Heat stress and sexual reproduction in maize: unveiling the most pivotal factors and the greatest opportunities

The escalation in the intensity, frequency, and duration of high temperature (HT) stress is currently unparalleled, which aggravates the challenges for crop production. Yet, the stage-dependent responses of sexual reproductive organs to HT stress at the morphological, physiological, and molecular levels, remain inadequately explored, particularly in pivotal staple crops. This review synthesized current knowledge regarding the mechanisms by which HT stress induces abnormalities and aberrations in reproductive growth and development, as well as alters the morphology and function of florets and their constituents, flowering patterns, and the processes of pollination and fertilization in maize (Zea mays L.). We identified the stage-specific sensitivities to HT stress through compiling and analyzing hundreds of lines of evidence, and accurately defined the sensitive period from days to hours timescale. The microspore tetrad phase of pollen development and anthesis (especially shortly after pollination) are most sensitive to HT stress, and even brief temperature spikes during these stages can lead to significant kernel loss. Unfortunately, these weak links are hidden and often neglected in practice. The impetuses behind the heat-induced impairments in seed set are closely related to carbon, reactive oxygen species, phytohormone signals, ion (e.g., Ca2+) homeostasis, plasma membrane structure and function, and others. At last, the recent advancements in understanding the genetic mechanisms underlying HT stress responses during maize sexual reproduction have been systematically summarized. This knowledge holds significant implications for the development of improved maize genotypes and effective crop management strategies to mitigate heat stress.