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

Liantai Su,Wuwu Wen,Yuan An,Aimin Lv,Peng Zhou

doi:10.3389/fpls.2020.00746

Liantai Su, Wuwu Wen + Show 3 more

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https://doi.org/10.3389/fpls.2020.00746

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Abstract

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 (gH+), and activities of ATP synthase and PM H+-ATPase significantly increased, and proton gradient (ΔpHpmf) between lumen and stroma decreased under Al stress. After removing apical buds of seedlings, the Y(I), Y(II), ETRI, ETRII, pmf, and gH+ under exogenous spraying IAA significantly increased, and ΔpHpmf 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 ΔpHpmf.

Highlights

Aluminum (Al) is the most abundant metal and is widely distributed in nature in the form of silicates or other deposits
Application of Mg and indole-3-acetic acid (IAA) significantly alleviated the Al-induced inhibition of growth, and the weights of shoots and roots were significantly higher under Mg and IAA application either alone or combination than that under Al treatments alone on days 3 and 6
Excess Al significantly decreased net photosynthetic rate (Pn) and contents of Chl a and Chl b, which were positively related to the reductions of maximum quantum yield of primary photochemistry (Fv/Fm), Y(II), photosynthetic electron flow through photosystem II (PSII) (ETRII), electron transport rate (ETR) through photosystem I (PSI) (ETRI), and rETRmax of PSI and PSII, indicating that Alinduced decreases in Pn were greatly attributed to the inhibition of photosystems, rather than stoma limitation

Summary

Introduction

Aluminum (Al) is the most abundant metal and is widely distributed in nature in the form of silicates or other deposits. Excess Al induced reduction in CO2 assimilation has been found in many crops like rye (Silva et al, 2012) and wheat (Julietta et al, 2016). The main photosynthetic apparatus consists of the light-harvesting PSI, PSII, and cytochrome b6f (Mikko and Aro, 2014). PSII, the “engine of life,” is the photosynthetic enzyme that uses sunlight energy to extract electrons from water for conversion of inorganic molecules into organic molecules (James, 2009). PSI is a multi-subunit pigment–protein complex embedded in the thylakoid membrane of the chloroplast. It catalyzes light-driven electron transfer from PC [a luminal mobile protein that receives electrons from cytochrome b6/f (Cyt b6/f)] to Fd located at the stromal side. The RC of PSI is P700, which is an electron donor that consists of a dimer of Chl a molecule (Hasni et al, 2015a)

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