Pod size regulator in soybean (Glycine max L. Merr.): heat shock protein 70 is associated with duration of cell proliferation in early pod development

Pod size varies among soybean cultivars, but the mechanism is largely unknown. We investigated pod size differences between two cultivars. The larger pod of ‘Tachinagaha’ was due to longer cell proliferation activity than in the shorter pod of ‘Iyodaizu’. Pod size of soybean 9  (GmPSS9), a member of the heat shock protein 70 family, was detected in a major QTL (qPSS on Chr. 2) as a candidate gene for determining pod length by QTL and expression QTL analysis. Expression of GmPSS9 in pods was higher in ‘Tachinagaha’ than in ‘Iyodaizu’ and was highest in early pod development. The difference in expression resulted from an indel polymorphism, Tachinagaha-specific-1, which has a 5′-UTR Py-rich stretch motif that boosts transcription. Treatment with an HSP70 inhibitor reduced plant height, pod length, and pod cell number. Our results identify GmPSS9 as a target gene for pod length which regulates cell number during early pod development. Differentially expressed genes in pod at 9 days after flowering between Tachinagaha and Iyodaizu cultivars in soybean.