Patterns of gene expression in pollen of cotton (Gossypium hirsutum) indicate down-regulation as a feature of thermotolerance.

Reproductive performance in plants is impaired as maximum temperatures consistently approach 40°C. However, the timing of heatwaves critically affects their impact. We studied the molecular responses during pollen maturation in cotton to investigate the vulnerability to high temperature. Tetrads, uninucleate and binucleate microspores, and mature pollen were subjected to SWATH-MS and RNA-seq analyses after exposure to 38/28°C (day/night) for 5 days. The results indicated that molecular signatures were down-regulated progressively in response to heat during pollen development. This was even more evident in leaves, where three-quarters of differentially changed proteins decreased in abundance during heat. Functional analysis showed that translation of genes increased in tetrads after exposure to heat; however, the reverse pattern was observed in mature pollen and leaves. For example, proteins involved in transport were highly abundant in tetrads whereas in later stages of pollen formation and leaves, heat suppressed synthesis of proteins involved in cell-to-cell communication. Moreover, a large number of heat shock proteins (HSPs) were identified in heat-affected tetrads but these proteins were less abundant in mature pollen and leaves. We speculate that the sensitivity of tetrad cells to heat is related to high rates of translation targeted to pathways that might not be essential for thermotolerance. Molecular signatures during stages of pollen development after heatwaves could provide markers for future genetic improvement.