Characterization and cloning of SMALL GRAIN 4, a novel DWARF11 allele that affects brassinosteroid biosynthesis in rice

Brassinosteroids (BRs) are important hormones that regulate plant development and physiology. Substantial progresses have been made in BR-related studies, and especially an increasing number of new genes involved in BR biosynthesis have been identified. Here, we characterize a BR-related rice mutant, small grain 4 (sg4), obtained from callus culture of japonica cultivar Nipponbare. This mutant showed multiple phenotypes such as dark green, rugose erect leaves and small round grains. It was higher than the wild type, different from the majority of BR- and gibberellin-related mutants. Genetic analysis showed that the mutant phenotypes are controlled by a single recessive locus. The gene was fine-mapped to a 90.7-kb interval with 1,100 F2 recessive individuals by means of map-based cloning. Totally 11 open reading frames were found in this interval, only one of which was detected with an 8-bp insertion in the 5ʹUTR region by sequencing. Functional complementation test revealed that a DWARF11 allele, LOC_Os04g39430, is answer for the mutant phenotype of sg4. Tissue-specific response to BR and decreased expression levels of BR biosynthetic genes suggest that sg4 is a weak BR-deficient mutant. These results are beneficial to understanding the physiological action of sg4 in a more comprehensive way.