Both lutein and zeaxanthin, along with β-carotene , also serve to to transfer incoming light back to the chlorophyll molecules for greater light harvesting efficiency.
In both photosynthetic and non-photosynthetic tissues, light and phytochrome signaling pathways are important for setting or entraining the plant circadian clock, a mechanism that produces self-sustained rhythms of ca. 24 h to enable anticipation of changes associated with the daily light and dark cycles (; ). In Arabidopsis, several studies have shown that carotenoid biosynthetic genes (including PSY and most MEP pathway genes) are clock-regulated, usually showing a peak phase of transcript abundance at around dawn (; ; ; ; ; ; ; ). Among the oscillating genes, only that encoding VDE shows peak transcript levels at dusk (). Diurnal oscillations have been found for the accumulation of several groups of MEP-derived plastidial isoprenoids, including the hormones gibberellins and carotenoid-derived ABA (; ; ; ), whereas chlorophyll levels (and plant growth) increase when the circadian clock period is matched to the external light-dark cycles (). Although these data suggest that carotenoid production might also be controlled by the circadian clock, perhaps allowing the plant to be more efficiently protected against photooxidative damage, experimental evidence is still missing.
Carotenoids serve as anti-oxidants in one of two ways.
With a slight 'S' angularity across the backbone and minor rotation at the ring interface, both to relieve steric strain, the majority of carotenoids achieve a highly functional delocalized system.
Genes and enzymes of carotenoid biosynthesis in plants.
Besides its role in the transcriptional control of the pathway, light also regulates carotenogenesis at multiple post-transcriptional levels. Light-driven processes in functional chloroplasts that result in non-enzymatic isomerization can substitute, at least in part, the activities of the Z-ISO and CRTISO isomerases in photosynthetic tissues (; ; ; ; ). Light also influences the activity of the carotenoid biosynthetic enzymes modulated by photosynthetic redox systems. The MEP pathway enzymes DXR, HDS and HDR appear to be targets of thioredoxin (; ), a member of the ferredoxin/thioredoxin system that is chemically reduced in photosynthetically-active chloroplasts to upregulate the activity of target proteins through the reduction of specific disulfide groups (; ). Because the carotenoid desaturases PDS and ZDS use plastoquinone as hydrogen acceptor, their activity is connected to the photosynthetic electron transport chain (). Both PDS and ZDS, as well as other carotenoid biosynthetic enzymes such as CRTISO, LCYB, LCYE, and ZEP, contain a conserved FAD-binding motif that suggests the involvement of redox balance in the corresponding enzymatic reactions (; ; ; ; ; ; ; ). There is also evidence for a redox control of the expression of some carotenoid biosynthetic genes in tobacco chloroplasts and tomato fruit chromoplasts (; ). Most interestingly, the activities of ZEP and VDE, which control the levels of zeaxanthin and violaxanthin, are tightly regulated by the light and photosynthesis status of the plant. Under light levels that exceed the maximum that can be productively used for photosynthesis, many plants adjust the carotenoid composition of leaves for photoprotection and transform violaxanthin into zeaxanthin to more efficiently dissipate the excess excitation energy. Low light conditions result in the transformation of zeaxanthin back into violaxanthin, in what is known as the xanthophyll cycle (; ; ; ). Although changes in gene expression in response to modified light conditions might contribute to the described effect in the xanthophyll profile (; ; ; ), the major driving force for regulating the activities of the enzymes involved in the xanthophyll cycle, VDE and ZEP, appears to be the light-induced changes in the trans-thylakoid pH. Under low light or in the dark, when the pH of the thylakoid stroma is neutral, the VDE enzyme remains soluble (mostly inactive) in the thylakoid stroma. But under high light the photosynthetic proton pump increases the acidity of the lumen, hence stimulating the binding of VDE to the thylakoid membrane and its enzymatic activity, eventually resulting in an enhanced production of zeaxanthin. The reaction catalyzed by ZEP is slow compared to that of VDE but it best functions at neutral pH (; ; ; ). Together, light not only regulates carotenoid gene expression but it also activates the metabolic flux through the pathway by increasing the activity of biosynthetic enzymes.