Proto Kranz-like leaf traits and cellular ionic regulation are associated with salinity tolerance in a halophytic wild rice

Species of wild rice (<i>Oryza</i> spp.) possess a wide range of stress tolerance traits that can be potentially utilized in breeding climate-resilient cultivated rice cultivars (<i>Oryza sativa</i>) 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 (<i>O. latifolia, O. officinalis</i> and <i>O. coarctata)</i> were found to possess superior salinity stress tolerance. The underlying mechanisms, however, were strikingly different. Na⁺ accumulation in leaves of <i>O. latifolia</i>, <i>O. officinalis</i> and <i>O. coarctata</i> were significantly higher than the tolerant landrace, Pokkali. Na⁺ accumulation in mesophyll cells was only observed in <i>O. coarctata</i>, suggesting that <i>O. officinalis</i> and <i>O. latifolia</i> avoid Na⁺ accumulation in mesophyll by allocating Na⁺ to other parts of the leaf. The finding also suggests that <i>O. coarctata</i> 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, <i>O. coarctata</i> 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., <i>NADPME</i>, <i>PPDK</i>) and was a clear outlier with respect to salinity tolerance among the studied wild and cultivated <i>Oryza</i> species. The unique phylogenetic relationship of <i>O. coarctata</i> with C₄ grasses suggests the potential of this species for breeding rice with high photosynthetic rate under salinity stress in the future.