Sorghum bicolor (L.) Moench for the enhancement of marginal lands, identification of salt-tolerant hybrids and evaluation of the role of brassinosteroids in salt stress response

Marginal lands cover 2.7 billion ha worldwide, and salinity is one of the main factors contributing to soil degradation. Salinization phenomenon consists of an excessive accumulation of water-soluble salts in the rhizosphere. The release of Na+ and Cl-ions is the major problem for crop survival, since they cause ionic toxicity, induce osmotic stress, altering the absorption of water/nutrients, and increasing reactive oxygen species (ROS) formation. In this harsher environment it is required to find an alternative to traditional European crops. Sorghum bicolor is an important agronomic species which, thanks to its anatomical and physiological adaptations and its C4 metabolism, can remain productive under conditions of low water availability and high salinity. However, few differences in salttolerance among genotypes were reported. Therefore, in this Thesis I wanted to identify markers of tolerance/sensitivity that could serve as a guide for breeders. To this purpose, the effects of salt stress on different grain sorghum commercial hybrids were analysed during their earliest developmental stages. The results showed that modifications in root and leaf anatomy, differences in ROS scavenging activity and changes in the metabolome may be used as selective markers salt tolerance in sorghum. The second aim of this Thesis was to identify an environmentally friendly strategy to mitigate the effects of salt stress in this species, particularly in sensitive genotypes. Brassinosteroids (BRs) were selected, since they can activate the antioxidant system, alleviating oxidative damages caused by stress, improve chlorophyll content and photosynthetic parameters, and modulate stress related-genes expression. BRs can be exogenously applied via seed priming. Here, the effectiveness of 1µM 24-epibrassinolide seed priming in mitigating salt-induced damage on the leaves of in vivo cultured plants was demonstrated. In fact, the treatment improved the photosynthetic efficiency of the sensitive genotype and restored its leaf anatomy, thereby enhancing the management of water resources. Among the main mechanisms of salt stress tolerance there are compartmentalization and sequestration of Na+ ions via vacuolar NHX transporters and plasma membrane HKT transporters. Thus, the third aim was to verify which known salt tolerance mechanisms were positively influenced by BRs. The results demonstrated that the BRs priming enhances the expression of the SbNHX2, positively modulate the root system architecture in stress conditions, and reduce oxidative stress preventing membrane lipid peroxidation. Collectively, the results of the Thesis demonstrated that the seed priming with brassinosteroids is an effective and agronomically promising method to help Sorghum bicolor cope with salt stress.