FARRANT, JM; SEOIGHE, C; ILLING, N; University of Cape Town: Evolution of desiccation tolerance in vegetative tissues of angiosperms
Evolution of desiccation tolerant (DT) pollen and seeds was an adaptation enabling successful land invasion by spermatophytes. Subsequently, DT in vegetative tissue of angiosperms (resurrection plants) is believed to have evolved independently on several occasions. We propose two stages to DT. Initial water loss involves activation of a reversible generic stress response that also occurs in desiccation sensitive tissues in response to mild drying. Further water loss in resurrection plants results in activation of unique desiccation-specific responses that are adapted from the response of DT-seeds. Our data suggests that the initial response includes activation of generic antioxidants and enzymes (ascorbate,glutathione, SOD, GR, CAT, APX), some LEAs and some compatible solute accumulation. The desiccation response results in i) activation of a specialist suite of genes for maintenance of cellular integrity in desiccated cells, and includes expression of unusual antioxidants (peroxiredoxins, glyoxylase I and metallothionein type 4), desiccation-specific LEAs, and further compatible solute accumulation, and ii) transcription of genes encoding repair mechanisms. In seeds, DT is developmentally regulated with putative mechanisms of tolerance being accumulated at precise times after fertilisation. In contrast drought is stochastic and vegetative tissues must respond to environmental signals to activate protective mechanisms. Nonetheless, we propose that DT in vegetative tissue is largely an adaptation of the developmentally regulated mechanisms of seeds, and that environmental signals activate an existing repertoire of genes in vegetative tissue, rather than due to the induction of proteins with new properties. We tested this hypothesis by comparing 1) physiological mechanisms of DT in various resurrection plants and seeds and 2) gene expression profiles of seed and vegetative tissues in response to mild and extreme desiccation.
