Research Topic

1. Light signaling, utilization and dissipation

Dimitris Petroutsos

In photosynthetic organisms light is a signal perceived by photoreceptors, i.e. proteins that convert light radiation into biological signals regulating essential cell functions (i.e. stomata opening and closure, photo-orientation of chloroplasts, plant development). Light is also the energy source used to convert CO₂ into organic metabolites. When light is absorbed in excess to the photosynthetic capacity of the cells, photoprotective mechanisms known as NPQ (non-photochemical quenching, Figure 1) are activated in order to reduce photodamage.

Figure 1:
Chlorophyll (Chl) molecules after absorbing light are excited to the singlet excited state (Chl*). They can relax back to their fundamental state (Chl) either by photochemistry (photosynthesis), or by light reemission (fluorescence) or by non radiative dissipation (heat). The latter process allows dissipating excess absorbed light, thereby reducing production of harmful species like ROS. Light absorption is also regulated by State Transitions, a process involving a « slow » movement of light absorbing proteins between the two photocenters of oxygenic photosynthesis in order to optimize their use of absorbed light. In general, state transitions mostly occur at limelight intensities limiting photosynthetic activity, while NPQ is mostly active at oversaturating photon fluxes.

NPQ can be divided in mainly two components:

(1) qE (high-energy state quenching), the major and fastest component of NPQ that depends on the buildup of a high ΔpH across the thylakoid membrane (i.e., a low lumen pH), on specific carotenoids and, in the case of Chlamydomonas, on the protein LHCSR3. It is a nucleus-encoded, chloroplast-localized protein that is induced under strong illumination. Full induction of LHCSR3 and qE takes few hours at variance with higher plants that the protein effectors of qE are constitutively expressed (Allorent et al., 2013). The signal transduction process that controls the expression of LHCSR₃ involves Ca²⁺ signaling events and requires an active photosynthetic electron flow (Petroutsos et al., 2011) but the molecular mechanism behind this process in still largely unknown.

(2) state transitions (qT), a change in the relative absorption capacity of the two photosystems (PSII and PSI). (Figure 2). This results from reversible phosphorylation of light harvesting proteins. We have recently shown that the increased ability of Chlamydomonas to perform state transitions compensates for the slow induction of qE during the early stages of high light acclimation (Allorent et al., 2013, Nagy et al., 2014).