SbTT8, a New Sorghum bHLH Transcription Factor that Rescues Brown Seed Coat Phenotype in Arabidopsis tt8 Mutant Plants

Abstract

Flavonoids are polyphenolic compounds and color most flowers, fruits and seeds. The regulation of flavonoid biosynthetic genes is directed by ternary complexes composed of MYB-bHLH-WD40 transcription factors. DNA-binding bHLH proteins are widely distributed in higher plants and comprise the second largest transcription factor family. The regulation of flavonoid synthesis by bHLH has been extensively studied in vascular plants, most functionally characterized in Arabidopsis. Sorghum is one of the most economically important crops worldwide. Despite 169 bHLH genes were identified in this cereal, few studies have reported their roles in seed coat pigmentation. Here we identified and characterized SbTT8, a sorghum bHLH-domain protein, through molecular modeling and complementation studies in Arabidopsis tt8 mutant. SbTT8 partially rescues the brown seed color and dormancy in Arabidopsis tt8 mutants. We show that SbTT8 is significantly larger than AtTT8 and holds a Lys467 residue where AtTT8 has an Ala367, within the DNA-recognition motif conserved throughout the bHLH family. This change likely affects how SbTT8 interacts with the target DNA, as compared with AtTT8. Our results shed light on the different phenotypes witnessed for A. thaliana transgenic lines expressing SbTT8 from sorghum, as well as the regulation of genes encoding enzymes for flavonoid biosynthesis.