Abstract
Non-photochemical quenching (NPQ) is an important photoprotective mechanism in plants and algae. Although the process is extensively studied, little is known about its relationship with ultrastructural changes of the thylakoid membranes. In order to better understand this relationship, we studied the effects of illumination on the organization of thylakoid membranes in Monstera deliciosa leaves. This evergreen species is known to exhibit very large NPQ and to possess giant grana with dozens of stacked thylakoids. It is thus ideally suited for small-angle neutron scattering measurements (SANS)—a non-invasive technique, which is capable of providing spatially and statistically averaged information on the periodicity of the thylakoid membranes and their rapid reorganizations in vivo. We show that NPQ-inducing illumination causes a strong decrease in the periodic order of granum thylakoid membranes. Development of NPQ and light-induced ultrastructural changes, as well as the relaxation processes, follow similar kinetic patterns. Surprisingly, whereas NPQ is suppressed by diuron, it impedes only the relaxation of the structural changes and not its formation, suggesting that structural changes do not cause but enable NPQ. We also demonstrate that the diminishment of SANS peak does not originate from light-induced redistribution and reorientation of chloroplasts inside the cells.
Highlights
Oxygenic photosynthetic organisms protect themselves against photodamage under excess light conditions
Light-induced thylakoid membrane reorganizations associated with Non-photochemical quenching (NPQ)
The most dominant alteration of the scattering peak was the decrease of the integrated intensity of the Bragg peak; the variation was largely reversible upon a 30 min dark readaptation
Summary
Oxygenic photosynthetic organisms protect themselves against photodamage under excess light conditions. In algae and higher plants, one of the most important photoprotective mechanisms is the non-photochemical quenching (NPQ) of the first singlet excited state of chlorophyll-a. The underlying molecular mechanisms of NPQ are still debated [4,5,6,7,8,9]. In excess light, sustained acidification of the lumen is sensed by the PsbS protein in plants [10,11] and it activates the xanthophyll cycle, leading to the conversion of violaxanthin to zeaxanthin [12,13,14,15]; both of these effects contribute to the generation of NPQ [9,16]
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