Membrane-Mediated Decrease in Root Exudation Responsible for Phorphorus Inhibition of Vesicular-Arbuscular Mycorrhiza Formation

Abstract

The mechanism responsible for phosphorus inhibition of vesicular-arbuscular mycorrhiza formation in sudangrass (Sorghum vulgare Pers.) was investigated in a phosphorus-deficient sandy soil (0.5 micrograms phosphorus per gram soil) amended with increasing levels of phosphorus as superphosphate (0, 28, 56, 228 micrograms per gram soil). The root phosphorus content of 4-week-old plants was correlated with the amount of phosphorus added to the soil. Root exudation of amino acids and reducing sugars was greater for plants grown in phosphorus-deficient soil than for those grown in the phosphorus-treated soils. The increase in exudation corresponded with changes in membrane permeability of phosphorus-deflcient roots, as measured by K' (86Rb) efflux, rather than with changes in root content of reducing sugars and amino acids. The roots of phosphorus-deficient plants inoculated at 4 weeks with Glomusfascicuaww were 88% infected after 9 weeks as compared to less than 25% infection in phosphorus-sufficient roots; these differences were correlated with root exudation at the time of inoculation. For plants grown in phosphorusdeficient soil, infection by vesicular-arbuscular mycorrhizae increased root phosphorus which resulted in a decrease in root membrane permeability and exudation compared to nonmycorrhizal plants. It is proposed that, under low phosphorus nutrition, increased root membrane permeability leads to net loss of metaboiltes at sufficient levels to sustain the germination and growth of the mycorrhizal fungus during pre-and postinfection. Subsequently, mycorrhizal infection leads to improvement of root phosphorus nutrition and a reduction in membrane-mediated loss of root metabolites.During the past 20 years, there has been a growing appreciation of the importance of VAM' in the improvement of plant growth through increased uptake of phosphorus (P) and other mineral nutrients, especially in soils of low fertility (5, 9). As evidence has mounted for the role of VAM in the enhancement of P uptake from P-deficient soils, it has also been recognized that high soil P levels severely limit VAM infection (9, 10).Early work failed to distinguish whether P was inhibiting the activity of the mycorrhizal fungus in the soil or during the hostfungus interaction (10,16 of sudangrass inoculated with Glomusfasciculatus (Thaxt.) Gerd. and Trappe only became heavily infected if the host was not receiving adequate P from the other half of the root system. It is now clear that the P content of the host plant is the critical factor controlling the mycorrhizal symbiosis.Ratnayake et al. (12) proposed that the mechanism of P control of VAM formation was associated with a membrane-mediated decrease in root exudation. They were able to correlate low P content of sudangrass and citrus roots with a decrease in phospholipid levels and a large increase in permeability of root membranes, which results in a greater net leakage of amino acids and sugars from the root. They suggested that under P-sufficient conditions, metabolites requi...

The plasma membrane proteome of Medicago truncatula roots as modified by arbuscular mycorrhizal symbiosis

“…Interestingly, a decrease in phospholipid levels in phosphatestarved roots has been correlated with an increase in permeability of root membranes (Ratnayake et al 1978). As this led to a greater leakage of amino acids and reducing sugars and increased mycorrhizal infection, Graham et al (1981) proposed a membrane-mediated decrease in root exudation might be responsible for Pi inhibition of mycorrhiza formation. The way by which phospholipid replacement affects the composition of the plasmalemmic proteome is largely unknown, but a proper phospholipid composition can be required for the recruitment of Arabidopsis proteins to the PM (Barbosa et al 2016).…”

Interaction of Light Intensity and Soil Temperature With Phosphorus Inhibition of Vesicular‐arbuscular Mycorrhiza Formation

“…After 5 weeks, two groups of three plants from each treatment were carefully washed free of soil, and analyzed for levels of soluble amino acids and total soluble sugars in root exudates, plant dry wt and total root P as previously described (Graham et al, 1981). Total soluble amino acids and total soluble sugar contents in root exudates were measured by standard procedures using ninhydrin (Spies, 1957) and sulphonated a-napthol (Denvor, 1950) reagents, respectively.…”

“…Inoculated and uninoculated soil at each P level was potted into 36 360 cm^ styrofoam cups. The microflora associated with the pot culture inoculum was established in non-mycorrhizal treatments by adding an inoculum water extract (Graham et al, 1981). Cups were seeded with sudangrass {Sorghum vulgare Pers.…”

“…Mycorrhizal formation was assessed for six plants per treatment by sampling a cross-section of the root system, and percentage root colonization was measured as previously described (Graham et al, 1981).…”

“…Phosphorus inhibition of vesicular-arbuscular mycorrhiza (VAM) formation may be associated with a membrane-mediated decrease in root exudation (Ratnayake, Leonard and Menge, 1978;Graham, Leonard and Menge, 1981). That is, when the phosphorus (P) content of the host is low, there is an increase in root membrane permeabihty and a greater loss of root cell metabohtes which are presumably necessary to sustain the germination and growth of the mycorrhizal fungus.…”