A molecular physiological review of vegetative desiccation tolerance in the resurrection plant Xerophyta viscosa (Baker).

MAIN CONCLUSION: Provides a first comprehensive review of integrated physiological and molecular aspects of desiccation tolerance Xerophyta viscosa. A synopsis of biotechnological studies being undertaken to improve drought tolerance in maize is given. Xerophyta viscosa (Baker) is a monocotyledonous resurrection plant from the family Vellociacea that occurs in summer-rainfall areas of South Africa, Lesotho and Swaziland. It inhabits rocky terrain in exposed grasslands and frequently experiences periods of water deficit. Being a resurrection plant it tolerates the loss of 95% of total cellular water, regaining full metabolic competency within 3 days of rehydration. In this paper, we review some of the molecular and physiological adaptations that occur during various stages of dehydration of X. viscosa, these being functionally grouped into early and late responses, which might be relevant to the attainment of desiccation tolerance. During early drying (to 55% RWC) photosynthesis is shut down, there is increased presence and activity of housekeeping antioxidants and a redirection of metabolism to the increased formation of sucrose and raffinose family oligosaccharides. Other metabolic shifts suggest water replacement in vacuoles proposed to facilitate mechanical stabilization. Some regulatory processes observed include increased presence of a linker histone H1 variant, a Type 2C protein phosphatase, a calmodulin- and an ERD15-like protein. During the late stages of drying (to 10% RWC) there was increased expression of several proteins involved in signal transduction, and retroelements speculated to be instrumental in gene silencing. There was induction of antioxidants not typically found in desiccation-sensitive systems, classical stress-associated proteins (HSP and LEAs), proteins involved in structural stabilization and those associated with changes in various metabolite pools during drying. Metabolites accumulated in this stage are proposed, inter alia, to facilitate subcellular stabilization by vitrification process which can include glass- and ionic liquid formation.

Taxonomically restricted genes of Craterostigma plantagineum are modulated in their expression during dehydration and rehydration.

MAIN CONCLUSION: Taxonomically restricted genes are known to contribute to the evolution of new traits. In Craterostigma plantagineum two of such genes are modulated during dehydration and rehydration and seem to contribute to a successful recovery after desiccation. The resurrection plant Craterostigma plantagineum can tolerate extreme water loss. Protective molecules linked to desiccation tolerance were identified in C. plantagineum but underlying mechanisms are far from being completely understood. A transcriptome analysis revealed several genes which could not be annotated and are, therefore, interesting candidates for understanding desiccation tolerance. Genes which occur only in some species are defined as orphan or taxonomically restricted genes (TRGs) and may be important for the evolution of new traits. Several of these TRGs are modulated in expression during dehydration/rehydration in C. plantagineum. Here we report the characterisation of two of these TRGs encoding a cysteine-rich rehydration-responsive protein 1 (CpCRP1) and an early dehydration-responsive protein 1 (CpEDR1). The involvement of CpCRP1 and CpEDR1 in different phases of the dehydration/rehydration cycle is shown by transcript and protein expression analysis. In silico sequence analyses predicted that both genes are likely to interact with other cellular components and are localised in two different cellular compartments. GFP fusion proteins demonstrated that CpCRP1 is secreted into the apoplasm, whereas CpEDR1 is imported into chloroplasts. Putative homologs of CpCRP1 and CpEDR1 were identified in Lindernia brevidens and Lindernia subracemosa which belong to the same family as C. plantagineum thus suggesting a recent evolution of the genes in this family. According to expression profiles, CpCRP1 may play a role in normal conditions and during rehydration, whereas CpEDR1 may be required for the acquisition of desiccation tolerance and protect photosynthetic structures during dehydration and rehydration.