Caterina Gerotto, Alessandro Alboresi, Andrea Meneghesso, Martina Jokel, Marjaana Suorsa, Eva-Mari Aro, and Tomas Morosinotto

Significance

Photosynthesis regulation is fundamental to responding to environmental dynamic changes and avoiding oxidative damage. These regulatory mechanisms were shaped during evolution from cyanobacteria to plants, as well as after the colonization of new habitats. Land colonization was a key phase in evolution with plants capable of adapting to drastically different conditions with respect to their aquatic ancestors. Valuable information on this adaptation can be obtained studying nonvascular plants, such as the moss Physcomitrella patens, that diverged from flowering plants early after land colonization. Here we show that in P. patens, Flavodiiron (FLV) proteins are acting as an electron sink to avoid photosynthetic electron transport chain over-reduction after any increase in illumination and are fundamental for protection under fluctuating light conditions.

Abstract

Photosynthetic organisms support cell metabolism by harvesting sunlight to fuel the photosynthetic electron transport. The flow of excitation energy and electrons in the photosynthetic apparatus needs to be continuously modulated to respond to dynamics of environmental conditions, and Flavodiiron (FLV) proteins are seminal components of this regulatory machinery in cyanobacteria. FLVs were lost during evolution by flowering plants, but are still present in nonvascular plants such as Physcomitrella patens. We generated P. patens mutants depleted in FLV proteins, showing their function as an electron sink downstream of photosystem I for the first seconds after a change in light intensity. flv knock-out plants showed impaired growth and photosystem I photoinhibition when exposed to fluctuating light, demonstrating FLV’s biological role as a safety valve from excess electrons on illumination changes. The lack of FLVs was partially compensated for by an increased cyclic electron transport, suggesting that in flowering plants, the FLV’s role was taken by other alternative electron routes.

See: http://www.pnas.org/content/113/43/12322.abstract.html?etoc

PNAS October 25 2016; Vol.113; no.43: 12322–12327

Fig. 1. Genotype characterization of flva KO and flvb KO lines. Effective homologous recombination with the insertion of the resistance cassette in FLVA or FLVB loci was first verified by PCR to prove the integration of resistance cassette in the expected position of the genome (SI Appendix, Fig. S2). (A) FLVA and FLVB gene expression was assessed by RT-PCR in WT and selected flva KO and flvb KO lines grown in control light. Amplification of Actin transcript is also reported as control. (B) Western blotting against P. patens FLVA and FLVB proteins. In each lane, 100 µg total proteins from CL-grown samples were loaded. In the case of WT, 0.5X and 2X indicates lanes loaded with 50 or 200 µg, respectively. WT and two independent lines for each flva KO and flvb KO mutants are reported, but additional independent lines were analyzed with indistinguishable results (SI Appendix, Fig. S3).