Photostasis in Plants, Green Algae and Cyanobacteria: The Role of Light Harvesting Antenna Complexes

Abstract

The structural, as well as the functional, light-harvesting antenna size of the photosystems in plants, green algae and cyanobacteria may vary as a function of either short term stress or long term acclimation to high or low irradiance, suboptimal or supraoptimal temperatures as well as limitations in nutrient and water availability. Modulation of antenna size in response to such environmental perturbations is part of a complex response called photoacclimation. A common effect of changes in these environmental factors is the creation of an imbalance between the energy absorbed through photochemistry and the energy utilized through the electrochemical reactions of electron transport which are coupled to the metabolic reduction of C, N and S. Either short term stress or long term acclimation to these environmental conditions, independently or in combination, may lead to irreversible photodamage or the induction of photoprotective mechanisms. Since there is a consensus in the literature that the structure and function of Photosystem II are generally more sensitive to changes in these environmental conditions than Photosystem I, we focus our discussion on the role of the light-harvesting antenna of Photosystem II in photoprotection through the maintenance of a balance between energy input through photochemistry and subsequent energy utilization through metabolism. The predisposition of photosynthetic organisms to maintain such a balance in energy budget is defined as photostasis. Any change in either photon flux, temperature, nutrient status or water availability may cause an imbalance in energy budget which occurs whenever $σ$ psII {\textbullet}I >n {\textbullet} $τ$ −1 where $σ$ PSII is the functional absorption cross section of PS II, I is the incident photon flux, n is the number of photosynthetic units and $τ$−1 is the rate at which metabolism consumes photosynthetically genereated electrons. Photosynthetic acclimation, induced by short and long term exposures to low or high light, low temperature, nutrient and water limitation, is discussed with respect to the modulation of $σ$ PSII, I, and metabolic sink capacity ($τ$ −1 ) to restore photostasis and minimize photodamage to PS II in plants, green algae and cyanobacteria. It appears that the plastoquinone pool and/or the Cyt b6f complex may act as the primary sensor for the maintenance of photostasis. We suggest that sensing/signaling associated with environmentally induced energy imbalances in terrestrial plants, green algae and cyanobacteria appears to exert a broad influence on diverse molecular, physiological and developmental process which is consistent with the concept of a `grand design of photosynthesis' initially proposed by Daniel Arnon in 1982.