Redesigning of Photosystem I for optimizing photo-synthesis/protection balance

BULTRI, JULIÁN; BRUGANARA, CANDELA; MIÑO, JULIÁN; ABRAHAM, MARÍA ; LOBAIS, CELINA; BLANCO, NICOLÁS E.

Congreso; SAIB 2023; 2023

Photosynthesis enables autotrophic organisms to convert absorbed light energy into ATP and reducingpower supporting the fixation of CO2 into biomass. These photochemical reactions are conducted bythe photosynthetic apparatus located in the thylakoid membranes, the Photosynthetic ElectronTransport Chain (PETC). It is composed by the light harvesting complexes, a series of protein complexes(two photosystems and the cytochrome b6f complex) and electron carriers, which couples redoxreactions for the generation of proton motive force (pmf), mainly composed of the ΔpH component. Thefinal steps of the photosynthetic electron flow involve the reduction of the stromal soluble carrierFerredoxin (Fd) and further production of NADPH during its regeneration. An emerging picture indicatesthe existence of a trade-off between photosynthetic efficiency and the activation of photoprotectivemechanisms. This is particularly pronounced in the frame of climate-change conditions, when plantsmust cope with high light intensity and drought stress. In such conditions, the availability of oxidizedreceptors at the end of Photosystem I is scarce, leading to what is referred to as “PSI acceptor-sidelimitation”. This triggers the induction of different photoprotective mechanisms with a concomitantreduction in photochemical yield.To explore approaches capable of adjusting photosynthesis/photoprotection trade-off for highlychangeable conditions, we conducted a study using transplastomic tobacco plants with overexpressedpea Ferredoxin (OeFd). Our hypothesis proposes that an increased content of electron acceptor, suchas Fd, can simultaneously act as a “safe valve” to avoid PSI acceptor-side limitation without affectingphotochemical efficiency.Under greenhouse conditions, OeFd plants exhibit reduced biomass, fewer leaves and smaller overallsize compared with wild type tobacco plants along with an easily recognizable variegated phenotype.To better understand the physiological implications of these modifications at PSI, plants were subjectedto various light conditions, ranging from 30 to 1200 μmol m-2 s-1, and photosynthetic and physiologicalparameters were analyzed. Low light conditions were not capable of reverting OeFd into wild typephenotype nor improve photochemical yield. Conversely, under high light conditions, overexpressorsdisplayed uniform leaf color together with an improvement in its photochemical capacity and growthrates. The analysis of these results in conjunction with measurements focusing on ATP synthase activitysuggested an alternative partition of electron flow at the PSI acceptor side, as initially hypothesized. Thisscenario presents new opportunities for reformulating photosynthesis/ photoprotective trade-off.Delving into the comprehension of this equilibrium certainly will allow us to engineerclimate-change-resilient plants.