Abstract
Species of wild rice (Oryza spp.) possess a wide range of stress tolerance traits that can be potentially utilized in breeding climate-resilient cultivated rice cultivars (Oryza sativa) thereby aiding global food security. In this study, we conducted a greenhouse trial to evaluate the salinity tolerance of six wild rice species, one cultivated rice cultivar (IR64) and one landrace (Pokkali) using a range of electrophysiological, imaging, and whole-plant physiological techniques. Three wild species (O. latifolia, O. officinalis and O. coarctata) were found to possess superior salinity stress tolerance. The underlying mechanisms, however, were strikingly different. Na+ accumulation in leaves of O. latifolia, O. officinalis and O. coarctata were significantly higher than the tolerant landrace, Pokkali. Na+ accumulation in mesophyll cells was only observed in O. coarctata, suggesting that O. officinalis and O. latifolia avoid Na+ accumulation in mesophyll by allocating Na+ to other parts of the leaf. The finding also suggests that O. coarctata might be able to employ Na+ as osmolyte without affecting its growth. Further study of Na+ allocation in leaves will be helpful to understand the mechanisms of Na+ accumulation in these species. In addition, O. coarctata showed Proto Kranz-like leaf anatomy (enlarged bundle sheath cells and lower numbers of mesophyll cells), and higher expression of C4-related genes (e.g., NADPME, PPDK) and was a clear outlier with respect to salinity tolerance among the studied wild and cultivated Oryza species. The unique phylogenetic relationship of O. coarctata with C4 grasses suggests the potential of this species for breeding rice with high photosynthetic rate under salinity stress in the future.
Highlights
Salinity tolerance is a polygenetic trait that has evolved multiple times in diverse genera (Bromham et al 2020) due to various modifications in plant physiological and anatomical traits (Chen and Soltis 2020; Munns et al.2020a; Solis et al 2020)
Significant reductions in biomass due to salinity stress were found in O. longiglumis, IR64, O. australiensis, and O. rufipogon (P < 0.05; salinity-sensitive lines) (Supplementary Figs. 1) but not for O. latifolia, Pokkali, and O. officinalis and O. coarctata; O. coarctata even showed a small increase in biomass under salinity treatment
Investigating gene expression, ion homeostasis, Na+ transport between the vascular system and mesophyll tissue in salt-tolerant, wild rice species may enable the identification of new mechanisms that contribute to salt tolerance
Summary
Salinity tolerance is a polygenetic trait that has evolved multiple times in diverse genera (Bromham et al 2020) due to various modifications in plant physiological and anatomical traits (Chen and Soltis 2020; Munns et al.2020a; Solis et al 2020). Phylogenetic analysis shows that salt-tolerant lineages exhibit a ‘tippy’ pattern (occurring on the tips of the phylogeny rather than internally) (Flowers et al 2010; Bromham et al 2020) This may suggest a potential loss or gain of salt-tolerant traits in different species during their evolutionary and ecological adaptation to saline conditions (Bromham et al 2020; Chen and Soltis 2020; Caperta et al 2020). Attempts to increase salinity tolerance have mostly focused on traits found in salt-tolerant genotypes such as Pokkali, Nona Bokra and FL468, that have poor reproductive performance resulting in low yields Salinity tolerance in these lines is mainly achieved by restricting Na+ accumulation in aboveground tissues and by maintaining higher K+ contents (Lutts et al 1996b, a; Prusty et al 2018; Gerona et al 2019). Development of salinity tolerant lines using these landraces has produced plants that have poor reproductive traits (Solis et al 2020), suggesting a negative trade-off between salt sensitivity and yield
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