Although light is essential for photosynthesis, when in excess, it may damage the photosynthetic apparatus, leading to a phenomenon known as photoinhibition. Photoinhibition was thought as a light-induced damage to photosystem II; however, it is now clear that even photosystem I may become very vulnerable to light. One main characteristic of light induced damage to photosystem II (PSII) is the increased turnover of the reaction center protein, D1: when rate of degradation exceeds the rate of synthesis, loss of PSII activity is observed. With respect to photosystem I (PSI), an excess of electrons, instead of an excess of light, may be very dangerous. Plants possess a number of mechanisms able to prevent, or limit, such damages by safe thermal dissipation of light energy (non-photochemical quenching, NPQ), slowing-down of electron transfer through the intersystem transport chain (photosynthesis-control, PSC) in co-operation with the Proton Gradient Regulation (PGR) proteins, PGR5 and PGRL1, collectively called as short-term photoprotection mechanisms, and the redistribution of light between photosystems, called state transitions (responsible of fluorescence quenching at PSII, qT), is superimposed to these short term photoprotective mechanisms. In this manuscript we have generated a number of higher order mutants by crossing genotypes carrying defects in each of the short-term photoprotection mechanisms, with the final aim to obtain a direct comparison of their role and efficiency in photoprotection. We found that mutants carrying a defect in the ΔpH-dependent photosynthesis-control are characterized by photoinhibition of both photosystems, irrespectively of whether PSBS-dependent NPQ or state transitions defects were present or not in the same individual, demonstrating the primary role of PSC in photoprotection. Moreover, mutants with a limited capability to develop a strong PSBS-dependent NPQ, were characterized by a high turnover of the D1 protein and high values of Y(NO), which might reflect energy quenching processes occurring within the PSII reaction center.

Domesticated rice Oryza sativa L. is a major staple food worldwide, and the cereal most sensitive to salinity. It originated from the wild ancestor Oryza rufipogon Griff., which was reported to possess superior salinity tolerance. Here, we examined the morpho-physiological responses to salinity stress (80 mM NaCl for 7 days) in seedlings of an O. rufipogon accession and two Italian O. sativa genotypes, Baldo (mildly tolerant) and Vialone Nano (sensitive). Under salt treatment, O. rufipogon showed the highest percentage of plants with no to moderate stress symptoms, displaying an unchanged shoot/root biomass ratio, the highest Na⁺ accumulation in roots, the lowest root and leaf Na⁺/K⁺ ratio, and highest leaf relative water content, leading to a better preservation of the plant architecture, ion homeostasis, and water status. Moreover, O. rufipogon preserved the overall leaf carbon to nitrogen balance and photosynthetic apparatus integrity. Conversely, Vialone Nano showed the lowest percentage of plants surviving after treatment, and displayed a higher reduction in the growth of shoots rather than roots, with leaves compromised in water and ionic balance, negatively affecting the photosynthetic performance (lowest performance index by JIP-test) and apparatus integrity. Baldo showed intermediate salt tolerance. Being O. rufipogon interfertile with O. sativa, it resulted a good candidate for pre-breeding towards salt-tolerant lines.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the major catalyst in the conversion of carbon dioxide into organic compounds in photosynthetic organisms. However, its activity is impaired by binding of inhibitory sugars such as xylulose-1,5-bisphosphate (XuBP), which must be detached from the active sites by Rubisco activase. Here, we show that loss of two phosphatases in Arabidopsis thaliana has detrimental effects on plant growth and photosynthesis and that this effect could be reversed by introducing the XuBP phosphatase from Rhodobacter sphaeroides. Biochemical analyses revealed that the plant enzymes specifically dephosphorylate XuBP, thus allowing xylulose-5-phosphate to enter the Calvin-Benson-Bassham cycle. Our findings demonstrate the physiological importance of an ancient metabolite damage-repair system in degradation of by-products of Rubisco, and will impact efforts to optimize carbon fixation in photosynthetic organisms.

Arabidopsis single and multiple knockout mutants generated for the Oxygen Evolving Complex (OEC) proteins of Photosystem II (PSII) contributed in the past to understand specific roles of these subunits in the functioning of PSII. Here, by applying a comparative phenotyping approach based on fast chlorophyll a fluorescence induction and JIP-test to the single psbr (ΔR) and psbo1 (ΔO), and multiple psbq1-psbq2 (ΔQ), psbq1-psbq2-psbr (ΔQR) and psbo1-psbp2-psbq1-psbq2-psbr (ΔOQR) Arabidopsis knockout mutants we dissected the defects occurring at different steps of the photosynthetic flux induced by these mutations. The psbq1 psbq2 mutations induced a clear impairment of the PSII donor side, as evidenced by the lowered F_WOC index in ΔQ. Conversely, the psbr mutation led to a predominant impairment of the PSII acceptor side, as all JIP-test parameters associated with the electron transfer further than Q_A (ET_O/RC, ET_O/TR_O, ET_O/ABS) were negatively affected in ΔR, possibly due to a defect in the exchange between reduced and oxidized quinones at the Q_B site. These mutations in ΔQR had an additive effect on the perturbation of the function of PSII. The psbo1 mutation induced dramatic effects on the PSII donor side, as evidenced by the strong decrease of the F_WOC index and the presence of a pronounced positive K-band in the OJIP transient in ΔO, leading to a marked reduction of the PSII photochemistry. The higher order mutant ΔOQR shared many characteristics with ΔO, indicating that the effect of the psbo1 mutation is preponderant even in the presence of additional psbq1 psbq2 and psbr mutations.