Regulation of soybean stem growth habit: A ten-year progress report

Stem growth habit dictates plant architecture and influences flowering and podding (seed setting), making it an essential morphological and breeding agronomic trait of soybean (Glycine max). Stem growth habit in soybean is affected by photoperiod and environment and is determined by genetic variation at major genes. Classical genetic analysis identified two critical loci, designated Determinacy 1 (Dt1) and Determinacy 2 (Dt2). Dt1 is an ortholog of Arabidopsis thaliana TERMINAL FLOWER1 (TFL1) and specifies an indeterminate stem growth habit, whereas Dt2 specifies a semi-determinate growth habit. MADS-box proteins, including Dt2, SUPPRESSOR OF OVEREXPRESSION OF CO1 (GmSOC1) and MADS-box genes downregulated by E1 (GmMDE), repress Dt1 expression. Photoreceptors encoded by the E3 and E4 loci regulate the expression of soybean FLOWERING LOCUS T (GmFT) orthologs via circadian clock genes and E1, and GmFTs compete with Dt1 to regulate stem growth habit. Study of the molecular mechanism underlying the regulation of stem growth habit in soybean has focused on the repression of Dt1 expression. Here we provide an overview of progress made in elucidating the genetic and molecular bases of stem growth habit in soybean, with emphasis on the molecular components responsible for integrating photoperiodic flowering and stem growth habit.