A study published in the Proceedings of the National Academy of Sciences shows that engineering plants to produce high levels of a protein known as PYL9 may boost drought tolerance in rice and the model plant Arabidopsis.
Because plants can’t flee drought, they deploy an array of survival strategies while awaiting better growing conditions. Their drought responses are controlled by a hormone known as abscisic acid (ABA), which regulates growth and development and directs plants’ reaction to stress. Plants’ short-term drought responses include closing their stomata—holes that “exhale” water—and creating extra wax to seal moisture within leaves. Long-term drought conditions cause plants to go into dormancy and redirect water and nutritional resources away from leaves to sink tissues such as seeds and buds, reservoirs for new growth. This process, called senescence, may result in a plant that looks shriveled and leafless but is just executing a line of defense.
The researchers found that altering plants to overexpress the protein PYL9 made them highly sensitive to ABA. A stress-responsive promoter protein controlled the level of PYL9 expression in the plants. The gene alterations enabled Arabidopsis and rice to better withstand severe drought stress and caused older leaves to yellow sooner compared with the plants’ wild type counterparts. PYL9 transgenic rice had a 50% survival rate after a two-week drought compared with 10% survival in wild type rice.
However, the researchers cautioned that the spike in survival rate does not mean that the yield of the transgenic plants under drought conditions would equal that of conventional rice varieties under good growing conditions. The study did not test for yield.
Unexpectedly, when transgenic plants were treated with ABA under normal conditions, the old leaves started to wilt, even though the plants received enough water. This suggests that the plants had blocked their old leaves’ access to water, preferentially driving water to developing tissues instead.
The research team concluded that during severe drought conditions, hypersensitivity to ABA leads to increased senescence and death of old leaves but protects young tissues by sending them into dormancy. The study also suggests that the ABA core signaling pathway plays a crucial role in plant survival during extreme drought and that senescence is a beneficial drought defense strategy, previously points of contention among plant scientists.