CO2-concentrating mechanisms in Egeria densa, a submersed aquatic plant.

Egeria densa is an aquatic higher plant which has developed different mechanisms to deal with photosynthesis under conditions of low CO2 availability. On the one hand it shows leaf pH-polarity, which has been proposed to be used for bicarbonate utilization. In this way, at high light intensities and low dissolved carbon concentration, this species generates a low pH at the adaxial leaf surface. This acidification shifts the equilibrium HCO3-/CO2 towards CO2, which enters the cell by passive diffusion. By this means, E. densa increases the concentration of CO2 available for photosynthesis inside the cells, when this gas is limiting. On the other hand, under stress conditions resulting from high temperature and high light intensities, it shows a biochemical adaptation with the induction of a C4-like mechanism but without Kranz anatomy. Transfer from low to high temperature and light conditions induces increased levels of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) and NADP-malic enzyme (NADP-ME, EC 1.1.1.40), both key enzymes participating in the Hatch-Slack cycle in plants with C4 metabolism. Moreover, one PEPC isoform, whose synthesis is induced by high temperature and light, is phosphorylated in the light, and changes in kinetic and regulatory properties are correlated with changes in the phosphorylation state of this enzyme. In the present review, we describe these two processes in this submersed angiosperm that appear to help it perform photosynthesis under conditions of extreme temperatures and high light intensities.