Desiccation sensitive seeds: understanding their evolution, genetics and physiology

Desiccation tolerance is a mechanism that allows plant species to avoid extinction by spreading their genetic information through time in a robust and safe vault: their seeds. However, species inhabiting long-stable environments in which the conditions are conducive for immediate germination may produce desiccation sensitive (DS) seeds. In this thesis I was committed to study the relatively unexplored physiology and genetics of these species. First, in Chapter 2 the literature was  reviewed concerning the physiology, evolution and molecular aspects of seed desiccation sensitivity.

Based on meta-analysis of available data on the seed’s lowest safe water content for diverse species, I revealed two remarkable limits for dehydration survival, which sheds light on the controversial separation of the categories ‘recalcitrant’ and ‘intermediate’ seeds. In this chapter, I also assembled the available knowledge to draw a generalized picture of the evolution of DS-seeded species, placing it in an ecological context and zooming in on the molecular mechanisms underlying it.

I decided to look further into the molecular mechanisms underlying this evolutionary hypothesis. Noticing a gap of knowledge on transcriptomes of seed development and maturation in relation to desiccation tolerance, I decided to use the Arabidopsis abi3-6 mutant as a model for seed desiccation sensitivity. In Chapter 3, it was shown that abi3-6 seeds had increased transcriptional activity that does not fit with a suppression of metabolic activity, generally observed in DT seeds. A clear association was found of its desiccation sensitivity with the absence of expression of genes responsible for dormancy and germination arrest.

In the next step, I asked whether the gene expression patterns during seed development of the Arabidopsis abi3-6 mutant and other model systems could also be observed in wild DS-seeded species. For this, I explored the genome and transcriptome of C. australe DS-seed development in comparison with other DS- and DT-seeded species (Chapter 4). I observed a lack of gene expression related to protection from desiccation damage and to dormancy during late seed development. In their genomes, specific relevant alterations were found in C. australe and other DS-seeded species associated with the regulation of seed development and control of germination that can lead to seed desiccation sensitivity.

Finally, in Chapter 5 I describe a protocol for suppression of gibberellin signalling by paclobutrazol for induction of desiccation tolerance in DS seeds of Citrus limon and studied the associated transcriptional changes. The acquisition of desiccation tolerance appeared to be mainly caused by the suppression of metabolism as a consequence of down-regulation of genes related to multiple hormonal pathways, except ABA.

Despite the large variation for seed desiccation sensitivity in nature, common physiological and molecular alterations have been revealed. With this we set the basis for further studies of the underlying mechanisms of seed desiccation sensitivity that may help to develop effective conservation strategies.