Nonphotochemical quenching in plants: Mechanisms and mysteries

Plants are vulnerable to photodamage when exposed to light intensities that exceed their photosynthetic capacity. To protect themselves, they activate nonphotochemical quenching (NPQ), a set of processes that dissipate excess excitation energy as heat. NPQ has been studied extensively; however, the field remains conceptually fragmented, and consensus on the underlying mechanisms has yet to be reached. Interest in NPQ has recently intensified due to studies showing that tuning NPQ regulation can lead to substantial improvements in photosynthetic efficiency and even crop yield increases of up to 30%. In this review, we aim to bring structure to the diverse and sometimes contradictory NPQ literature by framing the discussion around a set of key mechanistic questions. We focus on the fastest component of NPQ, known as qE, which is activated within the first minutes of excess light exposure. Topics addressed include the molecular properties and roles of PsbS and zeaxanthin, potential conformational changes in light-harvesting complexes, reorganization of the thylakoid membrane, and the interplay among these factors. We synthesize the available evidence into a working model in which qE arises largely from a localized conformational switch in a small number of antenna complexes, triggered by PsbS, whereas zeaxanthin increases the domain size of the antenna that can be quenched by each of these quenchers.