Transcriptional Regulation of Starch Biosynthesis in Sorghum Grain by a MIKC-Type MADS-Box Transcription Factor: An In Vitro Analysis

The MADS-box transcription factor (TF) family constitutes a critical class of transcriptional regulators in plants, playing pivotal roles in diverse developmental processes. MIKC-type proteins represent Type II MADS-box TFs that widely function in regulating floral organ development and reproductive growth in plants. In this study, a total of 38 MIKC-type MADS TFs were identified from the sorghum genome, distributed across nine chromosomes. Based on sequence alignments and phylogenetic analysis, these 38 SbMIKC genes (SbMIKCs) were further classified into 10 distinct subfamilies. The expression profiling of these SbMIKCs across multiple tissues revealed five major patterns, among which SbMIKC17 exhibited relatively abundant transcript levels during grain development in sorghum. Further assays confirmed that the protein encoded by SbMIKC17 localizes to the nucleus without self-transactivation activity in yeast. Integrated results from DNA affinity purification sequencing (DAP-seq), dual-luciferase assays, and yeast one-hybrid experiments demonstrate that SbMIKC17 binds to the promoter of SbAGPS1 and activates its activity, as well as enhance the promoter activities of SbBt1, SbGBSSI, SbSSIIa, and SbISA1 simultaneously. Collectively, these findings suggest that the MIKC-type MADS member of SbMIKC17 serves as a potential transcriptional regulator in starch biosynthesis in sorghum.