Expression of the Arabidopsis thaliana BBX32 Gene in Soybean Increases Grain Yield

Crop yield is a complex quantitative trait.  Successful breeding for improved grain yield has led to crop plants with improved source capacity, altered plant architecture, and increased resistance to abiotic and biotic stresses.  To date, transgenic approaches towards improving crop grain yield have primarily focused on protecting plants from herbicide, insects, or disease.  We have identified an Arabidopsis thaliana B-box domain gene (AtBBX32) that significantly increases soybean grain yield year after year in multiple transgenic events in multi-location field trials.   In order to understand the underlying changes that are associated with increased yield, we examined phenotypic differences in two AtBBX32-expressing lines and found increases in plant height and node, flower, pod, and seed number.  We propose that these phenotypic changes are likely the result of changes in the timing of reproductive development in transgenic soybean that lead to increased duration of the pod and seed development period.  Consistent with the role of BBX32 in A. thaliana in regulating light signaling, we show that the constitutive expression of AtBBX32 in soybean alters the abundance of a subset of gene transcripts in the morning, including the soybean clock genes GmTOC1 and GmLHY.  We propose that through expression of AtBBX32 and modulation in the abundance of circadian clock genes during the transition from dark to light, the timing of critical phases of reproductive development are altered.  These findings demonstrate a specific role for AtBBX32 in modulating soybean development, and demonstrate the validity of expressing single genes in crops to deliver increased agricultural productivity.