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.
Highlights
Soil salinization and the associated land degradation are major and growing ecological problems, in arid and semiarid areas (Porcel et al, 2012, 2015)
The dry masses of the shoots and roots of both NM and Arbuscular mycorrhizal (AM) plants were reduced by salt stress, AM plants grew better than NM plants at all salinity levels, which suggests that the AM symbiosis mitigated salt stress in black locust and indicates a high symbiosis efficiency of R. irregularis
The improvement in photosynthesis by AM fungi was related to a lower reduction in stomatal conductance (Gs), a higher PSII operating efficiency (PSII) efficiency and higher expression of three chloroplastic genes (RprbcL, RppsbA, and RppsbD) compared with NM plants
Summary
Soil salinization and the associated land degradation are major and growing ecological problems, in arid and semiarid areas (Porcel et al, 2012, 2015). The D1 and D2 proteins, which are in the PSII reaction center and are pivotal in the phosphorylationcoupled linear electron flow (Jansen et al, 1996), degrade under salt stress (Sudhir et al, 2005; Neelam and Subramanyam, 2013). Another challenge faced by plants exposed to salt stress is acquiring sufficient amounts of water from the soil (Ouziad et al, 2006). Plant aquaporins are categorized into five subfamilies (Ruiz-Lozano et al, 2012), of which plasma membrane intrinsic proteins (PIPs) and tonoplast intrinsic proteins (TIPs) are the most abundant in plant plasma and vacuolar membranes, respectively (Luu and Maurel, 2005), and serve as the primary path for transcellular and intracellular water movement, respectively (Maurel et al, 2008)
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