|Looking down into a membrane a rough sketch of PSII core (green) surrounded by the various antenna complexes. |
These antennas are several other protein complexes called Light Harvesting Complexes (LHC). The LHC have many chlorophyll and accessory pigments that can absorb light. The energy absorbed is then to their neighbor chlorophylls all the way to the special chlorophyll in the reaction core so that ATP can be made. Think of these pathways as a line of people standing next to each other, shoulder to shoulder, one standing at the goal line. Balls (photons) are tossed randomly at the line and whoever catches one can pass it down along the line of people to the goal. The greater amount people in the line, or even lines, the higher percentage of balls they will catch. For the plant, the increase in surface area due to the LHC allows a high percentage of light to actually reach the goal line of photosystem II.
Carotenes and xanthophylls are two important light capturing pigments. They extend the wavelength range of light the plant can use. Previously, we looked at carotenes. There is only difference between carotenes and xanthophylls. Carotenes are made of only carbon and hydrogen, while xanthophylls also contain oxygen. All xanthophylls are derived from carotene precursors in the same fashion as described in my previous post. A few specific xanthophylls are incredibly important for photosynthetic regulation. Their names are: violaxanthin. antheraxanthin, and zeaxanthin. But as my son would say, we'll just call them V A Z "for short".
Photosynthetic stressors generate excess amounts of reactive oxygen species, including singlet chlorophyll. This decrease the pH triggering the conversion of V to Z. The conversion happens quickly, allowing the plant to get rid of excess energy before it can cause irreparable harm.
|Violaxanthin cycle ▪ Yikrazuul ▪ Public Domain ▪ Wikipedia|
Another type of non-photochemical quenching involves LHCII aggregation. Aggregating LHC results in the removal of antenna from PSII reducing damage to the core. This occurs when more Z than V is present in the membrane. The mechanism behind this was examined in a paper that came out this month (Janik et al 2016). Both V and Z can bind to a pocket in LHCII which causes either the stabilization or destabilzation of LHCII complexes. When light is high, stress is high, and more Z is made resulting in more Z in the LHCII pocket which destabilizes the LHCII trimers into monomers. The researchers suggest that this could allow the monomers to form an as of yet unknown complex of LHCII that increase quenching. It is an interesting proposition.
Regardless of how it works, xanthophylls are critical for the survival of plants due to their ability to remove reactive oxygen species formed during normal, but especially stressed, photosynthesis.
- Goss R, Lepetit B (2015) Biodiversity of NPQ. Journal of Plant Physiology 172:13-32
- Janik et al, (2016) The xanthophyll cycle pigments, violaxanthin and zeaxanthin, modulate molecular organization of the photosynthetic antenna complex LHCII. Archives of Biochemistry and Biophysics In Press http://www.sciencedirect.com.proxy.ulib.uits.iu.edu/science/article/pii/S0003986116300030
- Ruban AV, Johnson MP, Duffy CDP (2012) The photoprotective molecular switch in the photosystem II antenna. Biochimica et Biophysics Acta 1817:167-181