The Control Effect And Induced Resistance Of Endophytic Fungus Phomopsis Liquidambari B3 On Peanut Leaf Spot And Root Rot

Peanut（Arachis hypogaea L.）is an important oil crop in China.Plant diseases have been considered as the limiting factors for peanut production and oil market.Peanut plants have generally been monocropped continuously due to limited arable land in agriculture production,leading to increased disease pressure,such as leaf spot and root rot.In the current management methods for peanut diseases,the strategy of inducing crop resistance to disease is advocated by green agriculture.Our previous studies have reported a broad-spectrum endophytic fungus Phomopsis liquidambari B3,can colonize peanut root and significantly increase peanut nodulation and yield under continuous cropping conditions.However,it is still unclear whether P.liquidambari can play a certain role in alleviating peanut diseases.Therefore,this study intends to explore the potential capacity of P.liquidambari in controlling peanut disease and the mechanism of P.liquidambari-induced resistance of peanut plant,from the perspective of the ability of endophytic fungi in controlling plant diseases.In order to establish a disease model of peanut for further study,the isolation and identification of pathogenic fungi were first carried out on diseased plants grown in continuous cropping soil.The optimal conditions for pathogenic fungi infection were screened,including vaccination time,components and inoculation methods.Results showed that the pathogenic strains isolated from the diseased plants were identified as Colletotrichum gloeosporioides（C）and Fusarium oxysporum（F）.Disease condition of peanut plant was evaluated according to the disease incidence（DI）and percentage disease index（PDI）.The optimal condition for leaf spot was confirmed by rub-inoculation of the spore suspension of C.gloeosporioides in 21-day-old peanut seedlings（DI,62.50%;PDI,29.38%）,and for root rot was confirmed by pouring the wound root with spore suspension of F.oxysporum（DI,77.50%;PDI,36.88%）.Together,C.gloeosporioides and F.oxysporum can be used as pathogenic fungi for further research.In order to explore the effects of P.liquidambari on prevention and control for peanut diseases,outdoor pot experiments were carried out,in which peanut seedlings were planted in continuous cropping soil in a natural condition.The experiments were designed with six treatments:sterile water control（CK）;plant inoculated with C.gloeosporioides alone（C）;plant inoculated with F.oxysporum alone（F）;plant inoculated with B3 alone（B3）;plant pre-inoculated with B3 one week before C.gloeosporioides inoculation（B3+C）;and plant pre-inoculated with B3 one week before F.oxysporum inoculation（B3+F）.Results showed that the control effect of leaf spot and root rot were 56.67%and 61.73%,respectively,in peanut inoculation with P.liquidambari B3.And plant height,fresh weight,dry weight,pod number and yield were increased by different degrees under continuous cropping conditions.To explore the natural factors,peanut seedlings were planted in sterile vermiculite under greenhouse conditions.Results showed that,under greenhouse conditions,B3 inhibited the invasion and colonization of pathogenic fungi,and reduced the DI and PDI of peanut.Root inoculation with B3 promoted callose deposition and cell lignification in peanut root;enhanced catalase（CAT）in roots and shoots,and helped to stabilize pathogenic fungi-drived excessive superoxide dismutase（SOD）and peroxidase（POD）;promoted the accumulation of phytoalexin,such as polydatin,reveratrol,quercetin and luteolin.These results indicated that peanut pre-inoculation with B3 can enhance peanut defense ability during subsequent pathogen infection,resulting in decreased diseases,which is associated with endophytic fungi-induced resistance of host plant.To further explore the potential relationship between B3 inoculation and induction of plant resistance,F.oxysporum was chosen as pathogenic fungi for the next experiment,as B3showed better effects on peanut root rot in our earlier study.Our results indicated that pre-inoculation with B3 induced systemic resistance of peanut plants by promoting the accumulation of hydrogen peroxide（H₂O₂）and salicylic acid（SA）in plant tissues,which played a central role in systemic resistance.Meanwhile,increased H₂O₂ and SA in peanut enhanced by B3 inoculation displaying synergistic action.In addition,exogenous application of SA increased the transcription level of genes related to plant defense,such as resistance-related gene NPR1,chitinase gene CHI and stilbene synthase gene STS1.These results indicated that root inoculation with B3 enhances peanut resistance against pathogen invasion by activating defense signal pathway of peanut plants.In conclusion,this study demomstrated that endophytic fungus B3 can change the level of physiological and biochemical of peanut,enhancing transcription level of defense genes,which are important to control peanut diseases.Moreover,the greenhouse experiment also demonsteated that the the potential mechanism of induced resistance when inoculated of B3.Therefore,endophytic fungus P.liquidambari B3 could be considered a green microbial agent for practical applications to control the peanut diseases.