The Chlamydomonas reinhardtii chloroplast envelope protein LCIA transports bicarbonate in planta

Authors: FÖRSTER, BRITTA - Australian National University; ROURKE, LORAINE - Australian National University; WEERASOORIYA, HIRUNI - Louisiana State University; PABUAYON, ISAIAH - Louisiana State University; AU, ENG KEE - Csiro, Black Mountain Laboratories; BALA, SOUMI - Australian National University; Bajsa-Hirschel, Joanna; KAINES, SARAH - Australian National University; KASILI, REMMY - Louisiana State University; LAPLACE, LILLIAN - Louisiana State University; MACHINGURA, MARYLOU - Georgia Southern University; MASSEY, BAXTER - Australian National University; ROSATI, VIVIANA - University Of York; STUART-WILLIAMS, HILARY - Australian National University; BADGER, MURRAY - Australian National University; PRICE, G. DEAN - Australian National University; MORONEY, JAMES - Australian National University

Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/28/2023
Publication Date: 3/29/2023
Citation: Förster, B., Rourke, L., Weerasooriya, H.N., Pabuayon, I.C., Au, E., Bala, S., Bajsa Hirschel, J.N., Kaines, S., Kasili, R., Laplace, L., Machingura, M.C., Massey, B., Rosati, V.C., Stuart-Williams, H., Badger, M.R., Price, G., Moroney, J.V. 2023. The Chlamydomonas reinhardtii chloroplast envelope protein LCIA transports bicarbonate in planta. Journal of Experimental Botany. https://doi.org/10.1093/jxb/erad116.
DOI: https://doi.org/10.1093/jxb/erad116

Interpretive Summary: The green alga Chlamydomonas reinhardtii has a CO2 concentrating mechanism (CCM) that enhances CO2 fixation in low CO2 conditions. Recently, there has been substantial interest in utilizing components of algal CCMs to improve photosynthesis in higher plants. Modeling work indicates that photosynthetic CO2 fixation and crop yield could increase up to 9% if a Ci pump was placed in the chloroplast envelope of higher plants, and greater than 15% when expression of two envelope-located transporters with complementary kinetics was considered. In this communication, we present evidence using heterologous systems and protein modeling that the component of CCM LCIA is an HCO3- channel. Furthermore, we report on our studies of LCIA expression in wild-type tobacco (N. tabacum) which demonstrate that the protein does not confer any photosynthetic enhancements to the plants.

Technical Abstract: LCIA is a chloroplast envelope protein associated with the CO2 concentrating mechanism of the green alga Chlamydomonas reinhardtii. It is a member of the FocA family of formate/nitrite channel proteins and a putative HCO3- transporter that has been shown to increase inorganic carbon uptake in oocytes. When expressed in tobacco (Nicotiana tabacum), LCIA was correctly localized to the chloroplast envelope but did not confer enhanced photosynthetic performance as would be expected from active HCO3- transport into chloroplasts. Therefore, LCIA activity was investigated more directly in two heterologous systems: an E. coli mutant (DCAKO) lacking both native carbonic anhydrases and an Arabidopsis mutant (ßca5) missing the plastid carbonic anhydrase ßCA5. Both DCAKO and ßca5 cannot grow in ambient CO2 conditions, as they lack carbonic anhydrase-catalyzed production of the necessary HCO3- concentration for lipid and nucleic acid biosynthesis. Expression of LCIA restored growth in both systems in ambient CO2 conditions, which strongly suggests that LCIA is facilitating HCO3- uptake in each system. To our knowledge, this is the first direct evidence that LCIA moves HCO3- across membranes in bacteria and plants. Furthermore, the ßca5 plant bioassay used in this study is the first system for testing HCO3- transport activity in planta, which is an experimental breakthrough that will be valuable for future studies aimed at improving the photosynthetic efficiency of crop plants using components from algal CO2 concentrating mechanisms.