Expansion mechanisms and functional divergence of the glutathione S-transferase family in Sorghum and other higher plants

Glutathione S-transferases (GSTs) exist in various eukaryotes and function in detoxification of xeno-biotics and in response to abiotic and biotic stresses. We have carried out a genome-wide survey of this gene family in 10 plant genomes. Our data show that tandem duplication has been regarded as the major expansion mechanism and both monocot and dicot plants may have practiced different expansion and evolutionary history. Non-synonymous substitutions per site (Ka) and synonymous substitutions per site (Ks) analyses showed that N- and C-terminal functional domains of GSTs (GST_N and GST_C) seem to have evolved under a strong purifying selection (Ka/Ks < 1) under different selective pressures. Differential evolutionary rates between GST_N and GST_C and high degree of expression divergence have been regarded as the major drivers for the retention of duplicated genes and the adaptability to various stresses. Expression profiling also indicated that the gene family plays a role not only in stress-related biological processes but also in the sugar-signalling pathway. Our survey provides additional annotation of the plant GST gene family and advance the understanding of plant GSTs in lineage-specific expansion and species diversification. Key words: abiotic stress; biotic stress; comparative genomics; functional divergence; glutathione transferase 1