Ion transporters are essential for photosynthesis

Photosynthesis is the key biochemical reaction in plants but the molecular mechanisms of K+ transport across chloroplast membranes and their relevance for chloroplast function and photosynthesis remained unknown. We have identified and characterized three K+/ H+ transporters in chloroplast membranes and demonstrate their vital role for chloroplast osmoregulation and thylakoid density and photosynthesis. Surprisingly, high Na+ restored the photosynthetic activity in the mutant plants.

HFSP Long-Term Fellow Hans-Henning Kunz and colleagues
authored on Thu, 22 May 2014

K+ is the most abundant cation found in plant tissues and a major player in plant cell physiology. It fulfills numerous essential roles, e.g., in osmoregulation, as a pH regulator, in membrane polarization, in photosynthetic activity and in motor cell movements. To maintain appropriate K+ levels in cellular compartments and different tissue types throughout the plant, membrane transport proteins and channels are mandatory. Several K+ transporters and channels and corresponding plant mutants have been studied at the plasma membrane and organelle membranes of plant cells. However, although a well-balanced K+ homeostasis was assumed to play an important role for chloroplast function, the molecular identity of chloroplast K+ transporters remained unsolved. This limitation has made it difficult to study the importance of K+ transporters for chloroplast function and photosynthesis.

The work supported by the HFSP fellowship to Hans-Henning Kunz at the University of California, San Diego has now identified CPA2 antiporter superfamily proteins that are targeted to the inner envelope membrane or thylakoid membranes of chloroplasts. Higher order but no single loss-of-function mutants showed increasingly impaired photosynthesis along with pale green leaves and severely stunted growth. The pH component of the proton motive force across the thylakoid membrane was significantly decreased in these mutants, also indicating an altered chloroplast pH homeostasis. Detailed electron microscopy of leaf cells revealed severe chloroplast damage including disrupted envelope membranes and reduced thylakoid membrane density. Unexpectedly, exogenous salt application reversed all observed phenotypes. Furthermore we could show that the mutant backgrounds enable functional significance analyses of other chloroplast ion transporters which provide an important framework for future discoveries.

Results gained from this research strongly increase the knowledge of chloroplast ion transport. The identified genes provide a basis to understand chloroplast function and photosynthesis under salt stress conditions. This phenomenon still remains poorly understood but will become more and more important since soil salinity and its plant toxicity is an increasingly prominent global issue. An in-depth understanding of the molecular processes involved along with targeted fertilization may provide a significant contribution to fighting plant salt stress in the future.

Reference

Plastidial transporters KEA1, -2, and -3 are essential for chloroplast osmoregulation, integrity, and pH regulation in Arabidopsis. Kunz HH1, Gierth M, Herdean A, Satoh-Cruz M, Kramer DM, Spetea C, Schroeder JI. Proc Natl Acad Sci U S A. 2014 May 2.

Pubmed link

Link to abstract