Natural Variation in 9-Cis-Epoxycartenoid Dioxygenase 3 and ABA Accumulation.

The stress hormone abscisic acid (ABA) is critical for drought resistance; however, mechanisms controlling ABA levels are unclear. At low water potential, ABA accumulation in the Arabidopsis () accession Shahdara (Sha) was less than that in Landsberg (L) or Columbia. Analysis of a L × Sha recombinant inbred line population revealed a single major-effect quantitative trait locus for ABA accumulation, which included () as a candidate gene. encodes a rate-limiting enzyme for stress-induced ABA synthesis. Complementation experiments indicated that Sha has a reduced-function allele. Compared with L, Sha did not have reduced gene expression or protein level but did have four amino acid substitutions within NCED3. Sha differed from L in the apparent molecular mass of NCED3, indicative of altered NCED3 proteolytic processing in the chloroplast. Site-directed mutagenesis demonstrated that substitution at amino acid 271 was critical for the altered NCED3 molecular mass pattern, while the other Sha NCED3 polymorphisms were also involved in the reduced ABA accumulation. Sha did not have a reduced level of thylakoid-bound NCED3 but did differ from L in the apparent molecular mass of stromal NCED3. As Sha was not impaired in known factors critical for NCED3 function in ABA synthesis (expression, chloroplast import, and thylakoid binding), the differences between L and Sha NCED3 may affect NCED3 activity or other factors influencing NCED3 function. These results identify functionally important sites on NCED3 and indicate a complex pattern of NCED3 posttranslational regulation in the chloroplast.