AUTHOR=Rubio Lourdes , García-Pérez Delia , Davies Julia M. , Fernández José A. 

TITLE=Short-Term Response of Cytosolic NO3− to Inorganic Carbon Increase in Posidonia oceanica Leaf Cells

JOURNAL=Frontiers in Plant Science

VOLUME=Volume 11 - 2020

YEAR=2020

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2020.00955

DOI=10.3389/fpls.2020.00955

ISSN=1664-462X

ABSTRACT=The concentration of CO2 in the atmosphere has increased over the past 200 years, and is expected to continue rising in the next 50 years at a rate of 3 ppm·year-1. This increase has led to a decrease in seawater pH that has changed inorganic carbon chemical speciation, increasing the dissolved HCO3-. Posidonia oceanica is a marine angiosperm that uses HCO3- as an inorganic carbon source for photosynthesis. An important side effect of the direct uptake of HCO3- is the diminution of cytosolic Cl- (Cl-c) in mesophyll leaf cells due to the efflux through anion channels and, probably, to intracellular compartmentalization. Since anion channels are also permeable to NO3- we hypothesize that high HCO3-, or even CO2, would also promote a decrease of cytosolic NO3- (NO3-c). In this work we have used NO3-- and Cl--selective microelectrodes for the continuous monitoring of the cytosolic concentration of both anions in P. oceanica leaf cells. Under light conditions, mesophyll leaf cells showed a NO3-c of 5.7±0.2 mM, which rose up to 7.2±0.6 mM after 30 min in the dark. The enrichment of natural seawater (NSW) with 3 mM NaHCO3 caused both a NO3-c decrease of 1±0.04 mM and a Cl-c decrease of 3.5±0.1 mM. The saturation of NSW with 1000 ppm CO2 also produced a diminution of the NO3-c, but lower (0.4±0.07 mM). These results indicate that the rise of dissolved inorganic carbon (HCO3- or CO2) in NSW would have an effect on the cytosolic anion homeostasis mechanisms in P. oceanica leaf cells. In the presence of 0.1 mM ethoxyzolamide, the plasma membrane-permeable carbonic anhydrase inhibitor, the CO2-induced cytosolic NO3- diminution was much lower (0.1±0.08 mM), pointing to HCO3- as the inorganic carbon species that causes the cytosolic NO3- leak. The incubation of P. oceanica leaf pieces in 3 mM HCO3--enriched NSW triggered a short-term external NO3- net concentration increase consistent with the NO3-c leak. As a consequence, the cytosolic NO3- diminution induced in high inorganic carbon could result in both the decrease of metabolic N flux and the concomitant biomass N impoverishment in P. oceanica and, probably, in other aquatic plants.