Flowering Under Heat: Linking Phenological Adaptation, Reproductive Resilience, and Yield Stability in Plants
The reproductive stage is a critical time in a plant’s life history when dealing with heat stress as the specific processes of meiosis, gametogenesis, anthesis, pollination, fertilization, and early seed development all occur within comparatively narrow temperature limits. Much of the importance of flowering is tied directly to temperature, both for the timing of reproductive transition and the ability for male and female reproductive tissues to survive exposure to damaging heat. Ambient-temperature sensing is linked to flowering via regulatory modules that involve phytochrome B, EARLY FLOWERING 3 (ELF3), PHYTOCHROME INTERACTING FACTOR 4 (PIF4), FLOWERING LOCUS T (FT), FLOWERING LOCUS M (FLM), SHORT VEGETATIVE PHASE (SVP), and the light–circadian components. Conversely, when temperature is harmful, protective responses involve other cellular mechanisms such as activation of heat-shock transcription factors (HSFs), heat-shock proteins (HSPs), endoplasmic-reticulum protein quality control, calcium and reactive oxygen species (ROS) signaling, antioxidant systems, hormone regulation, metabolic reprogramming, autophagy and DNA-repair pathways. Male reproductive development is often very sensitive, especially at meiosis, during formation of the tetrad, microspore development, during the maturation of pollen, and during the growth of the pollen tubes, although injury to pistils, ovules and the post-fertilization tissues may solely have an effect on the restriction of fertilization and seed set. Phenological heat escape and intrinsic reproductive thermotolerance are genetically separable but can be complementary aspects of adaptation, as revealed by natural allelic variation, QTL mapping, genomic prediction and marker assisted selection. This review summarizes molecular, genetic and physiological evidence, to propose that, to ensure stable yields at high temperatures, there is a need to coordinate optimization of reproductive timing, cell and development thermotolerance, and post-fertilization sink stability.