Molecular and physiological hallmarks of seed longevity in crops and crop wild relatives

Conserving the genetic richness embedded in plant genetic resources is of paramount importance for the selection of the high-yielding, improved varieties of the future. In the context of climate change, biodiversity conservation becomes a tool not only for crop improvement in breeding programmes but also in reforestation and habitat restoration actions. Ex situ conservation within germplasm banks is considered one of the most effective and convenient strategies for ensuring preservation and availability of this genetic richness. In particular, seeds can be stored in relatively small spaces and with low economic efforts, and their viability can be preserved for the long-term, at the same time providing a good sample of the genetic diversity within the conserved taxon and gene pool. It follows that the study of seed longevity, i.e., the ability of seeds to remain viable over time, allows the optimization of conservation techniques and viability monitoring, e.g., clarify differences among different species or accessions of the same species. Therefore, reliable artificial ageing techniques, as well as molecular and physiological markers associated to seed longevity can be used in quality control, to find materials more vulnerable to seed ageing and that therefore need more frequent monitoring and regeneration/recollection. The aim of this thesis was to study the different eco-physiological and molecular aspects of seed longevity, still poorly explored, in crops and wild relatives. The investigation focused on the following crop species: maize, garden pea (Pisum sativum L.) and wheat wild relatives, analysed in the context of seed banking. The different experimental systems were selected in order to obtain a wider picture of some common physiological processes involved in shaping seed longevity, i.e., oxidative stress in terms of antioxidant activity, ROS accumulation, oxidative damages and protective mechanisms. The different seed accessions were studied with a multidisciplinary approach, from the analyses of germination profiles and seed phenotype to the exploration of nuclear/nucleolar ultrastructure and gene expression. The obtained results enlarged the current knowledge about the ageing behaviour and mechanisms under genebank conditions, underlining the effectiveness of cold storage in preserving PGRs for the long term in all the three projects, involving material conserved in two of the world’s major seed banks (IPK and CIMMYT). In the maize and wheat wild relatives’ projects, it emerged the need of applying different viability monitoring intervals based on the accessions’ characteristics (i.e., grain type in maize and morph in wheat wild relatives). The accuracy of artificial ageing as a predictive tool for longevity rankings was questioned by the results obtained comparing ageing in cold storage and artificial ageing in wheat wild relatives, adding novel information to the emerging literature on the topic. These results emphasize the current need for new ageing methods (alternative to cold storage and AA) and/or molecular and physiological hallmarks for fast and accurate predictions of seed lifespan and rankings in storage. Moreover, the biochemical, molecular and structural hallmarks used in this thesis improved the characterization of the seed longevity of the considered accessions in both the wheat wild relatives and the garden pea. Features related to the oxidative stress status and the antioxidant response correlated with the germination capability after storage, further strengthening the prominent role of ROS and ROS buffering in shaping seed longevity. In this work we tested approaches still poorly used in this research field, such as the measurement of alternative ageing hallmarks (e.g. proline, reducing sugars) and the ultrastructural analysis of chromatin compaction and genome integrity. Indeed, multidisciplinary approaches are fundamental in the characterization of such a multi-faceted biological process.