Symbiotic Characteristics Of Pinus Massoniana And Suillus Glutinosus Under Low Phosphorus And Aluminum Stres

Acid soil is the main soil in South China.Low phosphorus and aluminum toxicity are the main factors restricting plant growth in acid soil.Aluminum is easy to be activated in acidic soil,and the activated aluminum is easy to combine with phosphorus in the soil,so as to reduce the availability of phosphorus and make the originally barren soil more deficient in nutrition.Ectomycorrhiza(ECM)fungi generally exist in forest ecosystems and can form reciprocal symbionts with most trees.It has an important contribution to promoting tree growth and improving the stress resistance of trees.Based on the widespread distribution of ECM tree species in acid soil environment,we speculate that ECM plays an important role in helping trees adapt to low phosphorus and aluminum acid environment.At present,the adaptation mechanism of ECM to low phosphorus and aluminum phosphate environment(especially the metabolic adaptation of ECM fungi and ECM to low aluminum phosphate stress)is still largely unknown.Based on this,this study took Pinus massoniana,the main tree species in southern China,and its dominant ECM Fungus Suillus bovinus under the forest as the research object,and studied the effects of low phosphorus,aluminum acid and combined stress of low phosphorus and aluminum acid on the growth and metabolism of S.bovinus,Formation of symbiosis between P.massoniana and S.bovinus and its effects on metabolism of symbiosis between P.massoniana and S.bovinus.The main results are as follows:(1)Effects of low phosphorus and aluminum acid stress on growth and metabolism of S.bovinus.Low phosphorus and aluminum acid stress are the main factors limiting plant growth in acidic soil.Studies have pointed out that ectomycorrhiza(ECM)can improve the adaptability of host plants to aluminum toxicity and low phosphorus stress.However,the adaptation mechanism of ECM fungi to low phosphorus and aluminum acid environment is not clear.Based on this,this study took the ECM Fungus S.bovinus,which is widely distributed in acid soil in South China,as the research object,and studied the effects of low phosphorus and aluminum acid stress on its growth,nutrient absorption and mycelial secretion under pure culture conditions.The results showed that S.bovinus was an aluminum tolerant fungus.Acid aluminum stress(1 mmol / L)did not affect its mycelial growth,but low phosphorus stress(20 μmol / L)significantly limited its mycelial growth(P < 0.05).It should be noted that the inhibitory effect of low phosphorus stress can be reversed by Aluminate stress.Low phosphorus stress significantly reduced the absorption of phosphorus by S.bovinus(P < 0.05),while acid aluminum stress promoted the absorption of potassium by mycelium.Low phosphorus and aluminum acid stress also significantly changed the composition of mycelial secretion.Under low phosphorus stress,the accumulation of a large number of phenolic acids,organic acids and lipid metabolites decreased;Under aluminum acid stress,a large number of phenolic acids were up-regulated,and the number of up-regulated metabolites in organic acids and lipids was also higher than that in down-regulated ones;The accumulation of phenolic acid and organic acid metabolites were significantly up-regulated under low aluminum phosphate compound stress.In addition,indole-3-acetic acid(IAA)was significantly up-regulated under all stress.(2)Effects of low phosphorus and aluminum acid stress on the formation of symbiosis between S.bovinus and P.massoniana.Under the environment of phosphorus and aluminum stress,S.bovinus can form symbiosis with P.massoniana.After 7 days of inoculation,the mycelium of each treatment had been in contact with the root system.At 28 days,the root system of P.massoniana had been successfully colonized,and typical Haversian net and bacterial sheath structure had been formed.In contrast,the infection process of S.bovinus on P.massoniana roots under low phosphorus stress is relatively backward(only a small amount of hyphae gather outside the roots at 7 days),while the infection process of S.bovinus on P.massoniana roots under low phosphorus and aluminum acid combined stress is not inhibited,which is consistent with the experimental results of the effects of low phosphorus and aluminum acid stress on the growth of Suillus bovinus.It was also found that inoculation with S.bovinus improved the root tip activity under various stress treatments,and the number of dead cells in the root tip of P.massoniana under various stress treatments after inoculation was greatly reduced compared with that without S.bovinus.In addition,inoculation with S.bovinus significantly reduced the aluminum deposition in the root tip of P.massoniana,but the aluminum deposition in the mycelium outside the root is obvious.The mycelium of S.bovinus can reduce the aluminum deposition in the root tip of P.massoniana by enriching aluminum,and further reduce the effect of aluminum toxicity on the root tip activity.(3)Effects of low phosphorus and aluminum acid stress on the function of symbiosis between P.massoniana and S.bovinus.Inoculation of S.bovinus under phosphorus and aluminum stress can significantly promote the growth of P.massoniana.Compared with non bacterial root seedlings,the biomass,number of root branches,total root length and total root surface area of P.massoniana inoculated with S.bovinus were significantly higher than those of non inoculated treatment.Inoculation of S.bovinus under phosphorus and aluminum stress changed the accumulation of 12 metabolites such as organic acids,flavonoids,lipids and phenolic acids in P.massoniana roots.These differential metabolites are mainly involved in regulating the metabolic pathways such as flavonoid biosynthesis,glucose metabolism and Zeatin synthesis.The growth promoting and stress resistant effect of S.bovinus on P.massoniana may be realized through metabolic regulation.In conclusion,this study systematically explained the mechanism of ECM alleviating phosphorus and aluminum stress resistance of P.massoniana in acidic soil from the aspects of pure culture of fungi under stress,the formation process of symbiont with host and its effect on symbiont function.The relevant research results enrich the stress resistant physiological mechanism of mycorrhizal fungi and provide a scientific basis for the mycorrhizal cultivation of P.massoniana in phosphorus and aluminum stress areas.