Rice Genomics and Genetics 2024, Vol.15, No.3, 142-152 http://cropscipublisher.com/index.php/rgg 148 2020). For biofortified rice varieties to be promoted and used, they should have a zinc content of≥35 mg/kg without yield loss and have ideal cooking quality. However, this combination rarely occurs in germplasm, thus requiring conscious breeding efforts to develop biofortified rice varieties with desirable attributes (Figure 1). Salt-alkali stress can have a significant impact on the growth and development of rice. High salt concentrations lead to high permeability and ionic imbalance in the rhizosphere environment. Plants absorb a large amount of Na+, which enters the outer cells of the roots from the soil solution, then enters the root xylem, and is finally transported to the leaves through the stems (Munns and Tester, 2008). The large accumulation of Na+ will inhibit the absorption of other nutritional elements such as K+, leading to ion toxicity (Assaha et al., 2017). Low external water potential causes osmotic stress in root cells, prompting cells to accumulate compatible solutes to reduce their water potential, thereby maintaining cell volume and turgor pressure. This process also helps to keep stomata open, increase CO2 concentration, and alleviate the inhibition of photosynthesis (Türkan and Demiral, 2009). However, salt-alkali stress will destroy this balance, leading to the accumulation of reactive oxygen species (ROS) in plants, damaging the biofilm system, and ultimately causing plant wilting. In the study of Ganapati et al. (2022), the ion regulation and signaling mechanisms of rice seedlings under salt-alkali stress were demonstrated. Na+ is absorbed from the roots into the plant through specific channels and transporters, and then transported to different tissues through xylem and phloem. In this process, OsSOS1 excludes Na+ through Na+/H+ anti-transport on the plasma membrane to maintain a lower cellular Na+/K+ ratio, thereby improving salt tolerance. OsHKT1,5 regulates Na+ transport in roots and maintains Na+/K+ balance, which is a key factor in salt-alkali tolerance. Other transporters such as OsHKT2,1, OsHKT2,3a, OsHKT1,4, OsHAK1, OsAKT1, OsKAT1 and OsGORK work together in different parts and organelles to regulate the balance of Na+ andK+ and ensure the normal function of cells. Calcium signaling plays a key role in response to salt-alkali stress. Ca2 +-binding proteins adapt to environmental changes by regulating plasma membrane H+-ATPase and other ion channels. The SOS signaling pathway, including SOS2 and SOS3 proteins, is involved in regulating Na+ emission and calcium signaling. In addition, salt-alkali stress causes the accumulation of reactive oxygen species (ROS) and damages the cell membrane system. Plants use a series of antioxidant mechanisms to alleviate the toxic effects of ROS and maintain cell homeostasis (Figure 2) (Ganapati et al., 2022). The ion homeostasis regulation mechanism of rice under salt-alkali stress is a complex process involving multiple ion channels, transporters and signaling pathways. Research on these mechanisms not only helps to reveal the basic physiological mechanisms of plants in response to stress, but also provides important theoretical foundations and breeding strategies for the future development of salt-alkali tolerant crops. By delving deeper into these mechanisms, we can better understand how plants survive in saline environments and develop more resistant and productive crop varieties. 6 Challenges and Future Directions Balancing nutrient content and yield in rice presents a significant challenge due to the genetic complexity involved. The primary obstacle lies in the fact that many genes regulating nutrient content and yield are often intertwined and may exhibit pleiotropic effects, where a single gene influences multiple traits. Enhancing nutrient content, such as increasing zinc or iron levels, can sometimes inadvertently affect yield negatively. This phenomenon, known as the yield-nutrient trade-off, poses a critical hurdle for breeders. Therefore, it is essential to develop strategies that can simultaneously optimize both traits without compromising one for the other. Managing genetic trade-offs requires a nuanced understanding of the underlying genetic mechanisms and their interactions. Advanced molecular techniques, such as genome-wide association studies (GWAS) and quantitative trait loci (QTL) mapping, can help identify key genetic regions associated with both high nutrient content and high yield. Additionally, the integration of CRISPR/Cas9 gene-editing technology provides a precise tool to manipulate specific genes, potentially mitigating negative trade-offs. By targeting and modifying specific alleles, breeders can enhance desired traits while minimizing adverse effects on yield.
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