Protection from too much light: Photosynthetic organisms protect themselves from too much light using pigment photoswitches that absorb excess energy. Leverenz et al. analyzed the structure of an active, energy-dissipating form of the orange carotenoid protein (OCP) from a cyanobacterium. When activated by excess light, OCP moves its hydrophobic carotenoid pigment 12 Å within the protein to accommodate nonphotochemical quenching by the broader photosynthetic antenna complex. Science , this issue p. 1463

The photoprotective processes of photosynthetic organisms involve the dissipation of excess absorbed light energy as heat. Photoprotection in cyanobacteria is mechanistically distinct from that in plants; it involves the orange carotenoid protein (OCP), a water-soluble protein containing a single carotenoid. The OCP is a new member of the family of blue light-photoactive proteins; blue-green light triggers the OCP-mediated photoprotective response. Here we report structural and functional characterization of the wild type and two mutant forms of the OCP, from the model organism Synechocystis PCC6803. The structural analysis provides high resolution detail of the carotenoid-protein interactions that underlie the optical properties of the OCP, unique among carotenoid-proteins in binding a single pigment per polypeptide chain. Collectively, these data implicate several key amino acids in the function of the OCP and reveal that the photoconversion and photoprotective responses of the OCP to blue-green light can be decoupled.

Summary: Inactivation of Arabidopsis WAT1 (Walls Are Thin1), a gene required for secondary cell‐wall deposition, conferred broad‐spectrum resistance to vascular pathogens, including the bacteria Ralstonia solanacearum and Xanthomonas campestris pv. campestris, and the fungi Verticillium dahliae and Verticillium albo‐atrum. Introduction of NahG, the bacterial salicylic acid (SA)‐degrading salicylate hydroxylase gene, into the wat1 mutant restored full susceptibility to both R. solanacearum and X. campestris pv. campestris. Moreover, SA content was constitutively higher in wat1 roots, further supporting a role for SA in wat1‐mediated resistance to vascular pathogens. By combining transcriptomic and metabolomic data, we demonstrated a general repression of indole metabolism in wat1‐1 roots as shown by constitutive down‐regulation of several genes encoding proteins of the indole glucosinolate biosynthetic pathway and reduced amounts of tryptophan (Trp), indole‐3‐acetic acid and neoglucobrassicin, the major form of indole glucosinolate in roots. Furthermore, the susceptibility of the wat1 mutant to R. solanacearum was partially restored when crossed with either the trp5 mutant, an over‐accumulator of Trp, or Pro35S:AFB1‐myc, in which indole‐3‐acetic acid signaling is constitutively activated. Our original hypothesis placed cell‐wall modifications at the heart of the wat1 resistance phenotype. However, the results presented here suggest a mechanism involving root‐localized metabolic channeling away from indole metabolites to SA as a central feature of wat1 resistance to R. solanacearum.

Cyanobacteria have developed a photoprotective mechanism that decreases the energy arriving at the photosynthetic reaction centers under high-light conditions. The photoactive orange carotenoid protein (OCP) is essential in this mechanism as a light sensor and energy quencher. When OCP is photoactivated by strong blue-green light, it is able to dissipate excess energy as heat by interacting with phycobilisomes. As a consequence, charge separation and recombination leading to the formation of singlet oxygen diminishes. Here, we demonstrate that OCP has another essential role. We observed that OCP also protects Synechocystis cells from strong orange-red light, a condition in which OCP is not photoactivated. We first showed that this photoprotection is related to a decrease of singlet oxygen concentration due to OCP action. Then, we demonstrated that, in vitro, OCP is a very good singlet oxygen quencher. By contrast, another carotenoid protein having a high similarity with the N-terminal domain of OCP is not more efficient as a singlet oxygen quencher than a protein without carotenoid. Although OCP is a soluble protein, it is able to quench the singlet oxygen generated in the thylakoid membranes. Thus, OCP has dual and complementary photoprotective functions as an energy quencher and a singlet oxygen quencher.