Modulation of flowering by an alternatively spliced AGL18-1 transcript in Brassica juncea

Photoperiod and temperature are crucial factors that trigger flowering in Brassica juncea (B. juncea). However, the underlying regulatory mechanisms remain poorly understood. The MADS-box transcription factor AGL18 acts as a pivotal repressor of floral transition and functions redundantly with AGL15. In this study, we isolated BjuAGL18-1 from B. juncea and identified two unique transcripts, resulting in two distinct proteins: a full-length protein, BjuAGL18-1L, and a truncated protein, BjuAGL18-1S. Further investigation showed that the two isoforms had similar subcellular localizations but different expression patterns in various plant tissues. Notably, BjuAGL18-1L and BjuAGL18-1S were abundantly induced under short- and long-day photoperiods, respectively. BjuAGL18-1L overexpression in B. juncea and Arabidopsis thaliana (A. thaliana) led to late flowering, whereas BjuAGL18-1S overexpression resulted in early flowering. Yeast two-hybrid, bimolecular fluorescent complementation, and luciferase complementation assays showed that BjuAGL18-1L, but not BjuAGL18-1S (which lacked the EAR motif), interacted with the co-repressor BjuAFR2 and the histone deacetylase BjuHDA9 to form a multiprotein complex. Further analysis indicated that BjuAGL18-1L could also form a complex with BjuAGL15 and bind to the BjuFUL promoter, thus inhibiting its expression. However, BjuAGL18-1S interacted with BjuAGL18-1L to form heterodimers, which attenuated their activities, likely by disrupting their binding to target genes, resulting in accelerated flowering progression. These results suggest that BjuAGL18-1 is involved in photoperiod-induced flowering via different regulatory mechanisms in B. juncea.