Light-driven dynamics: unravelling thiol-redox networks in plants through proteomics

Unlike animals, plants cannot hunt for prey or seek shelter, instead they use photosynthesis by harves ng the energy from sunlight to fix atmospheric carbon dioxide into biomass, allowing them to grow and spread.Owing to their sessile nature, plants have evolved ingenious mechanisms to protect themselves from too much sunlight and to adapt their metabolism to fluctua ng environmental condi ons.One of the mechanisms is the thiolbased redox regula on of protein ac vi es, an ancient and evolu onary conserved mechanism of post-transla onal modifica on.Thiol groups of cysteine residues in proteins form disulfide bonds when oxidised but this bond is broken into two thiol groups upon reduc on by a group of small proteins called thioredoxins (Trxs) (Figure 1A).In plant chloroplasts, light energy is used to excite electrons, which are transferred along the electron transfer chain in the thylakoid membrane.The final step, ferredoxin (Fdx) transfers electrons to ferredoxin-NADP oxidoreductase (FNR), which catalyses the synthesis of NADPH.To effec vely respond to variable light environments, plants have evolved two Trx systems.The Fdx-dependent system involves the Fdx-dependent Trx reductase (FTR), which uses reduced ferredoxin (Fdx) as an electron source (reviewed in Buchanan et al., 2016).This Fdx-Trx system contains many different types, most important for photosynthesis are the fand m-types (Figure 1a) and they are most ac ve under moderate and high light (Thormählen et al., 2015;Geigenberger et al., 2017;Nikkanen et al., 2019).Because the Fdx-Trx system uses ferredoxin that is reduced in the light, another system is required to reduce and ac vate chloroplast proteins during dark-to-light transi ons.This role is fulfilled by the NADPH-dependent Trx reductase (NTRC) system (Serrato et al., 2004).NTRC can use electrons from photosynthe cally synthesised NADPH but can also use NADPH from the oxida ve pentose phosphate pathway in the dark.Hence, the NTRC system is ac ve in the dark or during low light (Figure 1a), so that during dark-to-light transi ons, photosynthe c proteins can be ac vated, and photosynthesis can be "kick-started".Redox regula on via the two Trx systems allows for balancing of light and carbon-fixa on reac ons.For example, the chloroplast NADH dehydrogenase-like complex (NDH) par cipates in one of the two cyclic electron transfer (CET) pathways and is ac vated under low light by NTRC.However, under higher light, TRXm (an Fdx-depdentent Trx) inhibits the NDH complex and ac vates the PROTON GRADIENT REGULATION 5-depdentent CET pathway and proteins in the CO 2 -fixing Calvin-Benson-Bassham cycle (CBBC).This illustrates how these two Trx systems ensure proper tuning of light and carbon reac ons in response to the light environment.Even though we have a good understanding of the Trx systems, a quan ta ve inves ga on into the thiol-disulfide proteome in response to different light treatments is lacking.In this issue of Plant Physiology, Hou et al. use a bio n-switch assay to quan fy the redox changes of proteins in response to photoperiod and light intensity.In this assay, proteins were extracted from Arabidopsis thaliana leaves in the presence of Nethylmaleimide, which reacts with free thiol residues in proteins to form a strong C-S bond, irreversibly blocking free thiols.Then, the disulfide bonds in proteins were reduced with