The control of endopolygalacturonase expression by the sugarcane RAV transcription factor during aerenchyma formation

The development of lysigenous aerenchyma starts with cell expansion and degradation of pectin from the middle lamella, leading to cell wall modification, and culminating with cell separation. Here we report that nutritional starvation of sugarcane induced gene expression along sections of the first 5 cm of the root and between treatments. We selected two candidate genes: a RAV transcription factor, from the ethylene response factors superfamily, and an endopolygalacturonase (EPG), a glycosyl hydrolase related to homogalacturonan hydrolysis from the middle lamella. epg1 and rav1 transcriptional patterns suggest they are essential genes at the initial steps of pectin degradation during aerenchyma development in sugarcane. Due to the high complexity of the sugarcane genome, rav1 and epg1 were sequenced from 17 bacterial artificial chromosome clones containing hom(e)ologous genomic regions, and the sequences were compared with those of Sorghum bicolor. We used one hom(e)olog sequence from each gene for transactivation assays in tobacco. rav1 was shown to bind to the epg1 promoter, repressing β-glucuronidase activity. RAV repression upon epg1 transcription is the first reported link between ethylene regulation and pectin hydrolysis during aerenchyma formation. Our findings may help to elucidate cell wall degradation in sugarcane and therefore contribute to second-generation bioethanol production.