Molecular Plant Breeding 2025, Vol.16, No.3, 165-179 http://genbreedpublisher.com/index.php/mpb 166 transgenic approaches have shown promise in enhancing drought tolerance by overexpressing specific genes or silencing negative regulators (Hu and Xiong, 2014; Pant et al., 2022). This review summarizes the molecular mechanisms underlying drought resistance in rice and explores their potential applications in breeding programs. Specifically, it seeks to identify and characterize key drought-responsive genes and their regulatory networks, investigate the physiological and biochemical pathways involved in drought tolerance and assess the effectiveness of genetic engineering and MAS in developing drought-resistant rice varieties. Additionally, the review will provide insights into integrating molecular and conventional breeding approaches to enhance drought resistance in rice. Ultimately, this review aims to support the development of rice varieties that can withstand drought stress, thereby ensuring stable rice production and food security in the face of increasing environmental challenges. 2 Drought Stress in Rice: Physiological and Molecular Responses 2.1 Impact of drought stress on rice growth and yield Drought stress is a major abiotic factor that severely affects rice production, leading to significant reductions in growth and yield. Under drought conditions, rice plants experience reduced water availability, which directly impacts various physiological processes, such as photosynthesis, transpiration, and nutrient uptake. The severity of drought stress can cause stunted growth, delayed flowering, and reduced grain filling, ultimately leading to lower grain yield and quality (Lafitte et al., 2006). During the vegetative stage, drought reduces plant height, biomass, and tiller numbers, and causes leaf rolling in rice. The stress results from reduced soil moisture, which limits nutrient absorption and inhibits cell division in meristem tissues. At the tillering stage, water stress significantly impacts growth by hindering food production. Drought during flowering is especially damaging, disrupting pollination, inducing flower abortion, and leading to higher rates of unfilled grains. Prolonged moisture stress during the panicle initiation stage can reduce rice yields by 53.7%~63.5%, a major loss for farmers. Drought also hampers grain development during the reproductive stage, leading to spikelet infertility, decreased tillering capacity, and reduced photosynthesis due to leaf shrinkage. Water stress during the grain-filling stage accelerates plant senescence, shortens the grain-filling period, and reduces yield (Patnaik et al., 2021). Studies indicate that drought conditions during the critical flowering and grain-filling periods can reduce rice yields by up to 50%. Drought stress also affects the root system, reducing root length and density, which further impairs the plant’s ability to absorb water and nutrients from the soil (Serraj et al., 2011). 2.2 Physiological responses to drought stress Rice plants have developed a range of physiological responses to cope with drought stress. Chlorophyll is a key element in the photosynthesis of green plants and is positively correlated with the photosynthetic rate. Under drought stress, chlorophyll pigments can degrade and oxidize, which are common indicators of oxidative stress, resulting in a reduction in chlorophyll content. Both chlorophyll a and chlorophyll b are impacted by drought conditions (Islam et al., 2021). One of the primary responses is the closure of stomata to reduce water loss through transpiration. This stomatal closure, however, also limits CO2 uptake, thereby reducing photosynthetic efficiency. Drought stress decreases the efficiency of photosystem II, impairing energy conversion in the chloroplasts (Flexas et al., 2006). Osmotic adjustment is another critical response, where plants accumulate compatible solutes such as proline, glycine betaine, and sugars to maintain cell turgor and protect cellular structures (Serraj and Sinclair, 2002). Relative water content (RWC) is an important indicator of a plant’s water status, reflecting the metabolic activity within tissues and playing a crucial role in evaluating dehydration tolerance. RWC is associated with both water absorption by roots and water loss through transpiration. Many studies have reported a decrease in RWC in response to drought stress in various plant species (Nayyar et al., 2006). Additionally, rice plants enhance the expression of antioxidant enzymes like superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) to mitigate oxidative damage caused by reactive oxygen species (ROS) generated during drought stress (Mittler, 2002). Drought stress often leads to an increase in root-to-shoot ratio as plants invest more in root growth to enhance water uptake. Deeper and more extensive root systems help rice plants access water from deeper soil layers, improving drought tolerance (Henry et al., 2011).
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