| Entomopathogenic fungi(EPF)are commonly used in the biological control of agricultural and forestry pests.Beauveria bassiana and Metarhizium anisopliae are the most widely used species of EPF,and play important roles in pest control of garden plants.Moreover,the discovery of the growth-promoting effect of EPF on many plants has lighted a new research field for the biological relationship between EPF and plants.Although significant growth-promoting effects of B.bassiana and M.anisopliae on plants have been reported,the growth-promoting mechanism,endobiochemical process,and disease resistance of EPF have not been thoroughly studied.In this study,corn is used as the model plant,we studied the changes of maize growth indices at different periods after inoculating B.bassiana Bb13 or M.anisopliae Ma73 strain in a hydroculture system,and after inoculating B.bassiana Bb202 strain in soil culture system.At the same time,tissue culture recovery method on selected medium and SCAR molecular labelling method were used to monitor the population dynamics and spatial distribution of B.bassiana and M.anisopliae around the root of maize and in different plant tissues.In addition,biological response mechanism and the interaction pathway between B.bassiana and maize were explored through analysis and comparison of maize metabolomics after inoculation of B.bassiana Bb202 under hydroponics conditions,by which a better understanding on fungal roles was achieved for entomopathogenic fungi as insecticides and growth promoters for garden plants.In this study,the three inoculated EPF strains Bb13,Ma73 and Bb202 under the two culture systems of hydroponics and soil culture had significant promotion effect on the growth of maize.On the 35 th day of hydroponics system,the plant height,root length,leaf length,leaf width,above ground fresh weight,underground fresh weight and total fresh weight of B.bassiana Bb13 treatment group were significantly higher than those of control group,and the growth rates were 17.87%,48.26%,20.37%,36.97%,85.54%,81.49% and 84.87%,respectively.The plant height,leaf length,aboveground fresh weight,underground fresh weight and total fresh weight of M.anisopliae Ma73 treatment group were significantly higher than those of control group,with the growth rates of 11.76%,17.84%,57.74%,92.71% and 63.45%,respectively.In the soil culture system,plant height,root length,leaf area and ground fresh weight of the treatment group were significantly higher than those of the control group when inoculated with B.bassiana strain Bb202 24 days later,with growth rates of21.19%,20%,33.98%,and 38.92%,respectively.The root showed the most obvious growth promotion.In addition,inoculation experiments showed that maize root inoculation with Bb13 and Ma73 could inhibit down the growth of Bipolaris maydis in the hydroponic system.After 7 days of inoculation,the lesion area of the Metarhizium treated group was smaller than that of the control group,while the B.bassiana treated group showed significant inhibition on the development of the lesion.EPF population dynamics in the hydroponic and soil culture systems showed that the EPF population in soil was more stable than that in hydroponic solution.EPF population in soil dicreased much slowly than that in the hydroponic.In the hydroponic system,the initial population densities of B.bassiana were 5.92×103 cells·m L-1 and M.anisopliae 8.56×103 cells·m L-1,respectively.The population densities of B.bassiana decreased to 7.20 cells ml-1 at 14 d and could not be detected at 21 d in the hydroponic solution.M.anisopliae showed similar tendency,and the population density decreased to 17 cells·m L-1 at 21 d and 0 at 28 d.In the soil culture system,the population of B.bassiana remained a level of 1.64×106 cells·g-1 at 31 d after an initial inoculation density of 2.17×107 cells·g-1.Colonization and endochemistry of EPF in maize tissues showed that M.anisopliae and B.bassiana could colonize systematically in maize tissues.Both in hydroculture and soil culture systems,B.bassiana and M.anisopliae could successfully colonize in maize roots,stems and leaves.The result showed the two fungi could be transferred between different tissues and made a systematic colonization.Conpared the two fungal detection techniques,SCAR molecular labeling technique was more sensitive than the tissue culturing technique.B.bassiana was only re-isolated from the root tissue in the hydroponics at the 28 th day after fungal inoculation by tissue culturing method,while the detection rate of B.bassiana was 100% at every checking stage by SCAR molecular marker.Therefore,SCAR molecular labeling technique is more suitable for sensitive detection of endobiochemical fungi.The metabolomics study showed that 181 differential metabolites(DAMs)were obtained from the root tissue of maize at 14 days after inoculation of B.bassiana spore suspension in the hydroculture system.Among them,82 DAMs were up-regulated and 99 DAMs were down-regulated.A total of 291 differentially expressed extracellular metabolites were screened from the rhizosphere,of which 268 were up-regulated and 23 down-regulated.In the soil culture system,a total of 236 DAMs(135 up-regulated and 101 down-adjusted)were selected from the root tissue at 10 d,111 DAMs(70 up-adjusted and 41 down-regulated)at17 d,and 41 DAMs(18 up-regulated and 23 down-selected)at 24 d.The DAMs show temporal characteristics.Identification of the DAMs revealed the DAMs were mainly belong to phenylpropanes,polyketones,lipids,lipid molecules,organic acids and their derivatives,aerobic compounds,benzenes,organic heterocyclic compounds,alkaloids and derivatives,nucleosides,nucleotides and analogues,lignans,neolignans and related compounds,and organic nitrogen compounds.According to the joint analysis of the rhizospheric extracellular metabolites and the DAMs in root tissues in the hydroponic system,the inoculation of B.bassiana up-regulated both the intracellular and extracellular phenylpropane,polyketones and benzene compounds of maize roots.In addition,lipid and lipid-like molecules were significantly up-regulated in plant roots.KEGG enrichment analysis showed that the regulation of maize roots mainly focused on amino acid-related metabolic pathways.The colonization response of maize to B.bassiana was temporally sequential.The metabolic pathway "ABC transporter" and "plant hormone signal transduction" pathway have temporal changes.A large number of amino acid metabolites were up-regulated in the "ABC transporter" pathway,among which glycine betaine was significantly up-regulated in the ABC transporter pathway at 10 and 17 d of hydroculture,soil culture.In the "plant hormone signal transduction" pathway,maize mainly regulates jasmonic acid(JA)and Jasmonate-isoleucine(JA-L-ILE).In soil culture sysetem,17 days after fungal inoculation,L-glutamic acid was up-regulated by Nitrogen metabolism and Histidine metabolism.γ-aminobutyric acid(GABA)was produced under the effect of glutamat decarboxylase.GABA,the end product of "arginine and proline metabolism",was also significantly up-regulated.It is concluded that GABA plays a key role in the physiological regulation mechanism of maize growth after B.bassiana inoculation.The present study provided better understanding on the biological and ecological relationships between the EFPs and the fungi colonized plants. |
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