulted in enhanced constitutive ABA signaling independent of ABA biosynthesis, indicating a role for BL in suppressing ABA signaling. I further showed through epistatic analysis that the farnesyltransferase, ERA1, also plays a role downstream of BZR1, the BR-induced transcription factor, in mediating the skotomorphogenic (dark response during seedling development) response. This study has unveiled previously unknown links between farnesylation and brassinosteroid biosynthesis that could lead to new strategies to maintain crop yield under challenging climatic conditions.