Asian Research Thesis Index

Abstract

Blackleg, a bacterial disease caused by a group of pathogens belonging to Pectobacterium species inflicts incredible losses to potato (Solanum tuberosum) in major potato growing areas of Punjab. Re-classification of blackleg causing bacteria Pectobacterium and occurrence of newly emerging blackleg pathogens such as Dickeya species have adversely affected the impact of potato trade world-wide. To determine the impact and extent of these pathogens, samples exhibiting blackleg disease were collected from 80 farmers‘ fields in five potato production regions of Punjab, Pakistan during the years 2014-2017. Bacteria isolated from diseased samples were gram negative, pectinolytic and were molecularly identified using gapA primers followed by genome sequencing of the representative isolates. Genomic and phenotypic analyses confirmed the diversity of blackleg pathogens in Pakistan. From a total of 36 representative isolates from all five potato production regions, approximately 58% were P. atrosepticum, 8% P. carotovorum, 5% P. polaris, 8% P. parmentieri, 11% D. dianthicola and 8% newly identified P. punjabense sp. nov. Thus, P. atrosepticum was shown to be the most important causal agent of blackleg of potato and poses a threat to potato production in Pakistan. Moreover, in these type of pectinolytic bacteria pathogenicity and virulence is regulated through quorum sensing (QS) in a density dependent manner utilizing N-acyl homoserine lactones (NAHLs) as QS signaling molecules. Interference with QS through Quorum quenching (QQ) is a demonstrated bio-control tool. Degradation of the QS signals (NAHLs) by bacteria present in the rhizosphere soil is a useful approach for protecting potato plants against the blackleg pathogen P. atrosepticum. Therefore, NAHLs produced by P. atrosepticum were extracted and then evaluated through biosensor based plate assays using Chromobacterium violaceum (CV026) and Agrobacterium tumefaciens (NTLR4) biosensors. Furthermore, several NAHL degrading bacteria were sorted out from rhizosphere of potato plants and their ability to degrade NAHLs was evaluated by a series of NAHL degradation assays followed by QQ tuber assay. Out of 24 NAHL degrading bacterial isolates, 16 actual QQ isolates were identified that effectively reduced Pectobacterium based infections in potatoes.

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