Pod size of soybean is an important factor in the determination of seed weight. However, little is known about pod growth of soybean. Brassinosteroid, a group of phytohormones, regulate the pod growth of faba bean. We therefore investigated the role of brassinosteroid in pod growth of soybean. We measured pod length and cell number and cell area in pods treated with a brassinosteroid biosynthesis inhibitor. The inhibitor suppressed pod growth through the reduction of cell area. We then examined pod morphology and the expression of brassinosteroid biosynthesis (GmCYP450 85A1, 2 and 3) and response (GmBZR1, GmBES1 and GmBRU1) genes in the pods of two cultivars that differ in pod size. The difference in pod size was attributable to cell area, and the expression of brassinosteroid biosynthesis and response genes in pods was higher in the cultivar that has large pods. These results suggest that pod size of soybean is regulated through cell hypertrophy caused by brassinosteroid.
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