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Plant Innate Immunity Against Human Bacterial Pathogens

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Review

Plant Innate Immunity Against Human Bacterial Pathogens

Maeli Melotto et al. Front Microbiol.

Abstract

Certain human bacterial pathogens such as the enterohemorrhagic Escherichia coli and Salmonella enterica are not proven to be plant pathogens yet. Nonetheless, under certain conditions they can survive on, penetrate into, and colonize internal plant tissues causing serious food borne disease outbreaks. In this review, we highlight current understanding on the molecular mechanisms of plant responses against human bacterial pathogens and discuss salient common and contrasting themes of plant interactions with phytopathogens or human pathogens.

Keywords: Escherichia coli O157:H7; Salmonella enterica; fresh produce; leafy vegetables; plant defense.

Figures

Figure 1
Figure 1
Schematic representation of human pathogen (HP) association with plants. (A) Pathogens are introduced to soil through contaminated irrigation water, fertilizers, manure, and pesticides (1). HPs are attracted to rhizosphere (2; Klerks et al., 2007a) and penetrate root tissues at the sites of lateral root emergence, root cracks as well as root-shoot transition area (3; Cooley et al., ; Dong et al., ; Klerks et al., ; Tyler and Triplett, 2008). HPs were found to live on the leaf surface near veins (Brandl and Mandrell, 2002), in the leaf apoplast (intercellular space) (Brandl and Mandrell, ; Solomon et al., ; Niemira, ; Kroupitski et al., ; Barak et al., ; Dinu and Bach, ; Gu et al., ; Roy et al., 2013), and sometimes with affinity for abaxial side of leaf (e.g., S. enterica; (Kroupitski et al., 2011) (4). Salmonella enterica Typhimurium can enter tomato plants via leaves and move through vascular bundles (petioles and stems) (5) into non-inoculated leaves (6) and fruits (8) (Gu et al., 2011). HPs are also found to be associated with flower (7; Guo et al., ; Cooley et al., 2003). Salmonella could travel from infected leaves (4), stems (5), and flowers (7) to colonize the fruit interior (the diagram represents a cross-section of a fruit) and fruit calyx (8) Guo et al., ; Janes et al., ; Barak et al., . Escherichia coli O157:H7 has also been observed in the internal parts of the apple and the seeds following contamination of the flower (8) (Burnett et al., 2000). Movement on the plant surface has also been observed (9; Cooley et al., 2003). Epiphytic Salmonella and E. coli O157:H7 can aggregate near stomata and sub-stomatal space (10; Shaw et al., ; Berger et al., ,; Golberg et al., ; Gu et al., ; Saldaña et al., 2011), reach the sub-stomatal cavity and survive/colonize in the spongy mesophyll (Solomon et al., ; Wachtel et al., ; Warriner et al., ; Jablasone et al., ; Franz et al., 2007). Salmonella cells were observed near trichomes (10; Barak et al., ; Gu et al., 2011). (B) Stem cross-section showing bacteria located in different tissues (Ep, epidermis; C, cortex; V, vascular tissue; Pi, pith) (Deering et al., 2011a,b). (C) Root cross-section showing bacteria on the root surface, internalizing between the epidermal cells, and colonizing root outer and inner cortex, endodermis (En), pericycle (P) and vascular system (Kutter et al., ; Klerks et al., ,; Jayaraman et al., 2014).
Figure 2
Figure 2
Plant cellular defense responses against human pathogens. (A) Upon reception of PAMP (flagellin, LPS) through PRR (FLS2 and putatively others), Salmonella spp. trigger downstream plant defense responses which include ROS production, MPK3/6, salicylic acid (SA) signaling through NPR1, jasmonic acid (JA) and ethylene (ET) signaling, defense-associated gene induction, and extracellular alkalinization. All these cellular events ultimately lead to stomatal closure, antimicrobial activity, and plant defense. (B) Escherichia coli PAMPs (curli, LPS, flagellin, EPS) are also perceived by PRRs (FLS2 and putatively others) present on plant cell surface which triggers the induction of the SA-dependent BGL2 promoter activity and PR1 gene expression. Only components that have been directly demonstrated experimentally are included in the diagram. Plant defense responses in case of both these human pathogens are strain specific as well as plant cultivar specific.

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