Pollen development in cotton (Gossypium hirsutum) is highly sensitive to heat exposure during the tetrad stage

The development of gametes in plants is acutely susceptible to heatwaves as brief as a few days, adversely affecting pollen maturation and reproductive success. Pollen in cotton (Gossypium hirsutum) was differentially affected when tetrad and binucleate stages were exposed to heat, revealing new insights into the interaction between heat and pollen development. Squares were tagged and exposed to 36/25°C (day/night, moderate heat) or 40/30°C (day/night, extreme heat) for 5 days. Mature pollen grains and leaves were collected for physiological and proteomic responses. While photosynthetic competence was not compromised even at 40°C, leaf tissues became leakier. In contrast, pollen grains were markedly smaller after the tetrad stage was exposed to 40°C and boll production was reduced by 65%. Sugar levels in pollen grains were elevated after exposure to heat, eliminating carbohydrate deficits as a likely cause of poor reproductive capacity. Proteomic analysis of pure pollen samples revealed a particularly high abundance of 70‐kDa heat shock (Hsp70s) and cytoskeletal proteins. While short‐term bursts of heat had a minor impact on leaves, male gametophyte development was profoundly damaged. Cotton acclimates to maxima of 36°C at both the vegetative and reproductive stages but 5‐days exposure to 40°C significantly impairs reproductive development. Pollen function is notoriously sensitive to heat stress, imposing a risk to all food and fiber crops. Maximum temperatures of 40°C are increasingly common and yet the mechanisms by which heat damages pollen remain largely unknown. We show that five days at 40°C during tetrad formation when meiosis is occurring led to failed dehiscence, smaller pollen grains and dramatic yield loss. Photosynthetic efficiency was not significantly compromised at 36 or even 40°C. Soluble sugar levels increased in pollen under heat stress, especially during early development, discounting carbohydrate deficits as a likely cause of impaired function. Investigation of the proteome of pollen after transient heat stress at either the tetrad or binucleate stage showed that heat shock proteins (HSPs), particularly HSP70s, were highly up‐regulated when heat was imposed after meiosis but fewer HSPs were seen after tetrads were heat‐treated. This suggested a direct role for HSPs in protein folding and heat tolerance. By contrast, cytoskeletal components were highly expressed when moderate heat was imposed on tetrads, indicating a specific role in sub