Abstract
A feature of arbuscular mycorrhiza is enhanced drought tolerance of host plants, although it is unclear whether host H+-ATPase activity and gene expression are involved in the physiological process. The present study aimed to investigate the effects of an arbuscular mycorrhizal fungus (AMF), Funneliformis mosseae, on H+-ATPase activity, and gene expression of trifoliate orange (Poncirus trifoliata) seedlings subjected to well-watered (WW) and drought stress (DS), together with the changes in leaf gas exchange, root morphology, soil pH value, and ammonium content. Soil drought treatment dramatically increased H+-ATPase activity of leaf and root, and AMF inoculation further strengthened the increased effect. A plasma membrane (PM) H+-ATPase gene of trifoliate orange, PtAHA2 (MW239123), was cloned. The PtAHA2 expression was induced by mycorrhization in leaves and roots and also up-regulated by drought treatment in leaves of AMF-inoculated seedlings and in roots of AMF- and non-AMF-inoculated seedlings. And, the induced expression of PtAHA2 under mycorrhization was more prominent under DS than under WW. Mycorrhizal plants also showed greater photosynthetic rate, stomatal conductance, intercellular CO2 concentration, and transpiration rate and better root volume and diameter than non-mycorrhizal plants under DS. AMF inoculation significantly increased leaf and root ammonium content, especially under DS, whereas it dramatically reduced soil pH value. In addition, H+-ATPase activity was significantly positively correlated with ammonium contents in leaves and roots, and root H+-ATPase activity was significantly negatively correlated with soil pH value. Our results concluded that AMF stimulated H+-ATPase activity and PtAHA2 gene expression in response to DS, which resulted in great nutrient (e.g., ammonium) uptake and root growth, as well as low soil pH microenvironment.
Highlights
Drought is one of the most serious environmental stresses, which severely restrains the growth and productivity of crop by destroying various physiological and biochemical processes such as nutrient absorption, photosynthesis, and cell metabolism (Zhang et al, 2020)
According to the “acid” growth theory, plant growth requires acidification of the cell wall space, and such acidification is derived by H+ effluxes of Plasma membrane (PM) H+-ATPase, indicating that the H+ efflux is regulated by the proton pump, and the acidification of the cell wall is important for root elongation (Staal et al, 2011)
Earlier studies showed that mycorrhizal plants had greater leaf gas exchange capacity than non-mycorrhizal plants subjected to abiotic stress (e.g., DS and salt stress) (Porcel and Ruiz-Lozano, FIGURE 5 | Effects of Funneliformis mosseae on soil pH value in trifoliate orange (Poncirus trifoliata) seedlings subjected to well-watered (WW) and drought stress (DS)
Summary
Drought is one of the most serious environmental stresses, which severely restrains the growth and productivity of crop by destroying various physiological and biochemical processes such as nutrient absorption, photosynthesis, and cell metabolism (Zhang et al, 2020). Studies have demonstrated that cells regulated ion balances by changing the transmembrane transport of both ions and small molecules to maintain cell osmotic pressure under drought stress (DS) (Mak et al, 2014). Based on an electrical gradient inside and outside, H+ enters and exits the cell PM providing a driving force for the transport of small molecules, which is associated with mineral nutrient (NH4+) absorption, metabolite discharge, cytoplasmic pH regulation, cell growth, and stomata opening (Gaxiola et al, 2007). In Arabidopsis, a total of 11 PM H+-ATPase genes are identified and defined as AHA1–AHA11 (Arabidopsis PM H+-ATPase isoforms) (Palmgren, 2001) Among these isoforms, AHA2 is a housekeeping gene expressed at high levels and is the predominant proton pump in plant roots, contributing to the pH homeostasis and root growth and development (Młodziñska et al, 2014)
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