Heat stress induced reactive oxygen species in maize silks are correlated with pollen tube growth arrest and sterility

The reproductive phase in flowering plants is highly sensitive to ambient temperature stresses, with even a single hot day sometimes being fatal to reproductive success. Many studies of heat stress on crop plants have shown that pollen development and fertilization belong to the most sensitive reproductive stages. Less attention has been paid to heat sensitivity of female reproductive organs. We applied moderate heat stress to study the sensitivity and contribution of female organs to reproduction under heat stress using maize as a model grass crop. We focused our studies on the elongated stigma tissue (silk), which is in direct contact with ambient environment at the silking stage. We show that moderate heat stress causes increased cell death of silk hair cells, but did not affect pollen germination, silk hair penetration and early pollen tube growth inside the silk. Instead late growth arrest of pollen tubes was observed ultimately leading to sterility. We further show that heat stress causes elevated levels of reactive oxygen species (ROS). Superoxide and H2O2 scavenger reduce heat stress induced ROS levels in silks and partly restore pollen tube growth defects. RNAseq studies of heat stressed silks show that a number of biological processes including hydrogen peroxide catabolic processes and bHLH transcription factor genes are downregulated, while NAC trancriptional factor genes are strongly upregulated. In conclusion, this study now provides a solid basis to select genes for eninering heat stress tolerant plants during the reproduction phase.