Pseudomonas syringae : enterprising epiphyte and stealthy parasite

Graphical abstract The lifecycle and pathogenicity mechanisms of Pseudomonas syringae . Transmission : P. syringae can be disseminated by rainsplash, aerosols and airborne plant particles (1), insect vectors (2) or as a seed-borne pathogen (3). If carried into the atmosphere (4), ice-nucleating P. syringae strains can contribute to ice nucleation in clouds and be disseminated in snow or rainfall (5). P. syringae can also be disseminated through terrestrial water systems (6, 8) and through plant debris in soil (7, 8). Symptoms : P. syringae infections are commonly characterized by chlorosis and necrosis of leaves, stem tips, buds and flowers (top); by necrotic lesions and delayed ripening or altered development of fruit (middle); and by cankers and galls of woody tissues (bottom). Mechanism : P. syringae enters plant tissues through wounds and natural openings such as stomata. Some strains of P. syringae can increase frost damage to plant tissues through ice nucleation promoted by proteins such as InaZ. Pathogen-associated molecular patterns (PAMPs) produced by P. syringae , such as flagellin, are recognized by plant pattern recognition receptors (PRRs), triggering the induction of plant immune responses. P. syringae counters plant immune responses through the production of toxins, and the secretion of effector proteins via a type III secretion system (T3SS). Effector proteins and toxins disable or subvert plant immune responses and alter plant metabolism and physiology to promote P. syringae infection. Effectors can also be directly or indirectly recognized by plant immune receptors, notably nucleotide-binding domain and leucine-rich repeat region (NLR)-containing proteins, thereby triggering plant immune responses. Picture: Nattapong Sanguankiattichai. Abstract Pseudomonas syringae is best known as a plant pathogenic bacterium that causes diseases in a multitude of hosts, and it has been used as a model organism to understand the biology of plant disease. Pathogenic and non-pathogenic isolates of P. syringae are also commonly found living as epiphytes and in the wider environment, including water sources such as rivers and precipitation. Ice-nucleating strains of P. syringae are associated with frost damage to crops. The genomes of numerous strains of P. syringae have been sequenced and molecular genetic studies have elucidated many aspects of this pathogen ’ s interaction with its host plants.


Abstract
Pseudomonas syringae is best known as a plant pathogenic bacterium that causes diseases in a multitude of hosts, and it has been used as a model organism to understand the biology of plant disease. Pathogenic and non-pathogenic isolates of P. syringae are also commonly found living as epiphytes and in the wider environment, including water sources such as rivers and precipitation. Ice-nucleating strains of P. syringae are associated with frost damage to crops. The genomes of numerous strains of P. syringae have been sequenced and molecular genetic studies have elucidated many aspects of this pathogen's interaction with its host plants.

PROPERTIES
P. syringae is a Gram-negative aerobic bacterium with rodshaped cells, which are typically 1.5 um long and 0.7-1.2 um in diameter. The cells are motile, using at least one polar flagellum. The optimal temperature for growth ranges from 22-30 C, and P. syringae is negative for oxidase and arginine dihydrolase activity. Many P. syringae strains produce the polysaccharide levan and elicit the hypersensitive response on tobacco, which are elements of the levan, oxidase, potato rot, arginine dihydrolase, tobacco hypersensitivity (LOPAT) test. The P. syringae species complex is subdivided into pathovars depending on the plant species they infect. Currently over 60 pathovars have been defined [1].

GENOME
The complete genome sequences of three pathovars of P. syringae were published between 2003 and 2005. These were pv. tomato strain DC3000 with a 6.4 Mb chromosome and two plasmids (74 and 67 kb); pv. syringae strain B728a, which has a genome size of 6.1 Mb; and pv. phaseolicola strain 1448A with a 5.9 Mb chromosome and two plasmids (131 and 51 kb) (http://www.pseudomonas-syringae.org/). With advances in DNA sequencing technologies, many more genome sequences are now available; for example, a recent study compared the genomes of 391 P. syringae strains [2]. The majority of P. syringae strains have a similar genome size of approximately 5-6 Mb [1] and contain a number of plasmids. A number of studies have compared and contrasted these genomes. For example, the gene gain and loss from 27 strains of 18 pathovars using wholegenome sequences of P. syringae has been investigated and it was found that the pan genome for all 27 lineages was 11 025 genes, with the core genome of 2595 genes [1].

