An intragenic suppressor of the arabidopsis floral organ identity mutant apetala3-1 functions by suppressing defects in splicing

Abstract

The Arabidopsis floral organ identity gene APETALA3 (AP3) specifies the identity of petals and stamens in the flower. In flowers mutant for the temperature-sensitive ap3-1 allele, the petals and stamens are partially converted to sepals and carpels, respectively. ap3-1 contains a single nucleotide change in the AP3 gene that alters both an amino acid in the AP3 protein and the 5' splice consensus site for intron 5. Surprisingly, the Ap3-1 mutant phenotype is not due to the missense mutation but instead is due to defects in splicing; specifically, exon 5 is frequently skipped by the splicing machinery at the restrictive temperature. In a screen for suppressors of ap3-1, we isolated an intragenic suppressor, ap3-11, that functions to suppress the splicing defects of ap3-1. Using a reverse transcriptase-polymerase chain reaction assay, we demonstrate that the percentage of full-length exon 5-containing AP3 RNAs correlates with the phenotype of the flowers in both ap3-1 and ap3-11. Rather surprisingly, the ap3-11 suppressor mutation is located in intron 4. One model explaining the function of ap3-11 is that the ap3-11 suppressor creates a novel branch point sequence that causes exon 5 to be more frequently recognized by the splicing machinery. The identification of such a suppressor strongly suggests that exon-scanning models of intron-exon recognition are operative in plants.