Advancing Thermotolerance in Beans: Integrating Physiology, Genomics, and Breeding for Climate Adaptation

Abstract

Rising global temperatures increasingly threaten bean production and the food security of millions of people who depend on these protein-rich legumes. Heat stress during the reproductive stages induces pollen sterility, flower abscission, and disruptions in source–sink relationships, resulting in yield losses of 30%–40% or more when critical temperature thresholds are surpassed. At the molecular level, thermotolerance involves coordinated responses mediated by heat shock proteins, transcription factors, antioxidant systems, and osmotic adjustment mechanisms, with substantial genetic variation both within and among species. Bean species within the genera Phaseolus, Vigna, and Macrotyloma exhibit remarkable interspecific differences in thermotolerance, ranging from the heat-sensitive common bean (Phaseolus vulgaris) to the moderately tolerant adzuki bean (V. angularis), mungbean (Vigna radiata), urdbean (Vigna mungo), cowpea (V. unguiculata), and highly thermotolerant species such as horse gram (M. uniflorum), moth bean (V. aconitifolia), and tepary bean (P. acutifolius). Recent genomic advances have enabled the identification of quantitative trait loci and candidate genes underlying heat tolerance, with complete genome sequences now available for most of these crops. Breeding strategies increasingly exploit interspecific hybridization, with germplasm from tepary bean, moth bean, and horse gram providing valuable thermotolerance alleles for incorporation through pre-breeding programs. Characterized genes, such as horse gram MuHSP70, represent promising targets for marker-assisted selection and genome editing. Symbiotic nitrogen fixation is particularly vulnerable to heat stress, although stress-adapted legumes can maintain nodulation under adverse conditions. This review synthesizes current knowledge of the physiological mechanisms, genomic resources, breeding strategies, and agronomic management approaches for improving thermotolerance in beans, with particular emphasis on harnessing genetic diversity from underutilized and opportunistic thermotolerant species.