Rice can be drowned! Although rice is a semi-aquatic plant, the seedlings will die if they are submerged completely under water and oxygen is depleted. Recent climate change has transformed once-in-a-lifetime typhoon or heavy rainfall into a frequent occurrence, which greatly reduced rice production in Taiwan.

Dr. Ming-Che Shih, a distinguished research fellow from Agricultural Biotechnology Research Center and Dr. Meng-Chia Ho, an assistant research fellow from the Institute of Biological Chemistry, led a team to investigate the rice oxygen sensing mechanism and submergence response and discover how SUB1A-1 avoids being degraded using plant physiology and biophysics approaches. Their result was published in Proceedings of the National Academy of Sciences of the United States.

Some rice varieties possessing an important transcription factor, called SUB1A-1, can survival up to 2 week of submergence. This anti-submergence gene, SUB1A-1, inhibits rice growth during flooding to conserve energy and increase its survival. This is a similar strategy to how bears hibernate during winter. However, it is unclear how SUB1A-1 activates the anti-submergence responses in rice.

Their research team discovered SUB1A-1 will activate two transcription factors, ERF66 and ERF67, which turn on the submergence response mechanism in rice to enhance survival. More importantly, the research team found that ERF66 and ERF67 possessing a special amino acid sequence (N-degron) that can function as an oxygen sensor in rice. In normoxia environment, the N-degron is oxidized followed by protein degradation via ubiquitin-proteasome system. In result, no submergence response is activated in rice when oxygen is available. Only when rice is drowned and oxygen is depleted, ERF66 and ERF67 are not degraded. The stabilized ERF66 and ERF67 will then activate various downstream genes to respond to submergence. Based on this finding, a SUB1A-1/ERF66/ERF67 signal cascade system for hypoxia sensing and submergence-resistance mechanism was proposed.

Current climate change severely threaten food security. Previous studies showed that SUB1A-1 play a key role in rice submergence resistance. The team of Dr. Ming-Che Shih and Dr. Meng-Chiao Ho further found that rice ERF66/67 function as oxygen sensors and response switches against submergence. Furthermore, their result also suggested that the mechanism by which SUB1A-1 can escape the degradation by ubiquitin-proteasome system in the presence of oxygen is by forming special interactions within itself. This degradation escape is important for rice recovery at post-submergence stage, when oxygen is present. Their findings extend current understanding about how rice cultivars withstand submergence and may help improve the hardiness of other crops in the future.

This paper is ahead of print on the February 5th in Proceedings of the National Academy of Sciences of the United States with the title “Regulatory cascade involving transcriptional and N-end rule pathways in rice under submergence”. Fulltext: https://www.pnas.org/content/early/2019/02/04/1818507116

Image: Rice plants after 14 d of recovery from submergence. Fourteen-day-old rice plants were submerged for 7 d in darkness. After submergence, plants were returned to normal growth conditions for 14 d of recovery and photographed.