The oxygen-evolving complex: a super catalyst for life on earth, in response to abiotic stresses

Abstract

The oxygen-evolving complex is integrated into photosystem (PSII). An essential part of oxygenic photosynthetic apparatus, embedded in the thylakoid membrane of chloroplasts. The OEC is a super catalyst to split water into molecular oxygen in the presence of light. The OEC consist of four Mn atoms, one Ca atom and five oxygen atoms (CaMn₄O₅) and this cluster is maintained by its surrounding proteins viz., PsbQ, PsbP, PsbO, PsbR. The function of this super catalyst with a high turnover frequency of 500 s^-1 in standard condition. Chlorophyll a fluorescence (OJIP transients) are used to understand structural and functional cohesion of photosynthetic apparatus. A further K-peak in OJIP curve reflects damage at the OEC donor site in response to salinity, drought, and high temperature. The decline in performance indices (PI, SFI) also revealed structural damage of photosynthetic apparatus that leads to disruption of electron transport rate under abiotic conditions. This review discusses the structural and function cohesion of the OEC in plant against variable abiotic conditions.

Keywords: OEC; OJIP; PSII; Salinity; molecular oxygen; water.

Figure 1.

Distribution of different proteins of photosynthetic apparatus embedded in the thylakoid membrane. Systematic scheme of conversion of light energy to chemical energy through photosynthetic electron transport chain between PSII and PSI (based on Gupta et al. 2020 and Huang et al., 2020)

Figure 2.

a: The OJIP transients of maize seedlings under seawater with or without exogenous manganese. Here O is for origin (minimal fluorescence Fo), J and I for two different inflexions (Fj and Fi) and P for peak (maximum fluorescence Fp or Fm). An additional K step in salinity treated maize seedlings observed after seven days of seawater exposure (based on Gupta, 2020). b: Effect of various abiotic factors viz., salinity, high and low temperature and high light intensity on extrinsic proteins i.e, PsbO, PsbP, PsbQ, and PsbR in higher plants. Instability of the OEC caused production of reactive oxygen species (ROS) that adversely affect photosynthetic efficiency in plants