PHYLOGENY
The P. syringae complex is currently divided into at least 13 phylogenetic groups (phylogroups), although this number is still debated. It is suggested that seven of these phylogroups can be considered 'primary' because they are monophyletic and distinct from the six 'secondary' phylogroups [1]. The features of these groups raise interesting questions about the biology and evolution of P. syringae. For example, P. syringae appears to undergo a moderately high rate of recombination over a large number of loci, both within and between phylogroups. However, a recent analysis of rates of recombination within and between phylogroups revealed a higher rate of recombination within primary phylogroups than between primary and secondary phylogroups [2]. Strains found in individual phylogroups do have common features, for example, strains in phylogroup 2 secrete a smaller number of effector proteins relative to phylogroups 1 and 3 and commonly produce a toxin known as syringomycin. Additional work needs to be done to define P. syringae as a species and to understand the processes that affect the evolution of P. syringae populations [1].

KEY FEATURES AND DISCOVERIES
P. syringae pathovars cause a range of diseases on a diverse array of plants, including diseases of annual crops, such as bacterial speck on tomato and halo blight on beans, as well as diseases of woody plants and trees, such as bleeding canker on horse-chestnut and a recent epidemic of kiwifruit canker. Symptoms include water-soaked lesions, chlorosis, blights and cankers. P. syringae colonizes a range of plant tissues, including leaves, seeds, seedlings, fruit and bark, and can persist in non-host environments [3].
Molecular genetic studies carried out on P. syringae strains have led to many significant discoveries regarding plantpathogen interactions. For example, P. syringae pv. phaseolicola has been studied extensively and a number of discoveries have been made using it [4]. These included the first reports of mutants that lead to the loss of pathogenicity to bean and the ability to elicit the hypersensitive response. This work lead to the identification of hypersensitive reaction pathogenicity (hrp) genes, which were shown to form a bacterial type three secretion system (T3SS) that is used to deliver effector proteins into plant cells. Other areas where P. syringae has significantly contributed to the understanding of plant-pathogen interactions include the elucidation of race structures controlled by the interactions of effector genes in the bacteria and resistance genes in the host, the function of effector proteins in pathogenicity, and the evolution of pathogenicity. Recently, P. syringae has inspired new developments in synthetic biology, such as the development of 'genetic amplifiers' based on its T3SS regulatory system [5].
Many studies have been carried out on a selected group of P. syringae pathovars that infect a limited group of plants. However, more recently attention has turned to P. syringae in the wider environment. Work by Morris et al. [6] investigating the bacterium's ecology has found P. syringae in diverse environments, including rain and snowfall.
There is an increasing understanding of the role that reservoirs of populations of P. syringae play in disease outbreaks. An example of this is the recent epidemic of P. syringae pv. actinidiae, which had a devastating effect of the New Zealand kiwifruit industry. It has now been shown by isolating Pseudomonas from cultivated and wild kiwifruit across six provinces in China, that China was the likely origin of the pandemic lineage that was introduced into New Zealand [7].

OPEN QUESTIONS
. To what degree and how does P. syringae manipulate host plants to create a microenvironment that is conducive to bacterial growth?
. What is the frequency and significance of co-infections by multiple strains of P. syringae or by P. syringae and other micro-organisms? . How do plant resistance mechanisms act to restrict the growth of P. syringae? . Can we predict and prolong the durability of resistance? . Where do outbreaks originate from? Can we predict the emergence of new epidemics?
Funding information D. A. and G. P. acknowledge support from the Biotechnology and Biological Sciences Research Council, United Kingdom.

Conflicts of interest
The authors declare that there are no conflicts of interest.