New idea on the adaptation of plant to land proposed by international team

The transition from an aquatic to a terrestrial environment is one of the milestones in the evolution of terrestrial life. In contrast with the aquatic environment, the terrestrial environment is fraught with more drought stress for terrestrial life. How to cope with droughts is thus one of the key issues in agricultural and botanical research.

Research teams led by Prof. ZHANG Guoping and Prof. CHEN Zhonghua from the College of Agriculture and Biotechnology, conducted a joint study with 27 researchers from Australia, USA, UK, Israel, Canada and Germany. They proposed and verified a new theory regarding the transition of plants from an aquatic to a terrestrial environment 450 million years ago. They discovered an early evolution of SAL1-PAP chloroplast retrograde signaling in stomatal regulation. These findings are conducive to a more insightful understanding of the evolution of plants’ drought tolerance and their adaptation to climate changes, thus opening up a new avenue for drought-tolerant breeding and cultivation. This research is published in the journal of PNAS.

The chloroplast retrograde signaling network is vital for chloroplast biogenesis, operation, and signaling, including excess light and drought stress signaling. To date, retrograde signaling has been considered in the context of land plant adaptation. When plants sense drought stress, this network will activate theiracclamatory molecular responses by regulating retrograde signaling protein.

The joint research team led by CHEN Zhonghua conducted a comparative genetic similarity analysis on predicted sequences of 61 protein families from 31 species representing the major clades of land plants, streptophyte algae, and chlorophyte algae. They discovered that the two streptophyte algae Klebsormidium flaccidum and Charabraunii showed higher similarity to embryophytes than to the chlorophyte algae. Significantly, many components that subsequently form the key guard-cell–signaling pathways were present in streptophyte algae predating the origin of land plants and the evolutionary emergence of stomata. 

During plant growths, the plastid genome and the nuclear genome coordinate to maintain the development and function of the organism. Experimental results indicate that this molecular mechanism originates from streptophyte algae and remains in most terrestrial plants. 

The pore is a vital switch for plants to exchange with the outside world. When plants are in an arid environment, stomata can control the exchange of water and carbon dioxide between plants and the environment, thus reducing the impact of drought stress on plants. Researchers made anelaborate study on how this pathway affects the stomatal switch. “Most terrestrial plants, such as Marchantia polymorpha, mosses, ferns, crops and other flowering plants, have similar retrograde signaling pathways, and this signaling pathway is also very conservative in regulating the stomatal closure of terrestrial plants,” said CHEN Zhonghua. Such molecular biological methods as gene editing can regulate related genes in this pathway, which will be a novel way for drought-tolerant breeding.

The shaping of the SAL-PAP retrograde signaling pathway predates the emergence of the first stomata, suggesting that intricate cellular communication networks were already in place to prime stomatal regulation. The evolutionary conservation and coordination of the SAL1-PAP pathway and ABA signaling during the diversification of land plants appear linked to the regulation of stomatal closure and adaptation to varying terrestrial habitats.

These findings suggest that the conquest of land by plants was empowed by rapid response to drought stress through the deployment of an ancestral SAL1-PAP signaling pathway, intersecting with the core abscisic acid signaling in stomatal guard cells.

“This research is based on big data and a comparative analysis of key genes facilitates a more precise perception of the evolution of terrestrial plants,” ZHANG Guoping remarked.