As the simplest alkene in nature, ethylene functions as a hormone in plants. Beside its most well-known role in stimulating fruit ripening, ethylene also regulates many aspects of plant growth, senescence and response to stress responses. In Arabidopsis, ethylene signal is perceived and transduced through a well-characterized ethylene receptor-CTR1-EIN2-EIN3/EIL1-ERFs pathway. However, knowledge about the ethylene signaling mechanism in crops are still limited.
A recent study from ZHANG Jinsong's lab and CHEN Shouyi's lab at the Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, provided mechanistic insight into the regulation of ethylene signaling in the cereal crop rice.
In darkness, exogenous ethylene inhibits the root growth of rice seedlings. Based on this fact, ZHANG and CHEN's labs developed an effective screening system to troll for ethylene-insensitive phenotypes with abnormal root growth in the presence of ethylene. They fished out mao huzi11 (mhz11, mao huzi meaning 'cat whiskers' based on the appearance of their roots), a novel ethylene response mutant in rice.
Through map-based cloning, the researchers found that the MHZ11 gene encodes a putative GDSL lipase. They further showed that MHZ11 protein is localized to the ER membrane. Overexpression of MHZ11 resulted in shorter roots in air or ethylene, suggesting that MHZ11 facilitates ethylene response.
They further explored the function of MHZ11 through lipidomic analysis, and identified several phospholipids, including phosphatidylcholine, as possible MHZ11 substrates. MHZ11 is demonstrated to have PLA2 activity, and this activity is further demonstrated to be required for MHZ11 function.
Next, they took an effort to fit MHZ11 into the linear ethylene signaling pathway. Through a series of genetic and biochemical experiments, they figured out that MHZ11 functions at the ethylene receptors and plays an essential role in promoting ethylene inhibition of OsCTR2 (rice homologue of Arabidopsis CTR1) phosphorylation/activity. They further showed that MHZ11 may function through its lipase ability to reduce the sterol level on the ER membrane, thus impairing the interaction between ethylene receptors and OsCTR2 to facilitate ethylene signaling.
The study discovered a novel modulator of the ethylene signaling pathway in rice, suggesting that lipid metabolism plays a role in ethylene signaling regulation. This work provides crucial mechanistic insight into the regulation of ethylene signaling in the cereal crop rice and opens the door for future studies that carry potential to improve agriculture.
This work was supported by the National Natural Science Foundation of China, 973 Projects and the State Key Lab of Plant Genomics.
A proposed working model for MHZ11-mediated ethylene signaling in rice (Image by IGDB)
