Abscisic Acid Regulates Axillary Bud Outgrowth Responses to the Ratio of Red to Far-Red Light1[C][W][OPEN]

ABA regulates bud outgrowth responses to the ratio of red to far-red light, extending the known hormonal pathways associated with the regulation of branching and shade avoidance . Low red light/far-red light ratio ( R:FR ) serves as an indicator of impending competition and has been demonstrated to suppress branch development. The regulation of Arabidopsis ( Arabidopsis thaliana ) rosette bud outgrowth by the R:FR and the associated mechanisms were investigated at several levels. Growth under low R:FR suppressed outgrowth of the third from topmost bud ( bud n-2 ) but not that of the topmost bud. Subsequently increasing the R:FR near the time of anthesis promoted bud n-2 outgrowth and reduced topmost bud growth. Buds from specific rosette positions, exhibiting divergent fates to increased R:FR , were harvested 3 h after modifying the R:FR and were used to conduct ATH1 microarray-based transcriptome profiling. Differentially expressed genes showed enrichment of light signaling and hormone-related Gene Ontology terms and promoter motifs, most notably those associated with abscisic acid ( ABA ). Genes associated with ABA biosynthesis, including the key biosynthetic gene NINE-CIS-EPOXYCAROTENOID DIOXYGENASE3 ( NCED3 ), and with ABA signaling were expressed at higher levels in the responsive bud n-2 , and increasing the R:FR decreased their expression only in bud n-2 . ABA abundance in responsive buds decreased within 12 h of increasing the R:FR , while indole-3-acetic acid levels did not change. A role for ABA in repressing bud outgrowth from lower positions under low R:FR was demonstrated using the nced3-2 and aba2-1 ABA biosynthesis mutants, which showed enhanced branching and a defective bud n-2 outgrowth response to low R:FR . The results provide evidence that ABA regulates bud outgrowth responses to the R:FR and thus extend the known hormonal pathways associated with the regulation of branching and shade avoidance.