Arbuscular Mycorrhizal Symbiosis Alleviates Salt Stress in Black Locust through Improved Photosynthesis, Water Status, and K+/Na+ Homeostasis

Abstract

Soil salinization and the associated land degradation are major and growing ecological problems. Excess salt in soil impedes plant photosynthetic processes and root uptake of water and nutrients such as K+. Arbuscular mycorrhizal (AM) fungi can mitigate salt stress in host plants. Although, numerous studies demonstrate that photosynthesis and water status are improved by mycorrhizae, the molecular mechanisms involved have received little research attention. In the present study, we analyzed the effects of AM symbiosis and salt stress on photosynthesis, water status, concentrations of Na+ and K+, and the expression of several genes associated with photosynthesis (RppsbA, RppsbD, RprbcL, and RprbcS) and genes coding for aquaporins or membrane transport proteins involved in K+ and/or Na+ uptake, translocation, or compartmentalization homeostasis (RpSOS1, RpHKT1, RpNHX1, and RpSKOR) in black locust. The results showed that salinity reduced the net photosynthetic rate, stomatal conductance, and relative water content in both non-mycorrhizal (NM) and AM plants; the reductions of these three parameters were less in AM plants compared with NM plants. Under saline conditions, AM fungi significantly improved the net photosynthetic rate, quantum efficiency of photosystem II photochemistry, and K+ content in plants, but evidently reduced the Na+ content. AM plants also displayed a significant increase in the relative water content and an evident decrease in the shoot/root ratio of Na+ in the presence of 200 mM NaCl compared with NM plants. Additionally, mycorrhizal colonization upregulated the expression of three chloroplast genes (RppsbA, RppsbD, and RprbcL) in leaves, and three genes (RpSOS1, RpHKT1, and RpSKOR) encoding membrane transport proteins involved in K+/Na+ homeostasis in roots. Expression of several aquaporin genes was regulated by AM symbiosis in both leaves and roots depending on soil salinity. This study suggests that the beneficial effects of AM symbiosis on the photosynthetic capacity, water status, and K+/Na+ homeostasis lead to the improved growth performance and salt tolerance of black locust exposed to salt stress.

Beneficial role of acetylcholine in chlorophyll metabolism and photosynthetic gas exchange in Nicotiana benthamiana seedlings under salinity stress

“…Photosynthesis is a key physiological process known to be sensitive to salt stress (Chen et al . ). Previous studies have shown that ACh regulates plant growth and development processes mediated by phytochromes (Tretyn & Kendrick ).…”

Arbuscular mycorrhizal fungi (AMF) enhance the tolerance of Euonymus maackii Rupr. at a moderate level of salinity

“…salt stress (100 and 150 mM) and similar Gs values between different treatments at 200 mM salt. The lower reduction of Gs in mycorrhizal seedlings than nonmycorrhizal seedlings under moderate salt stress suggested that the growth status may be better in mycorrhizal seedlings, resulting from less of a decrease in CO 2 diffusion through the stomata [34]. The Ci values were higher than those under unstressed conditions, which may indicate that enzymes in the photosynthesis apparatus were destroyed, resulting in a decrease in CO 2 assimilation and the accumulation of CO 2 in intercellular areas [35].…”

Root endophytes improve physiological performance and yield in crops under salt stress by up-regulating the foliar sodium concentration

“…This latter effect was partially reverted when the plants growing in sterile soils were inoculated with the AMF Funneliformis mosseae (Chialva et al, 2018), indicating a key role for that kind of root symbiont. Arbuscular mycorrhizal fungi also improved the general performance of Robinia pseudoacacia seedlings under stress by NaCl salt (Chen et al, 2017). The overall physiological status of the plants (photosynthetic rate, photosystem II quantum efficiency and relative water content) was boosted by the symbiont, at the time that genes encoding membrane transport proteins involved in K + /Na + homeostasis in roots were found to be upregulated (Chen et al, 2017).…”

“…Arbuscular mycorrhizal fungi also improved the general performance of Robinia pseudoacacia seedlings under stress by NaCl salt (Chen et al, 2017). The overall physiological status of the plants (photosynthetic rate, photosystem II quantum efficiency and relative water content) was boosted by the symbiont, at the time that genes encoding membrane transport proteins involved in K + /Na + homeostasis in roots were found to be upregulated (Chen et al, 2017). Although AMF is by far one of the soil microorganism groups most studied likely due to its importance as provider mutualist (Omacini et al, 2012; Pieterse et al, 2014), other root colonizers (e.g., Trichoderma spp., Piriformospora indica , Penicillium spp.)…”