Identifying the causal agent of floral malformation as Fusarium complex using metagenomic and metabolomic approaches

Abstract

Mango malformation disease (MMD) is one of the most important diseases that have defied proper understanding and management strategy. The disease is characterized by malformation of vegetative shoots and inflorescences, causing 50–60 % economic losses to nursery and young bearing trees. This study focuses on floral malformation (FM), as it is the most serious concern affecting the flowering and fruiting. The causal agents of floral malformation have been highly debated and Koch postulates could only be partially proven with Fusarium mangiferae. The current modern tools approaches such as metagenomic and metabolomic methods provide robust data for associated pathogen diversity and their metabolites within the malformed mango panicle (MP). Whole metagenome analysis of malformed panicles (cv. Beauty Maclean) showing characteristic symptoms were compared with healthy normal panicle described as control (C), that revealed maximum OTUs representing Fusarium mangiferae, followed by F. proliferatum, F. verticilloides, F. oxysporum and Giberrella fujikorrii. Functional analysis predicted Polyketide synthase gene (FUM1a) involved in Fumonisin/Fusaric acid biosynthesis. Pathway analysis showed unique key regulators of fungal pathogenicity viz., glucosylceramidase, chitin deacetylase etc. in the malformed tissue. Upregulated urea carboxylase and mercuric reductase in the MP sample only raises interesting questions on chemical impact on predisposition of MMD. Zn-peptidases, dipeptidases and cyanide permease were also identified in MP which is involved in fungal virulence and cyanide toxicity induced by Fusarium complex. These results confirm Fusarium complex (Fusarium mangiferae, F. proliferatum, F. verticilloides) as incitants of FM. This was corroborated by standard semi-quantitative PCR and quantitative real time PCR (q-RT-PCR) assay using FUM1a gene in MP and C in three mango cultivars. Differential metabolite analysis through gas chromatography-mass spectrometry (GC-MS) based volatile organic compounds (VOC) detected upregulation of metabolites such as esters and methyl esters of fatty acids that implicate induction of jasmonic acid and fatty acid pathways, fumonisin production and suppression of resistance metabolites in the malformed samples. This study gives clues to understand the genetic and molecular events related to the disease etiology besides confirming the incitant of MMD as Fusarium complex.