How do plants turn off photosynthetic activity at night?

Now, Keisuke Yoshida, Toru Hisabori and colleagues have identified two proteins, constituting the thioredoxin-like2 (TrxL2)/2-Cys peroxiredoxin (2CP) redox cascade, that help control the reoxidation of these photosynthetic proteins by modifying key parts of the molecular players. These two proteins appear to function as part of a cascade that siphons energy from the photosynthetic proteins to the always energy-hungry hydrogen peroxide. TrxL2, unlike similar, better-known proteins, seems to be specialized for the 'switching off' process; it's an efficient oxidizer of many proteins, but only reduces 2CP, allowing the energy drained by TrxL2 from several upstream reactions to pass to 2CP and thence hydrogen peroxide. This cascade thus keeps photosynthesis on standby until light is available again.

TrxL2/2CP do work in light as well, but are overshadowed by the normal activation machinery in plants and only take center stage in the absence of light. Interestingly, this cascade does not seem to affect photosynthesis itself, as mutant plants without 2CP behave normally in light; however, the 'switching off' mechanism is significantly less efficient in these mutant plants than in wild-type plants. Moreover, the fact that this process is less efficient, rather than absent altogether, suggests that other, as yet unknown, proteins serve similar functions in plants. These researchers thus shed light on how plants reserve the activity of photosynthetic proteins for when it's actually useful.