This branch of the immune system known as pathogen-triggered immunity is the first line of active defense against infection. Human pathogen on plants is an emerging field that only recently has caught the attention of plant biologists and phytopathologists. A few studies have been reported in the last 5–10 years, which focused on the most well studied PAMPs, flagellin and lipopolysaccharide , in the interaction of human pathogens with plants. Table 1 lists the plants, bacterial strains, and method details for such studies.Flagellin, the structural component of flagellum in bacteria, is involved in bacterial attachment and motility on the plant , is recognized by plant through the FLS2 receptor , and induces plant defenses . Similar to the well-studied PTI elicitor flg22 , the flg22 epitope of S. enterica serovar Typhimurium 14028 is also an effective PAMP and elicitor of downstream immune responses in Arabidopsis , tobacco, and tomato plants . Flagellum-deficient mutants of S. enterica serovar Typhimurium 14028 are better colonizers of wheat, alfalfa, and Arabidopsis roots as compared to the wild type bacterium further suggesting that the Salmonella flagellum induces plant defenses that may restrict bacterial colonization of several plant organs. However, the Salmonella flg22 peptide is not the only PAMP for elicitation of plant immune response as fls2mutant of Arabidopsis still shows a low level of PTI activation in response to this PAMP . Purified flagellin or derived epitopes of E. coli O157:H7 has not been used to induce plant defenses. However, flagellum-deficient mutant of this strain does not activate the SA-dependent BGL2 gene promoter as much as the wild type strain and shows larger population in Arabidopsis than the wild type strain further suggesting that surface structures in the bacterial cell are perceived by plants. The differences in responses observed could be attributed to the presence of other microbial signatures eliciting plant defense.Although flagellin sequences from S. enterica strains and other bacteria are highly conserved,best vertical garden system even a minor change of five amino acids in the flg22 epitope leads to reduced activation of PTI in Arabidopsis, tobacco, and tomato plants .
Adding to the specificity, it has also been shown that Brassicaceae and Solanocecae plants recognize specific flagellin . Hence, evolving variations in flagellin sequences could be a strategy employed by the pathogens to avoid plant recognition, which in turn leads to the development of pathogen- specific immune responses in the plant. Flagella also play an important role in bacterial behavior on the plant. Several studies have pointed out to the usefulness of flagella for attachment to leaf surfaces and movement on plant surfaces .Lipopolysaccharide is a component of the cell wall of Gram-negative bacterial pathogens of animals and plants. In the animal host, LPS is a well-characterized PAMP that is recognized by host Toll-like receptor 4 . In plants however, receptors for LPS have not been discovered yet. Nonetheless, current evidence suggests that human pathogen-derived LPS can be perceived by plants resulting in PTI activation. For instance, on the leaf surface, purified LPS from Pseudomonas aeruginosa, S. Minnesota R595, and E. coli O55:B5 induces strong stomatal closure in Arabidopsis . Purified LPS from Salmonella triggers of ROS production and extracellular alkalinization in tobacco cell suspension but not on tomato leaves suggesting that LPS recognition may be either dependent on experimental conditions or variable among plant species. Genetic evidence suggests that the high activity of SA dependent BGL2 gene promoter in Arabidopsis is dependent on the presence of LPS in E. coli O157:H7 as higher activity of this promoter was observed in the wild type bacterial as compared to its LPS mutant . However, LPS-dependent responses seem not to be sufficient to restrict bacterial survival on plants as the population titer of E. coli O157:H7 LPS mutant or wild type in plant is essentially the same . Additionally, live S. Typhimurium cells do not induce ROS in epidermal tissue of tobacco suggesting that, at least Salmonella, can suppress LPS-induced ROS and extracellular alkalinization. Similar to flagellin, the O-antigen moiety of LPS is not only important for plant perception of bacterial cells, but also for bacterial attachment, fitness, and survival on plants .
One of the earliest PTI responses in plants is stomatal closure that greatly decreases the rate of pathogen entry into plant’s internal tissues. This response requires molecular components of PTI including such as flagellin and LPS perception and hormone perception and signaling . Stomatal immunity is also triggered by the presence of human pathogens S. enterica serovar Typhimurium SL1344 and E. coli O157:H7 , albeit at various levels. For instance, E. coli O157:H7 induces a strong stomatal immunity and Salmonella SL1344 elicits only a transient stomatal closure in both Arabidopsis and lettuce suggesting that the bacterial strain SL1344 can either induce weaker or subvert stomata-based defense. Active suppression of stomatal closure by SL1344 may be unlikely because it cannot re-open dark-closed stomata . However, it is possible that signaling pathways underlying bacterium-triggered and dark-induced stomatal closure are not entirely overlapping and SL1344 acts on immunity- specific signaling to subvert stomatal closure.Recognition of PAMPs by PRRs leads to several hallmark cellular defense responses that are categorized based on the timing of response. Zipfel and Robatzek have discussed that early responses occur within seconds to minutes of recognition including ion fluxes, extracellular alkalinization, and oxidative burst. Intermediate responses occur within minutes to hours including stomatal closure, ethylene production, mitogen-activated protein kinase signaling, and transcriptional reprogramming. Late responses occur from hours to days and involve callose deposition, salicylic acid accumulation, and defense gene transcription. These hallmark plant cellular defenses have also been tested for both E. coli and S. enterica . In particular, S. enterica infection results in the induction of MPK3/MPK6 kinase activity and plant defense-associated genes PDF1.2, PR1, and PR2 in Arabidopsis leaves as well as PR1, PR4, and PR5 in lettuce . MPK6 activation in Arabidopsis is independent of FLS2 , indicating that flagellin is not the only active PAMP of Salmonella and plant response to other PAMPs may converge at MAPK signaling.
