Identification of the metabolites involved in resistance to Meloidogyne incognita in the‘Honggengansutao 1’peach

Abstract:【Objective】Root-knot nematode is the second largest class of microbes that seriously endangerplant growth. It is widely distributed but difficult to control. There are many species of root-knotnematode, including Meloidogyne incognita, M. hapla, M. arenaria, M. javanica, and M. Florida.Among them, M. incognita might result in the most serious damage to peach plants. In our laboratory,we have conducted long- term studies on the resistance of different peach rootstocks to M. incognitawith continuous inoculation.‘Honggengansutao 1’, which belongs to Prunus kansuensis, has beenfound to be immune to the nematode. To analyze the molecular mechanism of the resistance to rootknotnematodes in‘Honggengansutao 1’, several previous studies focused on the localization of the resistancegene using linkage analysis. The resistance locus was located at the top of chromosome 2, anda subsequent study identified a candidate gene, ppa021062m, containing an NBS-LRR (nucleotide bindingsite-leucine rich repeat) domain, which is putatively associated with the resistance. However, littleis known about downstream of the response pathway including signal transduction, and about the metabolitesinvolved in nematode resistance.【Methods】In order to understand the physiological mechanismof the resistance of‘Honggengansutao 1’to M. incognita, metabolites from roots of accessions withdifferent resistances to nematodes were analyzed in this study. The materials tested included‘Honggengansutao1’‘Baigengansutao 1’and‘Bailey’, which are immune, highly resistant, and highly susceptibleto M. incognita, respectively. The resistant and susceptible bulked seedlings of the BC1 population derived from‘Honggengansutao 1’and‘Bailey’were also sampled. In June 2018, the roots of 20 accessionsfrom the BC1 population with high resistance or high susceptibility were selected to constructtwo bulked pools. The samples were subjected to metabolite determination. The roots of the‘Honggengansutao1’and‘Bailey’peach were also taken for transcriptome analysis.【Results】A total of 427 metaboliteswere identified from all the samples. Flavonoids constituted the highest percentage (21.5%),followed by organic acids, lipids, phenylpropanoids, amino acids, and nucleotides. We screened the differentialmetabolites that showed a ratio greater than 2.0 or less than 0.5 between the two samples. A totalof 209 differential metabolites were identified from the two parents of the BC1 population. However,only 45 differential metabolites were identified from the two bulked pools of roots. Among them, 22were significantly higher in resistant than the susceptible accessions. Overall, a total of 14 differentialmetabolites were identified between the two types of resistant and susceptible germplasm. We also comparedmetabolites in roots of‘Honggengansutao 1’and‘Baigengansutao 1’. Four of the above 14 metabolitecomponents, including methyl quercetin O-hexoside, 4-methoxycinnamic acid, trans-4-hydroxycinnamicacid methyl ester, and 2’-hydroxygenistein, were found to be significantly different betweenthe two samples. Of the other differential metabolites, the content of methyl quercetin O-hexose was31.15 times greater in‘Honggengansutao 1’than in‘Baigengansutao 1’, suggesting that this substancemay participate in the response to M. incognita of‘Honggengansutao 1’. Subsequently, using the RNAseqdata, this study further revealed that only two genes encoding flavonol synthase, Prup.1G502300and Prupe.1G502500, were differentially expressed between the resistant and susceptible accessions.Prup.1G502300 should be recognized as a candidate gene, because its expression was 6.64 times higherthan that of Prupe.1G502500 in RNA-seq result. The finding was further validated by qPCR.【Conclusion】In this study, a key differential metabolite, methyl quercetin O-hexose, was identified from germplasmwith different resistances to root- knot nematode and might be associated with the immune responseof‘Honggengansutao 1’. Furthermore, a gene differentially expressed between the resistant andsusceptible accessions, Prup.1G502300, was found to be a candidate gene encoding flavonol synthase.This study provides a theoretical basis for the intrinsic defense mechanism of‘Honggengansutao 1’against M. incognita and is also of great significance for the integrated control of root knot nematodesand for disease-resistant breeding.