Tomato (Lycopersicon esculentum) Response to Heat Stress: Focus on Pollen Grains

Exposure to higher than optimal temperatures, heat-stress (HS), reduces yield and decreases the quality of many crops, including tomato. Pollen development is one of the most heat-sensitive developmental stages in a large variety of plant species. Impaired fruit set in response to HS conditions may thus be largely due to HS-induced changes in pollen characteristics. This review emphasizes the effect of HS on the developing pollen grains, though the available data are still very limited. Recently accumulated data on the molecular control of pollen development are included, which indicate the processes important for the production of functional pollen grains. These recent studies are put in the context of data on HS effects on the other parts of the tomato plant (whole plant, leaves, anther cells) likely to affect the development and functioning of the gametes. Among other effects, HS was found to affect membrane integrity, Ca 2+ flux, protein conformation and production of reactive oxygen species (ROS) which, in turn, elicit defense mechanisms that include the production of heat shock factors, heat shock proteins and ROS-scavenging enzymes. In the developing pollen grain HS directly affects sugar and starch metabolism – effects that are associated with reduced pollen quality. In parallel, defense mechanisms are elicited, including the induction of heat shock proteins. The full understanding of the heat sensitivity of pollen, on the one hand, and of the mechanisms that may enable the pollen to cope with HS, on the other hand, requires additional data, including data on other components of the chaperone network, as well as data on other protective mechanisms (such as ROS scavengers and compatible solute accumulation).