These gene variants will help protect against heat-stress-induced yield loss in corn and should help maintain productivity as climate change results in more frequent and more severe heat-stress events. Corn is the most popular grain crop produced globally, and is projected to be the most consumed cereal grain in the world by 2020. Although advances in breeding and farming techniques have greatly increased corn yield, the crop remains subject to drops in overall production due to heat-stress events. Heat stress, caused by temperature spikes at crucial moments in the crop’s development, reduces grain weight by limiting the activity of certain enzymes essential to proper kernel development. In particular, the isoenzyme PGD3 is integral to kernel starch accumulation processes and has been identified as a main factor limiting grain yield in corn under heat-stress conditions. Researchers at the University of Florida have engineered two plastid-targeted versions of maize heat-stable 6PGDH isozymes. The rationale is to provide extra heat-stable enzymes to the cellular compartment where starch is synthesized, as the endogenous plastidic isoenzyme PGD3 is extremely heat sensitive and critical for starch accumulation. A genetically-modified corn variant using these transgenes will protect the crop against yield loss due to heat stress.
Corn genetically modified with transgenes to protect against yield loss due to heat stress
The enzyme 6-phosphogluconate dehydrogenase (6PGDH), is a critical part of the Pentose Phosphate Pathway (PPP), one of the main sources of the cellular reductant and sugar phosphates required for grain fill in corn. Corn has three specific genes that produce 6PGDH enzymes: PGD1, PGD2 and PGD3. Each of these genes produces a correspondingly named enzyme that acts in a certain cell compartment. The plastid-localized version of PGD3 is extremely sensitive to heat and loses 40-80 percent of its activity after only 20 minutes at 108o F / 42o C, preventing proper grain fill under heat-stress conditions. While the activities of PGD1 and PGD2 remain stable under this same heat-stress condition, their location in the cytosolic compartment prevents them from acting as substitutes for PGD3. However, UF researchers designed two transgenes and tested that direct expression of the more heat-tolerant gene variants (PGD1 and PGD2) in the plastid. These enzymes then carry out the starch synthesis processes of PGD3 that are necessary for corn kernel formation. In multiple field trials, researchers tested transgenic events under heat stress conditions. Three of the events yielded gains greater than 30 percent, compared to heat-stressed controls. These data strongly suggest that the transgenes mitigate yield-loss during heat-stress exposure. Additionally, non-stressed transgenics did not have any yield penalty compared to non-transgenic siblings. Expression of the plastid-targeted PGD1 and PGD2 transgenes provides an abiotic stress tolerance that does not have any apparent cost for growth under more optimal conditions.
