Fried green tomatoes: Transgenic tomatoes reveal critical component of thermotolerance
Heat poses a danger to living organisms because it can denature, or unfold, the proteins and enzymes that carry out normal biological processes, thereby severely compromising protein function, and, in turn, cell viability. Organisms ranging from bacteria to humans have evolved a common biological response to dramatic increases in ambient temperature--called the heat stress response. The heat stress response entails the rapid production of heat stress proteins, which act as molecular chaperones to ensure that newly formed proteins are folded properly and misfolded proteins are removed from the cell.
Unlike animals, plants cannot relocate to escape the heat. The heat stress response is therefore essential for their survival during periods of high temperatures. Genome sequencing has revealed that plants have a more complex heat stress response components than animals, and scientists theorize that perhaps plant's sessile nature has necessitated the evolution of more sophisticated heat stress response machinery.
Although the tomato genome has not been fully sequenced, Dr. Lutz Nover and colleagues have already identified a protein called HsfA1 as a master regulator of the tomato heat stress response. HsfA1 is a heat stress transcription factor that activates the expression of genes that encode heat stress proteins. Dr. Nover and colleagues genetically engineered tomato plants to be either deficient in HsfA1, or to over-express HsfA1 at higher than normal levels. Both types of transgenic plants grew normally under ordinary conditions, but under heat stress, the importance of HsfA1 was readily apparent.
When HsfA1-deficient plant were exposed to high temperatures, the plants were unable to initiate the heat stress response and therefore died. Similarly, HsfA1-deficient fruit were unable to ripen at high temperatures. On the other hand, plants that over-express HsfA1 were actually more resistant to heat than unmodified plants.
Dr. Nover and colleagues have demonstrated that although each of the 17 tomato heat stress transcription factors identified thus far, may have a specific function, HsfA1 is the most important, playing an dominant role in the ability of tomatoes to respond to high temperatures. Further delineation of the tomato heat stress response will be of undoubted interest to agricultural biotechnologists as they look for ways to manipulate tomato growth and ripening to better suit the population's needs.