A transcription factor γMYB1 binds to the P1BS cis-element and activates PLA₂-γ expression with its co-activator γMYB2

Phospholipase A₂ (PLA₂) hydrolyzes phospholipid molecules to produce two products that are both precursors of second messengers of signaling pathways and signaling molecules per se. Arabidopsis thaliana PLA₂ paralogs (-β, -γ and -δ) play critical roles during pollen development, pollen germination and tube growth. In this study, analysis of the PLA₂-γ promoter using a deletion series revealed that the promoter region -153 to -1 is crucial for its pollen specificity. Using a yeast one-hybrid screening assay with the PLA₂-γ promoter and an Arabidopsis transcription factor (TF)-only library, we isolated two novel MYB-like TFs belonging to the MYB-CC family, denoted here as γMYB1 and γMYB2. By electrophoretic mobility shift assay, we found that these two TFs bind directly to the P1BS (phosphate starvation response 1-binding sequence) cis-element of the PLA₂-γ promoter. γMYB1 alone functioned as a transcriptional activator for PLA₂-γ expression, whereas γMYB2 directly interacted with γMYB1 and enhanced its activation. Overexpression of γMYB1 in the mature pollen grain led to increased expression of not only the PLA₂-γ gene but also of several genes whose promoters contain the P1BS cis-element and which are involved in the Pi starvation response, phospholipid biosynthesis and sugar synthesis. Based on these results, we suggest that the TF γMYB1 binds to the P1BS cis-element, activates the expression of PLA₂-γ with the assistance of its co-activator, γMYB2, and regulates the expression of several target genes involved in many plant metabolic reactions.