Decoding heat-induced chalkiness in rice: molecular mechanisms, genetic networks, and mitigation strategies for climate resilience.

Chalkiness is a critical trait that negatively affects rice grain appearance, milling quality, cooking properties, and consumer acceptance, and its incidence is increasing under climate change as elevated temperatures during grain filling become more frequent. As a complex quantitative trait shaped by strong genotype × environment interactions, heat-induced chalkiness arises from coordinated disruption of endosperm development, storage-substance metabolism, and stress-response pathways. This review synthesizes current understanding of the molecular, physiological, and genetic mechanisms underlying heat-induced chalkiness in rice. We distinguish constitutive regulators that govern grain development under all conditions from heat-responsive genes and pathways specifically activated under thermal stress, thereby providing a conceptual framework for temperature-dependent grain quality deterioration. We examine temperature-sensitive developmental windows, source-sink coordination, and heat-mediated disruption of starch, storage-protein, and lipid metabolism, with emphasis on the critical grain-filling stage during which elevated temperatures irreversibly impair endosperm structure. We integrate recent advances in heat sensing and signalling, including phytohormone, calcium, reactive oxygen species, endoplasmic reticulum stress, and membrane-lipid remodelling pathways and show how these signals converge through multilayered regulatory networks involving epigenetic, transcriptional, post-transcriptional, and protein-quality control. Finally, we discuss varietal variation in heat tolerance, the nutritional and post-harvest implications of chalkiness including its valorization across brewing, food-processing, and bio-economy markets, and emerging mitigation strategies spanning marker-assisted selection, genome editing, genomic selection, and optimized agronomic management. By integrating mechanistic insights with translational applications, this review provides a systems-level framework for developing climate-resilient rice varieties capable of maintaining grain quality in a warming climate.