| Cotton(Gossypium spp.)is an important economic crop and the main resource for the textile industry,which occupies an important position in the global and our national economy.Arbuscular mycorrhizal fungi(AMF)are the most widespread endomycorrhizal fungi,which can form mutualistic symbionts as so called arbuscular mycorrhiza(AM)with the majority of terrestrial plants.The development of AMF depends on the colonization of its host plants to acquire their carbon nutrition.In return,AMF play important roles in nutrition cycling processes as they provide access to phosphorus,which is poorly mobile in the soil and also to a lesser extent,nitrogen and other mineral nutrients.However,additional benefits arise from the symbiosis,including disease and stress resistance.Many reports emphasize that the mycorrhiza-induced resistance(MIR)plays a major role in plant defense against a broad spectrum of pathogens.However,a successful symbiotic relationship between AMF and its host involves the premise that the AMF will have positive effects on improving both plant growth and disease tolerance.At present,domestic and overseas researches regarding the symbiosis between cotton and AMF are primarily focused on its drought/salt tolerance and the effects of diverse AMF species on the plant growth promotion(PGP)of the same cotton variety.However,studies have not been reported concerning the impact of diverse plant genotypes on both the mycorrhizal colonization level of the specific AMF species as well as its roles in AMF-mediated PGP.To assess the potential of AMF Rhizophagus irregularis CD1 in promoting cotton growth,the symbiotic efficiency of AMF(SEA)was first investigated over a remarkable range of 3%to 94% in 17 cotton varieties under greenhouse conditions.Further studies showed that inoculation of AMF significantly promoted the growth of high-SEA subgroups under greenhouse conditions.In contrast,the low-SEA subgroups did not show any PGP effects.From these results,the highest-SEA variety of Lumian 1(SEA=94%)was selected for a two-year field assay.Consistent with the performance from the greenhouse,the AMF-mediated PGP of Lumian 1 also produced significant results in fields,including an increased plant height,stem diameter,the area of the functional leaf,the number of fruit branches,the number of bolls,and phosphorus content,etc.These results indicate that aneffective SEA is required for AMF-mediated PGP.Verticillium wilt caused by the soil-borne fungus Verticillium dahliae has become one of the most devastating diseases in cotton-growing areas,resulting in significant losses worldwide.Although recent studies have pointed out the potential of AMF to control several soil-borne diseases,improvement of Verticillium wilt resistance by applying AMF to a cotton variety with high SEA has not been reported.In our results,compared with the mock treatment,AM colonization obviously inhibited the symptom development of V.dahliae and more strongly altered the expression of pathogenesis-related genes and lignin synthesis-related genes.These results hint that AM colonization could lead to the MIR of Lumian 1 to V.dahliae.Meanwhile,AM colonization significantly increased the yield of Lumian 1 and improved the quality of cotton fiber under the stress of V.dahliae.Interestingly,our results indicated that the AMF endosymbiont could directly inhibit the growth of phytopathogenic fungi including V.dahliae by releasing undefined volatiles,which to our knowledge,is a novel AMF-mediated biocontrol mechanism.The specific antifungal substances derived from the mycorrhizal symbionts merit additional research to identify these compounds.In summary,our results illustrated that a high-SEA cotton variety could perform well on both growth promotion and wilt resistance following mycorrhizal colonization.However,the SEA of most cotton varieties in our study were less than 60%.These results suggest high-SEA is a key phenotype to accelerate the utilization of AMF that will contribute to research on the genetic improvement of SEA. |
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