PLoS Genet 14(2): e1007208. (Feb 15, 2018)
A molecular basis behind heterophylly in an amphibious plant, Ranunculus trichophyllus
Juhyun Kim, Youngsung Joo, Jinseul Kyung, Myeongjune Jeon, Jong Yoon Park, Ho Gyun Lee, Doo Soo Chung, Eunju Lee, Ilha Lee

Abstract
Ranunculus trichophyllus is an amphibious plant that produces thin and cylindrical leaves if grown under water but thick and broad leaves if grown on land. We found that such heterophylly is widely controlled by two plant hormones, abscisic acid (ABA) and ethylene, which control terrestrial and aquatic leaf development respectively. Aquatic leaves produced higher levels of ethylene but lower levels of ABA than terrestrial leaves. In aquatic leaves, their distinct traits with narrow shape, lack of stomata, and reduced vessel development were caused by EIN3-mediated overactivation of abaxial genes, RtKANADIs, and accompanying with reductions of STOMAGEN and VASCULAR-RELATED NAC-DOMAIN7 (VDN7). In contrast, in terrestrial leaves, ABI3-mediated activation of the adaxial genes, RtHD-ZIPIIIs, and STOMAGEN and VDN7 established leaf polarity, and stomata and vessel developments. Heterophylly of R. trichophyllus could be also induced by external cues such as cold and hypoxia, which is accompanied with the changes in the expression of leaf polarity genes similar to aquatic response. A closely-related land plant R. sceleratus did not show such heterophyllic responses, suggesting that the changes in the ABA/ethylene signaling and leaf polarity are one of key evolutionary steps for aquatic adaptation.

Author summary
Evolutionary adaptation into aquatic environment is widely observed in diverse clades of land plants. To understand the molecular basis behind such adaptation, we analyzed Ranunculus trichophyllus, an amphibious plant producing different leaf shape depending on the growth conditions. Aquatic leaves of this plant produce higher levels of ethylene, which causes overactivation of genetic circuits composed of EIN3, an ethylene signaling transducer, and abaxial genes that suppress genes regulating stomata and xylem development. In contrast, terrestrial leaves produce higher levels of ABA, which activates adaxial genes and causes activation of stomata and xylem developments. Such changes in the ABA/ethylene signaling and leaf polarity after submergence were not observed in the closely-related land plant R. sceleratus, indicating that they are key evolutionary steps towards aquatic adaptation.

Cook, CDK: Leaf form in Ranunculus (1969)

Arber: Heterophylly in water plants (1919)