Abstract
Light is a paramount parameter driving photosynthesis. However, excessive irradiance leads to the formation of reactive oxygen species that cause cell damage and hamper the growth of photosynthetic organisms. Xanthophylls are key pigments involved in the photoprotective response of plants and algae to excessive light. Of particular relevance is the operation of xanthophyll cycles (XC) leading to the formation of de-epoxidized molecules with energy dissipating capacities. Neoxanthin, found in plants and algae in two different isomeric forms, is involved in the light stress response at different levels. This xanthophyll is not directly involved in XCs and the molecular mechanisms behind its photoprotective activity are yet to be fully resolved. This review comprehensively addresses the photoprotective role of 9′-cis-neoxanthin, the most abundant neoxanthin isomer, and one of the major xanthophyll components in plants’ photosystems. The light-dependent accumulation of all-trans-neoxanthin in photosynthetic cells was identified exclusively in algae of the order Bryopsidales (Chlorophyta), that lack a functional XC. A putative photoprotective model involving all-trans-neoxanthin is discussed.
Highlights
Xanthophylls’ Role in PhotoprotectionSunlight is essential to power the photochemical reactions of photosynthesis, but a surplus of excitation energy produces various radicals and reactive oxygen species (ROS) that can damage the photosynthetic apparatus, leading to a decline in photosynthetic activity, growth, and productivity (i.e., “photoinhibition”) [1,2]
In addition to its indirect role in chlorophyll-to-carotenoid triplet transfer, 90 -cis-neoxanthin is directly involved in photoprotection as an antioxidant: antenna complexes lacking this molecule were reported to be more sensitive to photobleaching and showed a reduced capacity of ROS scavenging under excessive light [15,48]
Strong evidence has been found for a photoprotective role of the xanthophyll pigment neoxanthin in plants and algae
Summary
Sunlight is essential to power the photochemical reactions of photosynthesis, but a surplus of excitation energy produces various radicals and reactive oxygen species (ROS) that can damage the photosynthetic apparatus, leading to a decline in photosynthetic activity, growth, and productivity (i.e., “photoinhibition”) [1,2]. XCs are considered ubiquitous in the plant kingdom, but variations can be present, and different xanthophyll molecules can be involved: six different types of xanthophyll cycles have been described to date (for an extensive review on this topic see [11]) Despite their wide occurrence, the importance of XCs for photoprotection can vary between taxa: while their contribution to NPQ (in terms of qE) is known to be extremely relevant in higher plants, it was reported to be less significant in green algae [12]. In the late 30s, Strain [18] first described the presence of a common, widely distributed xanthophyll in barley leaves, which he named neoxanthin According to his experimental studies, this pigment (molecular formula: C40 H56 O4 ) was one of the three major carotenoids of plant leaves (together with lutein and violaxanthin) and had absorption properties close to that of violaxanthin (major peaks of absorption at 437 and 467 nm in ethanol). Epoxy xanthophylls (violaxanthin and antheraxanthin) [26]
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