Characterization of the structural changes and photochemical activity of photosystem I under Al(3+) effect

Interaction of zinc and IAA alleviate aluminum-induced damage on photosystems via promoting proton motive force and reducing proton gradient in alfalfa

BACKGROUND

In acidic soils, aluminum (Al) competing with Zn results in Zn deficiency in plants. Zn is essential for auxin biosynthesis. Zn-mediated alleviation of Al toxicity has been rarely studied, the mechanism of Zn alleviation on Al-induced photoinhibition in photosystems remains unclear. The objective of this study was to investigate the effects of Zn and IAA on photosystems of Al-stressed alfalfa. Alfalfa seedlings with or without apical buds were exposed to 0 or100 μM AlCl3 combined with 0 or 50 μM ZnCl2, and then foliar spray with water or 6 mg L- 1 IAA.

RESULTS

Our results showed that Al stress significantly decreased plant growth rate, net photosynthetic rate (Pn), quantum yields and electron transfer rates of PSI and PSII. Exogenous application of Zn and IAA significantly alleviated the Al-induced negative effects on photosynthetic machinery, and an interaction of Zn and IAA played an important role in the alleviative effects. After removing apical buds of Al-stressed alfalfa seedlings, the values of pmf, gH+ and Y(II) under exogenous spraying IAA were significantly higher, and ΔpHpmf was significantly lower in Zn addition than Al treatment alone, but the changes did not occur under none spraying IAA. The interaction of Zn and IAA directly increased Y(I), Y(II), ETRI and ETRII, and decreased O2- content of Al-stressed seedlings. In addition, the transcriptome analysis showed that fourteen functionally noted genes classified into functional category of energy production and conversion were differentially expressed in leaves of alfalfa seedlings with and without apical buds.

CONCLUSION

Our results suggest that the interaction of zinc and IAA alleviate aluminum-induced damage on photosystems via increasing pmf and decreasing ΔpHpmf between lumen and stroma.

Auxin Is Involved in Magnesium-Mediated Photoprotection in Photosystems of Alfalfa Seedlings Under Aluminum Stress

The objective of this study was to investigate the effects of Mg and IAA on the photosystems of Al-stressed alfalfa (Medicago sativa L.). Alfalfa seedlings with or without apical buds were exposed to solutions fully mixed with 0 or 100 μM AlCl₃ and 0 or 50 μM MgCl₂ followed by foliar spray with water or IAA. Results from seedlings with apical buds showed that application of Mg and IAA either alone or combine greatly alleviated the Al-induced damage on photosystems. The values of photosynthetic rate (Pn), effective quantum yields [Y(I) and Y(II)] and electron transfer rates (ETRI and ETRII), proton motive force (pmf), cyclic electron flow (CEF), proton efflux rate (g_H ⁺), and activities of ATP synthase and PM H⁺-ATPase significantly increased, and proton gradient (ΔpH _pmf ) between lumen and stroma decreased under Al stress. After removing apical buds of seedlings, the Y(I), Y(II), ETRI, ETRII, pmf, and g_H ⁺ under exogenous spraying IAA significantly increased, and ΔpH _pmf significantly decreased in Mg addition than Al treatment alone, but they were no significant difference under none spraying IAA. The interaction of Mg and IAA directly increased quantum yields and electron transfer rates, and decreased O₂ ^- accumulation in Al-stressed seedlings with or without apical buds. These results suggest that IAA involves in Mg alleviation of Al-induced photosystem damage via increasing pmf and PM H⁺-ATPase activity, and decreasing ΔpH _pmf .

Photosystem II Is More Sensitive than Photosystem I to Al3+ Induced Phytotoxicity

Aluminium (Al) the most abundant metal in the earth's crust is toxic in acid soils (pH < 5.5) mainly in the ionic form of Al3+ species. The ability of crops to overcome Al toxicity varies among crop species and cultivars. Here, we report for a first time the simultaneous responses of photosystem II (PSII) and photosystem I (PSI) to Al3+ phytotoxicity. The responses of PSII and PSI in the durum wheat (Triticum turgidum L. cv. 'Appulo E') and the triticale (X Triticosecale Witmark cv. 'Dada') were evaluated by chlorophyll fluorescence quenching analysis and reflection spectroscopy respectively, under control (-Al, pH 6.5) and 148 μM Al (+Al, pH 4.5) conditions. During control growth conditions the high activity of PSII in 'Appulo E' led to a rather higher electron flow to PSI, which induced a higher PSI excitation pressure in 'Appulo E' than in 'Dada' that presented a lower PSII activity. However, under 148 μM Al the triticale 'Dada' presented a lower PSII and PSI excitation pressure than 'Appulo E'. In conclusion, both photosystems of 'Dada' displayed a superior performance than 'Appulo E' under Al exposure, while in both cultivars PSII was more affected than PSI from Al3+ phytotoxicity.

Physiological changes in Chlamydomonas reinhardtii after 1000 generations of selection of cadmium exposure at environmentally relevant concentrations

Cadmium (Cd) is a nonessential and toxic trace element widely existing in waters through various anthropogenic activities such as mining and waste disposal. The physiological responses of aquatic organisms to long-term Cd exposure at environmentally relevant concentrations are still not well explored. In the present study, two strains of unicellular green algae Chlamydomonas reinhardtii, a walled strain CC125 and a wall-less strain CC406 were selected to investigate the physiological changes of aquatic organisms after long-term Cd exposure at environmentally relevant concentrations (4.92 and 49.2 μg L-1). After about 1000 generations of selection, all of the two strains showed higher intracellular lipid peroxidation and lower photosynthetic activities, and failed to evolve specific adaptation to high levels of Cd (4.92 mg L-1) compared to the control. However, short-term low dose Cd exposure exerted hormetic effects on C. reinhardtii and the hormetic stimulation of growth rate, chlorophyll contents and photochemical activities at the lower concentration of Cd (4.92 μg L-1) groups were more pronounced than those at higher ones (49.2 μg L-1). Taken together, this study confirmed that long-term exposure to Cd at environmentally relevant concentrations which were regarded as nontoxic in acute experiments would produce toxic effects on C. reinhardtii and should be paid more attention.