Genome-wide analysis of pectate-induced gene expression in Botrytis cinerea: identification and functional analysis of putative d-galacturonate transporters

The fungal plant pathogen Botrytis cinerea produces a spectrum of cell wall degrading enzymes for the decomposition of host cell wall polysaccharides and the consumption of the monosaccharides that are released. Especially pectin is an abundant cell wall component, and the decomposition of pectin by B. cinerea has been extensively studied. An effective concerted action of the appropriate pectin depolymerising enzymes, monosaccharide transporters and catabolic enzymes is important for complete d-galacturonic acid utilization by B. cinerea. In this study, we performed RNA sequencing to compare genome-wide transcriptional profiles between B. cinerea cultures grown in media containing pectate or glucose as sole carbon source. Transcript levels of 32 genes that are induced by pectate were further examined in cultures grown on six different monosaccharides, by means of quantitative RT-PCR, leading to the identification of 8 genes that are exclusively induced by d-galacturonic acid. Among these, the hexose transporter encoding genes Bchxt15 and Bchxt19 were functionally characterised. The subcellular location was studied of BcHXT15-GFP and BcHXT19-GFP fusion proteins expressed under control of their native promoter, in a B. cinerea wild-type strain. Both genes are expressed during growth on d-galacturonic acid and the fusion proteins are localized in plasma membranes and intracellular vesicles. Target gene knockout analysis revealed that BcHXT15 contributes to d-galacturonic acid uptake at pH 5~5.6. The virulence of all B. cinerea hexose transporter mutants tested was unaltered on tomato and Nicotiana benthamiana leaves.