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Team Characterizes Mango Pathogen With Genome Sequencing, Comparative Genomics

NEW YORK (GenomeWeb) - A Spanish research team has detected genetic factors that appear to influence the infectious capabilities of a Pseudomonas syringae strain known for infecting the mango tree.

As they reported online yesterday in PLOS One, the researchers started by sequencing and annotating a P. syringae pathovar (pv.) syringae strain called UMAF0158, which causes a condition called apical necrosis in mango trees.

Through extensive comparisons between the new genome sequence and sequences from P. syringae strains that produce disease in other host plants, the team characterized the mango pathogen's protein-coding repertoire and investigated its relationships to other P. syringae strains.

"This work provides the basis for further analysis on the specific mechanisms that enable this strain to infect mango trees and for the functional analysis of the factors governing host specificity in pv. syringae strains from different phylotypes," senior author Antonio de Vicente, with the University of Málaga, and his colleagues wrote.

Past research suggests P. syringae pv. syringae UMAF0158 is genetically related to the P. syringae pv. syringae strain B728a, a pathogen from another P. syringae pv. syringae phylotype that infects bean plants.

In an effort to untangle genetic features that distinguish the mango pathogen from B728A and the more than 100 P. syringae strains that infect various other plant hosts, the researchers had UMAF0158 sequenced with the Illumina HiSeq 2500 at BGI-HK.

Using reads generated from several UMAF0158 libraries, the team put together a finished genome assembly for the pathogen, which was subsequently annotated with help from the National Center for Biotechnology Information's Prokaryotic Genome Annotation Pipeline.

The nearly 5.8 million base UMAF0158 genome contained an estimated 5,017 protein-coding genes, while a 63,004 base pair plasmid carried by the bug housed 71 coding sequences.

When the researchers delved into the mango pathogen's past with a phylogenetic analysis of house-keeping genes from 25 previously sequenced P. syringae strains, they saw clustering between UMAF0158 and a strain called Cit 7 — first found on a seemingly healthy orange tree — in a phylotype known for pathogenicity in mango, lilac, tomato, and pear plants.

Meanwhile, the team's head-to-head comparison of P. syringae sequences revealed 105 genes within the P. syringae pv. syringae UMAF0158 genome that have not yet been identified in other P. syringae strains, with most of these UMAF0158-specific genes coding for hypothetical proteins.

The researchers turned up both hypothetical protein-coding genes and UMAF0158 genes involved in magnotoxin production, cellulose fabrication, and cell secretion that were missing in B728a. On the other hand, the bean pathogen carried a streptomycin resistance transposon that wasn't present in the UMAF0158 strain.

By combing through the new genome sequence, the team also got a look at some genes that seem to contribute UMAF0158's ability to become virulent in some situations and remain relatively harmless, or "epiphytic" in others.

"[O]ur data revealed a number of genetic factors that could be involved in the differential pathogenic and epiphytic lifestyle of UMAF0158, in comparison with the model strain B728a," authors of the study concluded.