The Chlamydomonas reinhardtii chloroplast envelope protein LCIA transports bicarbonate in planta.

LCIA (low CO2-inducible protein A) is a chloroplast envelope protein associated with the CO2-concentrating mechanism of the green alga Chlamydomonas reinhardtii. LCIA is postulated to be a HCO3- channel, but previous studies were unable to show that LCIA was actively transporting bicarbonate in planta. Therefore, LCIA activity was investigated more directly in two heterologous systems: an Escherichia coli mutant (DCAKO) lacking both native carbonic anhydrases and an Arabidopsis mutant (ßca5) missing the plastid carbonic anhydrase ßCA5. Neither DCAKO nor ßca5 can 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, 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.

Arabidopsis plastid carbonic anhydrase ßCA5 is important for normal plant growth.

Carbonic anhydrases (CAs) are zinc-metalloenzymes that catalyze the interconversion of CO2 and HCO3-. In heterotrophic organisms, CAs provide HCO3- for metabolic pathways requiring a carboxylation step. Arabidopsis (Arabidopsis thaliana) has 14 α- and ß-type CAs, two of which are plastid CAs designated as ßCA1 and ßCA5. To study their physiological properties, we obtained knock-out (KO) lines for ßCA1 (SALK_106570) and ßCA5 (SALK_121932). These mutant lines were confirmed by genomic PCR, RT-PCR, and immunoblotting. While ßca1 KO plants grew normally, growth of ßca5 KO plants was stunted under ambient CO2 conditions of 400 µL L-1; high CO2 conditions (30,000 µL L-1) partially rescued their growth. These results were surprising, as ßCA1 is more abundant than ßCA5 in leaves. However, tissue expression patterns of these genes indicated that ßCA1 is expressed only in shoot tissue, while ßCA5 is expressed throughout the plant. We hypothesize that ßCA5 compensates for loss of ßCA1 but, owing to its expression being limited to leaves, ßCA1 cannot compensate for loss of ßCA5. We also demonstrate that ßCA5 supplies HCO3- required for anaplerotic pathways that take place in plastids, such as fatty acid biosynthesis.