Pseudomonas syringae pv. syringae Associated With Mango Trees, a Particular Pathogen Within the "Hodgepodge" of the Pseudomonas syringae Complex

Abstract

The Pseudomonas syringae complex comprises different genetic groups that include strains from both agricultural and environmental habitats. This complex group has been used for decades as a "hodgepodge," including many taxonomically related species. More than 60 pathovars of P. syringae have been described based on distinct host ranges and disease symptoms they cause. These pathovars cause disease relying on an array of virulence mechanisms. However, P. syringae pv. syringae (Pss) is the most polyphagous bacterium in the P. syringae complex, based on its wide host range, that primarily affects woody and herbaceous host plants. In early 1990s, bacterial apical necrosis (BAN) of mango trees, a critical disease elicited by Pss in Southern Spain was described for the first time. Pss exhibits important epiphytic traits and virulence factors, which may promote its survival and pathogenicity in mango trees and in other plant hosts. Over more than two decades, Pss strains isolated from mango trees have been comprehensively investigated to elucidate the mechanisms that governs their epiphytic and pathogenic lifestyles. In particular, the vast majority of Pss strains isolated from mango trees produce an antimetabolite toxin, called mangotoxin, whose leading role in virulence has been clearly demonstrated. Moreover, phenotypic, genetic and phylogenetic approaches support that Pss strains producers of BAN symptoms on mango trees all belong to a single phylotype within phylogroup 2, are adapted to the mango host, and produce mangotoxin. Remarkably, a genome sequencing project of the Pss model strain UMAF0158 revealed the presence of other factors that may play major roles in its different lifestyles, such as the presence of two different type III secretion systems, two type VI secretion systems and an operon for cellulose biosynthesis. The role of cellulose in increasing mango leaf colonization and biofilm formation, and impairing virulence of Pss, suggests that cellulose may play a pivotal role with regards to the balance of its different lifestyles. In addition, 62-kb plasmids belonging to the pPT23A-family of plasmids (PFPs) have been strongly associated with Pss strains that inhabit mango trees. Further, complete sequence and comparative genomic analyses revealed major roles of PFPs in detoxification of copper compounds and ultraviolet radiation resistance, both improving the epiphytic lifestyle of Pss on mango surfaces. Hence, in this review we summarize the research that has been conducted on Pss by our research group to elucidate the molecular mechanisms that underpin the epiphytic and pathogenic lifestyle on mango trees. Finally, future directions in this particular plant-pathogen story are discussed.

Keywords: Pseudomonas syringae pv. syringae; epiphytic fitness; mango tree; mangotoxin; pPT23A family plasmid; ultraviolet radiation and copper resistance; virulence strategies.

FIGURE 1

Typical symptoms of bacterial apical necrosis (BAN) disease on mango trees. (A) Healthy mango tree. (B) Mango tree affected by BAN disease. (C) Healthy mango apical bud. (D) Typical gum exudes on mango apical bud. (E) Initial necrotic spots on mango apical bud. (F) Severe necrosis of mango apical bud. (G) Necrotic symptoms progression from apical bud to leaves through the petiole. (H) Dead mango apical bud and surrounded leaves. (I) Flower panicles. Yellow arrow: healthy mango flower panicle; red arrow: necrosis on mango flower panicle.

FIGURE 2

The life cycle of Pseudomonas syringae pv. syringae on mango trees. (A) Epiphytic phase of P. syringae pv. syringae on mango trees is developing mainly in spring/summer seasons, where high temperature and high UV radiation are present. At population level, P. syringae pv. syringae is present mainly on the buds and leaves surfaces forming microcolonies (1), that will subsequently form a mature biofilm with the biosynthesis of an extracellular matrix (2). At single cell level, rulAB operon encoded by 62-kb PFP plasmid involved in resistance to UV radiation, and wss operon present at the chromosome and involved in the biosynthesis of cellulose, are both highly expressed. On the contrary, copABCD operon encoded by 62-kb PFP plasmid involved in copper resistance, and mbo operon located at the chromosome and involved in mangotoxin biosynthesis, are less expressed. (B) Pathogenic phase of P. syringae pv. syringae on mango trees arise primarily in autumn/winter seasons, where low temperatures, low UV radiation and high rainfall are present. At population level, the infection process on mango leaves and buds is the following: (3) epiphytic survival and biofilm formation; (4) biofilm disassembly and bacterial migration; (5) Ice nucleation activity to damage mango surfaces; (6) bacterial entry into cells by microinjuries; (7) Bacterial entry into cells through stomata; (8) Release of phytotoxins; and (9) Release of type III effectors by using the type III secretion system. At single cell level, firstly, copABCD operon involved in detoxification of copper compounds is highly expressed in response to copper treatment applications by farmers. Then, all genes that encode virulence factors are highly expressed (Mangotoxin, lipodepsipeptidic toxins and type III secretion system and its effectors) to elicit the typical BAN disease symptoms.

FIGURE 3

Enzymatic target of mangotoxin and its biosynthesis regulation. (A) Representative scheme of the arginine-glutamine and polyamine biosynthesis pathways. The enzymatic targets of the different antimetabolite toxins, including mangotoxin, and the different pathovars involved in the production of the different antimetabolite toxins are depicted. OAT, ornithine N-acetyltransferase; OCT, ornithine carbamoyltransferase; ODC, ornithine decarboxylase, and GS, glutamine synthetase. Orange box: pathovars positives for the presence of mbo genes and positives for mangotoxin production; red box: pathovar positive for the presence of mbo genes, but negative for mangotoxin production; dotted lines box: pathovars positive for the presence of mbo genes, but not experimentally tested for mangotoxin production. (B) Mangotoxin biosynthesis regulation model. GacS/GacA two-component regulatory system regulates directly or indirectly the transcription of the mgo operon. Mgo operon could synthetize a positive regulator (signal molecule) to activate the mbo operon transcription. The mbo operon produces mangotoxin, which acts primarily as a virulence factor, although, other functions have been described.

FIGURE 4

Multilocus sequence typing analysis of strains belonging to the P. syringae complex. (A) The neighbor-joining tree was constructed with combined partial sequences of rpoD and gyrB housekeeping genes using MEGA 7 software. Bootstrap values (1,000 repetitions) are shown on branches and evolutionary distances are in units of nucleotide substitutions per site. One hundred and fifty strains belonging to the phylogenetic groups 1, 2, 3, 4, 5, 6, 7, and 11 of the P. syringae complex are depicted in the circular phylogenetic tree. Marked in blue are represented the strains belonging to the phylogenetic group 2, where the P. syringae pv. syringae strains isolated from mango are found. (B) Exclusive representation of the phylogenetic group 2. Three main groups are defined regarding the presence or not of the mbo genes necessary for mangotoxin production. The topology was similar among phylogenetic trees produced by the maximum-parsimony and maximum-likelihood methods. Supplementary Table S1 provides the phylogenetic groups, the host of isolation and the accession numbers of the DNA sequences used for each strain represented in this phylogenetic analysis.

FIGURE 5

Type III effectors repertoire. (A) Venn diagram comparing the putative type III effectors presence in selected strains of P. syringae including the model strain P. syringae pv. syringae isolated from mango UMAF0158. Eleven type III effectors shared by all the strains analyzed. (B) Presence of specific type III effectors. Gray boxes indicate type III effectors presence in all the strains analyzed. Red boxes indicate type III effectors specific for each strain analyzed. Green boxes represents the hopAX1effector, an effector present in UMAF0158 that shows a high-specificity for a few strains of different pathovars exclusively belonging to the Genomospecies 1-Phylogenetic Group 2.