Professor Dario Leister in Martinsried's greenhouse Credit: LMULMU biologists discovered that the "supervisor" protein and the "motivator" protein are necessary to allow a third factor to function in photosynthesis.For the synthesis sugars through photosynthesiscarbon dioxide, sunlight and water, plants, algae, and cyanobacteria only need three ingredients. The operation is much more complex than the simple list of ingredients may suggest. Professor Dr. Dario and his research group at LMU's Department of Biology I are studying the intricate regulation of photosynthesis. The latest findings revealed the roles of three proteins named PGRL1, PGRL2 or PGR5, that control one of two subsystems in the photosynthetic apparatus. The new study was the first to discover PGRL2.Photosynthesis is a series of coordinated reactions. The first step is when a portion of the electromagnetic radiation emitted from the Sun is absorbed into membrane-bound pigment protein complexes. These complexes are divided into two "photosystems", PSII and PSII. Two basic modeslinear or cyclic are used by the photosystems. PSIII and PSI work in series in the former. Light energy is used to dissociate electrons from water molecules and generate hydrogen ions (protons), and molecular oxygen. The protons are moved to the opposite membrane side, and the electrons are sequentially transferred from one complex to another, increasing in energy. This energy is then stored as ATP which drives the majority of biochemical transactions within cells. PSI provides the "reducing equivalents", which are required to convert carbon dioxide into sugars, via linear electron flow (LEF).Only photosystem I is involved in the second mode of photosynthesis. The electrons that are energized by sunlight are diverted to other proteins. They follow a cyclic path. This cyclic electron flow through PSI produces ATP only. Leister says that plants need both subsystems. Plants that are stressed need more ATP, which makes the cyclic pathway even more important. Without this mechanism, plants would not be able to survive in natural conditions.Photosynthesis in the Arabidopsis Model System. Credit: LMUOne supervisor, one motivatorHow is the regulation of the cyclic pathway? Japanese researchers began to study a group of mutants of Arabidopsis Thaliana (thale cress) 20 years ago. This is a popular model system for plant geneticists. One of these strains contained a mutation in a gene called PGR5. PGR stands for proton gradient and is the proton concentration gradient that results from the transfer of protons across a membrane during photosynthesis. The mutant caused the formation of the gradient to be disrupted. Leister says, "To our delight, the PGR5 gene did not have the sequences one would expect in an electron transporter." The idea was quickly borne out that there must be other factors involved in maintaining the proton gradient.Leister's 2008 experiments confirmed his suspicion. Leister discovered another protein, which he named "pgr5like 1" (PGRL1). This factor is coded by two genes in Arabidopsis, which explains the reason it didn't turn up in the original mutant screening that identified the PGR5 gene. He recalls, "At that time, we thought our hands had now been on the really important protein." The cyclic electron flow around the photosystem is reduced by inactivating either PGRL1 nor PGR5. Loss of PGRL1 can also destabilize PGR5, but vice versa. It seemed that PGRL1 was the central component of the cyclic photosynthesis. This idea was further supported by the fact it contained the structural elements that one would expect to see in an electron transporter.Later, however, regulation of cyclic electronic flow proved to be more complicated than this. Leister and his coworkers discovered PGRL2 which was a third protein that is involved in the regulation of cyclic electron flow. This discovery made matters more complicated. Photosynthesis is not affected by PGRL2 being removed, according to the team. Overproduction of PGRL2 caused PGR5 to become unstable, even when there was PGRL1. Surprise! PGRL1 & PGRL2 were both simultaneously activated. PGR5 resurfaced and was able to restore cyclic electron transportation on its own. These plants grew slower than the ones without PGR5 (and cyclic electronic transport). Leister provides a clear interpretation of these results. "PGR5 does the job. PGRL1 acts to motivate PGR5, while PGRL2 serves as PGR5's supervisor. PGR5 becomes inactive without a motivator. It works well without a supervisor. However, if the motivator or supervisor is absent, PGR5 becomes hyperactive and eventually destructive.Leister's group now plans to unravel the biochemical mechanisms behind these behaviors using cyanobacteria as a model system. Cyanobacteria are genetically simpler than Arabidopsis.See more about Photosynthesis: Living labsThilo Rhle et. al., PGRL2 triggers PGR5 degradation in the absence PGRL1, Nature Communications (2021). Information from Nature Communications Thilo Rhle et al., PGRL2 triggers PGR5 degradation in the absence PGRL1 (2021). DOI: 10.1038/s41467-242107-7
