Abstract
Peanut is one of the calciphilous plants. Calcium (Ca) serves as a ubiquitous central hub in a large number of signaling pathways. The effect of exogenous calcium nitrate [Ca(NO3)2] (6 mM) on the dissipation of excess excitation energy in the photosystem II (PSII) antenna, especially on the level of D1 protein and the xanthophyll cycle in peanut plants under heat (40°C) and high irradiance (HI) (1 200 µmol m−2 s−1) stress were investigated. Compared with the control plants [cultivated in 0 mM Ca(NO3)2 medium], the maximal photochemical efficiency of PSII (Fv/Fm) in Ca2+-treated plants showed a slighter decrease after 5 h of stress, accompanied by higher non-photochemical quenching (NPQ), higher expression of antioxidative genes and less reactive oxygen species (ROS) accumulation. Meanwhile, higher content of D1 protein and higher ratio of (A+Z)/(V+A+Z) were also detected in Ca2+-treated plants under such stress. These results showed that Ca2+ could help protect the peanut photosynthetic system from severe photoinhibition under heat and HI stress by accelerating the repair of D1 protein and improving the de-epoxidation ratio of the xanthophyll cycle. Furthermore, EGTA (a chelant of Ca ion), LaCl3 (a blocker of Ca2+ channel in cytoplasmic membrane), and CPZ [a calmodulin (CaM) antagonist] were used to analyze the effects of Ca2+/CaM on the variation of (A+Z)/(V+A+Z) (%) and the expression of violaxanthin de-epoxidase (VDE). The results indicated that CaM, an important component of the Ca2+ signal transduction pathway, mediated the expression of the VDE gene in the presence of Ca to improve the xanthophyll cycle.
Highlights
Plants are frequently subject to various environmental stresses
To investigate whether enhanced photosystem II (PSII) function is associated with less reactive oxygen species (ROS) accumulation under heat and high irradiance (HI) stress, we examined the accumulation of H2O2 and O2N
Heat stress often occurs in combination with light stress
Summary
Plants are frequently subject to various environmental stresses. During summer, high temperature and high irradiance (HI) are the common stresses which plants are always faced with. Excess excitation energy, when not dissipated harmlessly, would be transformed to O2 to form reactive oxygen species (ROS) which could damage the photosynthetic apparatus, e.g. D1 protein, encoded by the psbA gene, can be used to reflect the degree of photoinhibition of PSII [2,3,4]. The repair of damaged PSII centers involves the degradation and de novo synthesis of this polypeptide in mature chloroplasts [5,6]. This efficient repair mechanism is essential to maintain PSII in a functional state. The effects of exogenous calcium (Ca) on photosynthesis have been widely reported, its role on D1 protein under heat and HI stress requires further study
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