Interaction Between Root Metabolites And Microbial Community Of Rhododendron Delavayi Under Waterlogging Stress

Rhododendron delavayi is an evergreen shrub or dungarunga with high horticultural value.The tolerance and growth state of Rhododendron delavayi seedlings are affected by waterlogging,and the promotion of Rhododendron delavayi in the humid and rainy regions of southern China is greatly limited.Therefore,it is important to study the tolerance,root metabolites and microbial response of Rhododendron delavayi seedlings under simulated waterlogging conditions to provide a basis for the appropriate moisture conditions for its cultivation and expansion.In this study,three treatments including normal cultivation CK,waterlogging（WL10,WL20,WL30）and recovery（WL10R,WL20R,WL30R）were applied to 2-year-old Rhododendron delavayi seedlings using potted water control method to fully reveal the changes in root system,leaf photosynthesis,root metabolites and microbial changes in response to waterlogging stress.The main results were as follows.（1）The root morphology of Rhododendron delavayi showed no significant changes in the morphology of the root system at 10d of waterlogging stress and in the recovery state,but the root morphology of Rhododendron delavayi began to produce damage after 20d of waterlogging stress in the recovery culture,especially at 30d of waterlogging stress when the root system caused serious damage,deepened and darkened in color until it finally became black and rotten and could not resume growth.（2）The results showed that the net photosynthetic rate（Pn）,stomatal conductance（Gs）and transpiration rate（Tr）decreased significantly with increasing time of waterlogging stress compared with the control,and the intercellular carbon dioxide concentration（Ci）had no significant effect.The net photosynthetic rate and intercellular CO₂concentration increased significantly under 10d of recovery after waterlogging stress,and there was no significant change in photosynthetic physiological indexes of leaves at 20d and 30d of recovery from waterlogging stress.（3）Metabolomic analysis of Rhododendron delavayi roots based on high performance liquid chromatography-tandem mass spectrometry（HPLC-MS/MS）platform identified 693 metabolites,including lipids,phenolic acids,flavonoids,amino acids and their derivatives,organic acids,etc.Compared with the control group,there were 223 different metabolites in the roots of Rhododendron delavayi responding to waterlogging stress,which were mainly up-regulated with the increase of waterlogging time（10d,20d,30d）.Extremely（p<0.01）enriched in flavonoid biosynthesis,phenylpropanoid biosynthesis,stilbene,diaryl heptanoic acid and gingerol biosynthesis,carbon fixation of photosynthetic organisms,arginine and proline metabolism,glycine,serine and threonine metabolism,glycolysis/gluconeogenesis,purine metabolism,aminoacyltrna biosynthesis,flavonoid and flavonol biosynthesis,histidine and purine alkaloid biosynthesis metabolic pathways,mainly including There were 83 different metabolites in the waterlogging group（WL10）and the recovery group（WL10R）,which were mainly down-regulated and enriched in purine metabolism,flavonoid biosynthesis and phenylpropanoid biosynthesis pathway,mainly including nucleotides and their derivatives,flavonoids,etc.There are 104 different metabolites in the waterlogging group（WL20）and the recovery group（WL20R）,which are mainly down-regulated and enriched in flavonoid biosynthesis and arginine and proline metabolism pathways,mainly including flavonoids,amino acids and their derivatives.There are103 different metabolites in the waterlogging group（WL30）and the recovery group（WL30R）,which are mainly down-expressed.They are very significantly enriched in flavonoid biosynthesis,purine metabolism,phenylpropanoid biosynthesis,stilbene,diaryl heptanoic acid and gingerol biosynthesis pathway,mainly including flavonoids,nucleotides and their derivatives,phenolic acids,etc.Flavonoids and lipids are the main differential metabolites in control group,waterlogging group and recovery group,which may play an important physiological role in antioxidation and waterlogging resistance.（4）The microbial community composition and diversity of Rhododendron delavayi root were analyzed by Illumina high-throughput sequencing technique.Short-term waterlogging stress can increase the abundance of microbial community.With the extension of stress time,microbial community was inhibited,and the abundance of microbial community was significantly increased after 10d and 30d of waterlogging stress,but significantly inhibited after 20d of waterlogging stress.There is no obvious difference in microbial communities at phylum level.Proteobacteria is always the dominant group,accounting for an average of 55.91%,while the proportion of other phylum decreases in turn.Relative abundance analysis of the classification level of Rhododendron lantana root microbial genus,under flooding stress,with the increase of stress time on the root microbial community structure of Lantana Rhododendron aristus has a greater impact,flooding stress inhibits the growth of aerobic microorganisms,such as Ralstonella,Bremensus,etc.,but anaerobic bacteria and facultative anaerobic bacteria can grow under hypoxic conditions,the demand for nutrients is low,can glucose fermentation,produce acidic substances and gases,may have a toxic effect on the root system of Lantana Rhododendron,such as Salmonella,Acid fever fungus and so on.（5）Correlation between root metabolism and microbial community of Rhododendron delavayi.Under waterlogging stress（10d,20d,30d）,it was found that flavonoids,amino acids and their derivatives,nucleotides and their derivatives,phenolic acids and other metabolites were significantly positively correlated with Nevskia and Bradyrhizobium,but significantly negatively correlated with Ralstonia and Salmonella.In the waterlogging group（WL10）and the recovery group（WL10R）,there was a significant positive correlation between lipid,flavonoids,nucleotides and their derivatives,phenolic acids,etc.,and a significant negative correlation with Others,but no significant correlation with the relative abundance of bacteria.In the waterlogging group（WL20）and the recovery group（WL20R）,it was found that metabolites such as lipids,phenolic acids,flavonoids and organic acids were significantly positively correlated with Puia,Sphingomonas and Rolston,and were associated with Bradyrhizobium,unidentified＿Mitochondria,unidentified＿Chlor-oplast and Acidothermus.In the waterlogging group（WL30）and the recovery group（WL30R）,it was found that metabolites such as lipids,flavonoids,lignans and coumarin were positively correlated with Sphingomonas and Rolston,and negatively correlated with Nevskia and Others.Under waterlogging stress and recovery conditions,lipids and flavonoids may have an important effect on changes in the root microbiota（as Ralstonella）.