Towards transcript profiling of desiccation tolerance in Xerophyta humilis:: Construction of a normalized 11 k X. humilis cDNA set and microarray expression analysis of 424 cDNAs in response to dehydration

Xerophyta humilis (Bak.) Dur and Schinz is an indigenous Southern African resurrection plant which is able to protect itself from the stresses associated with extreme dehydration. For the purpose of analysing global patterns of gene expression in response to desiccation and recovery on rehydration, we have generated a normalized 10 900 library, representing genes from root and leaf tissue that are expressed during the dehydration-rehydration cycle. For the small-scale microarray analysis described here, 424 cDNAs were sequenced, annotated, arrayed and hybridized with hydrated and dehydrated, leaf-specific RNA. Reverse Northern blots were used as an alternative method to compare the expression results. A total of 55 dehydration-inducible cDNAs were identified combining the results of both methods. Northern blot analysis of 14 of the 55 the dehydration-upregulated cDNAs verified the expression status of all 14 genes. Dehydration-upregulated cDNAs included those homologous to known dehydration stress-responsive genes encoding metallothioneins, galactinol synthases, an aldose reductase and a glyoxalase. A large number of genes encoding late embryonic abundant proteins (LEAs), dehydrins and desiccation-related proteins were also identified, suggesting that proteins that provide mechanical and antioxidant protection against water loss dominate the mRNA population in desiccated X. humilis leaf tissue. Dehydration-upregulated genes identified in this study, and not previously implicated in the dehydration response, include cDNAs encoding a putative chloroplast RNA-binding protein and a protein containing SNF2/helicase domains. Comparisons with microarray data, which profile the dehydration response in desiccation-sensitive plants, reveal differences in expression patterns between X. humilis and Arabidopsis and rice that could provide clues as to the mechanisms underlying desiccation-tolerant and -sensitive phenotypes. The X. humilis library promises to be a useful resource for transcript profiling the dehydration and rehydration response in a desiccation-tolerant plant.