Functional domains of SPATULA, a bHLH transcription factor involved in carpel and fruit development in Arabidopsis

The SPATULA (SPT) gene is involved in generating the septum, style and stigma: specialized tissues that arise from carpel margins. By matching sequences within the extended bHLH region of AtSPT across species databases, twelve orthologues were identified in eudicots, rice and a gymnosperm. Two conserved structural domains were revealed in addition to the bHLH region: an amphipathic helix and an acidic domain. These are conserved in the tomato orthologue, which can restore carpel function to spt mutants of Arabidopsis. The acidic domain is essential for SPT carpel function, and the amphipathic helix supports it. A bipartite sequence overlapping the bHLH domain is required for nuclear localization, and a mutation in the conserved beta strand adjacent to the bHLH C terminus results in the loss of SPT function. SPT apparently acts as a transcriptional activator, as the addition of the SRDX repression domain phenocopies the spt mutant phenotype. Expression of an artificially activating 35S:SPT-VP16 construct can induce carpelloid properties in sepals, and new defects in the gynoecium. These disruptions are associated with ectopic expression of the STYLISH2 gene, although STYLISH2 expression does not require SPT function. Ectopic expression of unmodified SPT does not induce such changes, implying that SPT acts in association with essential coactivators present only in regions where SPT is normally active. Because the VP16 activation domain can compensate to some extent for the loss of the amphipathic helix and acidic domain, these domains may normally interact with such co-activators.