Photosynthetic acclimation of terrestrial and submerged leaves in the amphibious plant Hygrophila difformis

Generally, higher land plants do not grow under submerged conditions and aquatic plants cannot grow in terrestrial conditions. On the other hand, amphibious plants are able to live under both terrestrial and submerged conditions. The amphibious species Hygrophila difformis develops serrated and dissected leaves when grown in terrestrial and submerged conditions, respectively. Furthermore, dissected leaves develop when H. difformis is treated with ethylene under terrestrial growth conditions. Although this morphological change is regulated by ethylene, it remains unknown whether ethylene induces the leaves to functionally deal with underwater conditions.

Hygrophila difformis develops serrated and dissected leaves when grown in terrestrial and submerged conditions (top-left and bottom-right). These leaves are optimised to the conditions and underwater allow the leaves to utilise HCO3- in photosynthesis (top-right and bottom-left). Image credit: Horiguchi et al.

A recent study by Horiguchi et al. published in AoBP reports on the plasticity of morphological and physiological responses to submergence in an amphibious plant Hygrophila difformis. The authors analysed leaf morphology, measured underwater photosynthetic rates and HCO3− affinity in H. difformis to determine if there are differences in acclimation ability dependent on growth conditions: terrestrial, submerged, terrestrial treated with ethylene and submerged treated with an ethylene inhibitor. H. difformis was found to acclimate to a submerged environment by developing leaves with a characteristic morphology that are capable of photosynthesis optimised for the submerged environment by utilising HCO3. Ethylene regulates the submergence response both by changing leaf morphology and inducing HCO3− utilisation. Processes requiring carbon concentrating mechanism (CCM) proteins such as HCO3− transporters and carbonic anhydrase are also involved in photosynthetic acclimation. H. difformis likely harbours genes and regulatory mechanisms that will prove to be valuable resources for discovering CCM genes in higher plants. Further characterization and understanding of this acclimation mechanism will provide novel resources for discovering CCM regulatory systems in higher plants.

Researcher highlight

Genki Horiguchi and Naoki Hirotsu

Genki Horiguchi (left in photo) graduated from Toyo Univeristy, Japan in 2019 (MSc Life Science), where he studied photosynthetic acclimation of amphibious plants under the supervision of Professor Naoki Hirotsu. Genki will continue this study in his PhD research where he will be more focused on the molecular mechanisms of underwater acclimation.

Professor Naoki Hirotsu (right in photo) is based at Toyo University, Japan where his research aim is to understand the genetic and molecular mechanisms determining grain yield and quality of rice. He has also been interested in environmental responses of photosynthesis since completing his PhD.

Further reading

Horiguchi, G., Nemoto, K., Yokoyama, T., & Hirotsu, N. (2019). Photosynthetic acclimation of terrestrial and submerged leaves in the amphibious plant Hygrophila difformis. AoB PLANTS, 11(2). https://doi.org/10.1093/aobpla/plz009