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 ‘Rinrei’) shows dwarfism in many
organs including pods and seeds. ‘Rinrei’ 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 ‘Rinrei’ and the wild-type plant.
? Key Results ‘Rinrei’ 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
betweenseedsina pod,thewild-type fababean seedshada nearlyconstant size regardlessofseed numberperpod or
pod length. ‘Rinrei’ 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 ‘Rinrei’.
? Conclusion Seed size of ‘Rinrei’ 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.