The rice asura mutant exhibits abnormal stigma morphology caused by a T-DNA insertion in the gene for serine carboxypeptidase SCP5, a putative regulator of homeotic proteins.

Abstract

The rice asura mutant, which shows an increased number of stigmas, was found to contain a T-DNA insertion in one of the two alleles of the Os01g0833500 gene, encoding a serine carboxy peptidase II-like protein, SCP5. In the mutant allele, a T-DNA was inserted in the region between Chains A and B domains, resulting in the generation of an aberrant transcript with a stop codon downstream of the Chain A domain region. In the asura mutant, the transcripts corresponding to the Chain A domain were more abundant than those corresponding to the Chain B domain. From the mutant gene, an additional protein consisting only of the Chain A domain was expected to be produced. This was predicted to recognize the substrates but did not exhibit catalytic activity. The same phenotype of increased stigma numbers was detected in both the transformants overexpressing the Chain A domain region in SCP5 and in genome-edited mutants in which the downstream region of the Chain A domain was disrupted. SCP5 interacted with several regulatory proteins involved in flower morphogenesis. Furthermore, a protein consisting only of the Chain A domain interacted with these proteins. These findings imply that excess production of the Chain A domain protein in the asura mutant impairs the function of these regulatory proteins, resulting in insufficient control of stigma number within spikelets.