Desiccation tolerance in seeds and plants

The interest of research groups in desiccation tolerance (DT) has increased substantially over the last decades. The emergence of germinated orthodox seeds and resurrection plants as main research models has pushed the limits of our knowledge beyond boundaries. At the same time, new questions and new challenges were posed. The work presented in this thesis aims at shedding light on some of these questions, deepening our understanding of DT and providing relevant information to improve stress resistance in crops. Chapter 2 is a survey of the literature and discusses the ecological and evolutionary significance for seeds to be able to re-acquire DT after germination. This chapter also discusses recent progress in DT studies using developing and germinated seeds of the model plants Arabidopsis thaliana and Medicago truncatula. In Chapter 3 I used microarray data from a time series of DT re-acquisition, together with network analysis of gene expression, to gain temporal resolution and identify relevant genes involved in the re-acquisition of DT in germinated A. thaliana seeds by incubation in abscisic acid (ABA). Overall, genes related to protection, response to stresses, seed development and seed dormancy were up-regulated, whereas genes related to cell growth and photosynthesis were down-regulated with time. Genes that respond early to exogenous ABA were related to wax biosynthetic processes, lipid storage, seed development and response to ABA stimulus. Genes that respond late to exogenous ABA were related to syncytium formation and response to abiotic stimulus (mainly light stimulus). The robustness of the network was confirmed by the projection of sets of genes – related to the acquisition of DT, seed dormancy, drought responses of adult plants and re-induction of DT by polyethylene glycol – on this network. In Chapter 4 the relation between DT in germinated seeds and drought resistance in adult plants is analysed, using rice (Oryza sativa) as experimental model. Considering the predictions of a future with lower availability of fresh water, efforts to increase rice drought tolerance without reducing yield are increasingly important. The results presented in this chapter suggest that the intrinsic mechanisms of drought tolerance in adult plants are part of the mechanisms used by seeds to tolerate desiccation, but the molecular nature of these mechanisms remains elusive. Chapter 5 explores the relation between DT and longevity in germinated seeds of the Brazilian tree species Sesbania virgata as experimental model. DT and longevity are acquired by orthodox seeds during the maturation phase of development and lost upon germination. DT can be re-induced in germinated seeds by an osmotic and/or ABA treatment, but there is no information on how these treatments affect seed longevity. S. virgata seeds lose DT slowly upon radicle growth. The radicle appeared to be the most sensitive organ and the cotyledons the most resistant. The ability to produce lateral roots was key for whole seedling survival. An osmotic treatment improved DT in germinated S. virgata seeds, but not longevity. This implies that DT and seed longevity can be uncoupled. Xerophyta viscosa is one of the best studied resurrection species. Despite the fact that adult plants and mature seeds display DT, young X. viscosa seedlings are sensitive to fast drying. A treatment with ABA can induce DT early in shoots of these seedlings, but not in roots. Chapter 6 addresses the changes in the transcriptome and proteome of X. viscosa seedlings during induction of DT. A draft genome sequence of X. viscosa was used to improve transcript and protein identification and annotation. Differences in ABA signalling and the cross talk between ABA and ethylene were presented as determinant for shoot and root responses. Moreover, differences in the accumulation of late embryogenesis abundant proteins were also shown as being key for DT in shoots and roots. In Chapter 7, DT-transcriptomes of distantly related organisms are compared and surveyed for a core set of genes representing the signatures of critical adaptive DT mechanisms. A shortlist of 260 genes emerged, with a significant number of genes under the control of ABI3 and related to dormancy. The results reinforced the idea that core mechanisms and key regulators involved in DT developed early in the history of life and were carried forward by diverse species and life forms in a conserved manner and in conjunction with dormancy. In Chapter 8, the findings of this thesis are integrated, showing how they can contribute to future improvement of stress tolerance in crops. The ability of germinated seeds to re-acquire DT is discussed in combination with dormancy and longevity and related to seed survival under unfavourable environmental conditions. The relationship between drought- and desiccation tolerance and the role of ABA are presented briefly. Possible approaches to mine for new genes for crop improvement, such as searching for conserved genes and analysing new genome sequences, are addressed. Finally, a new perspective of the way to consider the evolution of DT is proposed.