PhD Defence by Anett Stéger: Terminal Autoinhibitory Domains In Plant P-type ATPases

To regulate, or not to regulate, that is the question: Investigating the evolution and the physiological importance of terminal autoinhibitory domains in plant P-type ATPases

P-type ATPases are a superfamily of proteins present in all kingdoms of life that transport a variety of ligands across membranes at the expense of ATP. Some members of this family, including plant plasma membrane (PM) H+-ATPases (P3A ATPases) and autoinhibited Ca2+-ATPases (P2B ATPases), are equipped with a terminal regulatory (R) domain that modulates the activity of the enzyme. As a starting point for the present thesis, it was observed that P3A and P2B ATPases were not equipped with an R domain in green algae, but it evolved in the streptophyte algal predecessors of land plants. This could implicate that the regulation of the pump activity through the R domain was necessary for the successful terrestrialization of plants.

To explore the possible physiological consequences of losing the R domain, I have examined in vivo C-terminal truncations of a model pump: the autoinhibited PM H+-ATPase2 (AHA2) in Arabidopsis thaliana. I observed that, as more residues are deleted from the C-terminal end of AHA2, the Arabidopsis plants become progressively more effective in pumping out protons and they show elevated expansion growth. Major drawbacks of the constitutive activation of the pump were that plants became more susceptible to pathogens and drought due to their more open stomata. I also report an unexpected smaller seed size, likely due to unfavorable acidosis of the filial tissue, and shorter root hairs. I hypothesize that the latter is the consequence of an energy-saving mechanism as the mutant plant had increased membrane potential and rhizosphere acidification which resulted in more effective nutrient uptake. High external concentrations of potassium, arginine, and cesium led to higher accumulation of these compounds and an earlier toxic effect.

Taken together, my results suggest that the R domain of P-type ATPases is essential for plant fitness on land. Thus, careful consideration is needed before applying the P-type ATPase upregulation strategy to enhance crop production in the future.

Supervisor

Professor Michael Broberg Palmgren, PLEN

Assessment Committee 

Snr Scientist Markus Geisler, University of Freiburg

Professor Jens Preben Morth, DTU

Associate Professor Rosa Lopez Marques, PLEN (Chair)