The Putative GATA Transcription Factor SbGATA22 as a Novel Regulator of Dhurrin Biosynthesis

Cyanogenic glucosides are specialized metabolites produced by over 3000 species of higher plants from more than 130 families. The deployment of cyanogenic glucosides is influenced by biotic and abiotic factors in addition to being developmentally regulated, consistent with their roles in plant defense and stress mitigation. Despite their ubiquity, very little is known regarding the molecular mechanisms that regulate their biosynthesis. The biosynthetic pathway of dhurrin, the cyanogenic glucoside found in the important cereal crop sorghum (Sorghum bicolor (L.) Moench), was described over 20 years ago, and yet no direct regulator of the biosynthetic genes has been identified. To isolate regulatory proteins that bind to the promoter region of the key dhurrin biosynthetic gene of sorghum, SbCYP79A1, yeast one-hybrid screens were performed. A bait fragment containing 1204 base pairs of the SbCYP79A1 5' regulatory region was cloned upstream of a reporter gene and introduced into Saccharomyces cerevisiae. Subsequently, the yeast was transformed with library cDNA representing RNA from two different sorghum developmental stages. From these screens, we identified SbGATA22, an LLM domain B-GATA transcription factor that binds to the putative GATA transcription factor binding motifs in the SbCYP79A1 promoter region. Transient assays in Nicotiana benthamiana show that SbGATA22 localizes to the nucleus. The expression of SbGATA22, in comparison with SbCYP79A1 expression and dhurrin concentration, was analyzed over 14 days of sorghum development and in response to nitrogen application, as these conditions are known to affect dhurrin levels. Collectively, these findings suggest that SbGATA22 may act as a negative regulator of SbCYP79A1 expression and provide a preliminary insight into the molecular regulation of dhurrin biosynthesis in sorghum.