Physical Restriction of Pods Causes Seed Size Reduction of a Brassinosteroid-deficient Faba Bean (Vicia faba)

Background and Aims A brassinosteroid-deficient mutant faba bean (Vicia faba ʹRinreiʹ) shows dwarfism in many organs including pods and seeds. ʹRinreiʹ has normal-sized seeds together with dwarf seeds, suggesting that dwarfism in the seed may be indirectly caused by brassinosteroid deficiency. The mechanism of seed size reduction in this mutant was investigated. Methods The associations between seed orientation in the pod, seed numbers per pod and pod lengths with seed sizes were analysed in ʹRinreiʹ and the wild-type plant. Key Results ʹRinreiʹ seeds are tightly arranged in pods containing two or three seeds. Seed size decreased as the number of seeds per pod increased or as the length of the pod decreased. Where no physical restriction occurred between seeds in a pod, the wild-type faba bean seeds had a nearly constant size regardless of seed number per pod or pod length. ʹRinreiʹ seeds in pods containing single seeds were the same size as wild-type seeds. Brassinolide treatment increased the seed size and the length of pods containing three seeds in ʹRinreiʹ. Conclusion Seed size of ʹRinreiʹ is mainly regulated through a reduction of pod length due to brassinosteroid deficiency; physical restriction within pods causes a reduction in seed size. These results suggest a possible mechanism for increasing faba bean yields to optimal levels.