Direct comparison of the PR1 gene expression in Arabidopsis indicated that both E. coli O157:H7 and Salmonella SL1344 are able to induce this defense marker gene, however at difference levels . The PR1 gene induction is low in SL1344-infected plants indicating that immune responses are either weaker or are suppressed by Salmonella.The ethylene-insensitive mutant of Arabidopsis, ein2, supports higher Salmonella 14028 inside whole seedlings as compared to the wild type Col-0 plants . Furthermore, addition of a specific inhibitor of ethylene mediated signaling, 1- methylcyclopropene , to the growth medium resulted in increased S. enterica 14028 endophytic colonization of Medicago truncatula, but not M. sativum, roots and hypocotyls suggesting that the role of endogenous ethylene signaling maybe be specific to each plant-bacterium interaction. However, ethylene signaling may play a contrasting role during fruit contamination. Tomato mutants with defects in ethylene synthesis, perception, and signal transduction show significantly reduced Salmonella proliferation within their fruits as compared to the wild type control .Similar to the ein2 mutant, the coronatine-insensitive mutant of Arabidopsis, coi1-16, also supports high Salmonella 14028 inside whole seedlings . Along with the induction of the jasmonate-responsive gene PDF1.2 addressed in the same study and mentioned above, it seems that jasmonate signaling isalso an important component to restrict Salmonella infection in, at least, Arabidopsis. These results are surprising as coi1 mutants are well known to have increased resistant to various bacterial pathogen of plants, such as P. syringae, but not to fungal or viral pathogens .Two genetic lines of Arabidopsis has been extensively used to determine the role of salicylic acid in plant defenses against phytopathogens,frambuesa cultivo the transgenic nahG plant that cannot accumulate SA and the null mutant npr1 that is disrupted in both SA-dependent and -independent defense responses . Both of these plant lines support higher populations of Salmonella 14028 inside their roots and seedlings as compared to the wild type plant. NPR1-dependent signaling is important reduce the population of the curli-negative strain of E. coli O157:H7 43895 but not for the curli-positive strain 86-24 in Arabidopsis leaves . Although only a few strains of Salmonella and E. coli have been used, there is an emerging patterns suggesting that SA itself and activation of SA-signaling can potentially restrict HPOP. In attempts to understand the overall cellular transcriptional response to human bacterial pathogens, global transcriptomic analyses have been used. Thilmony et al. showed that E. coli O157:H7 regulates PTI-associated genes in Arabidopsis leaves, albeit in a flagellin-independent manner. A similar transcriptomic analysis with medium-grown Arabidopsis seedlings 2h after inoculation with S. enterica serovar Typhimurium 14028, E. coli K-12, and P. syringae pv. tomato DC3000 showed a strong overlap among genes responsive to each bacterial infection suggesting a common mechanism of plant basal response toward bacteria . Gene expression analysis of Medicago truncatula seedlings root-inoculated with only two bacterial cells per plant indicated that 83 gene probes were commonly regulated in response to S. enterica and E. coli O157:H7 . All together, these studies indicate that each human pathogenic bacterium can modulate specific plant genes beyond a basal defense response; however the mechanisms for plant-bacterium specificity are largely unknown.Successful virulent pathogens of plants are able to defeat this army plant defense by employing its own set of artillery and cause disease in the host plant .
In incompatible interactions , the host plant already has pre-evolved molecules that recognize these effectors and cause a specific defense response to this pathogen. This specific response is called effector-triggered immunity . Because the type 3-secretion system is important for the virulence of both animal and plant pathogenic bacteria on their natural hosts as evidenced by the use of bacterial mutants, it is reasonable to expect that T3SS would be important for HPOP as well. However, animal and plant cell surfaces are structurally different; the plant cells wall seems to be impenetrable by the secretion needle of the extracellular animal pathogens as discussed by He et al. raising the question of how these effectors can reach the plant cytoplasm and interfere with plant defenses. To date, there is no evidence for the ability of human pathogens to inject T3SS effectors inside plant cells. It is possible that the T3SS is still active on the plant cell surface and the effectors are secreted into the plant apoplast. If that is the case, however, plant membrane receptors would be necessary to recognize the effectors and trigger plant cellular responses. Nevertheless, it has been observed that the T3SS mutant of E. coli O157:H7, escN, has reduced ability to attach to and colonize baby spinach leaves similar to the fliC mutant . Furthermore, apoplastic population of T3SS structural mutants of S. enterica serovar Typhimurium 14028 is smaller than that of the wild type bacterium in Arabidopsis leaves and plant defense-associated genes are up-regulated for longer time by the prgH mutant than wild type Salmonella in Arabidopsis seedlings . Contrary to these findings, Iniguez et al. reported that two Salmonella 14028 T3SS-SPI1, the structural mutant spaS and the effector mutant sipB, hypercolonize roots and hypocotyls of M. sativum and fail to induce SA-dependent PR1 promoter in Arabidopsis leaves. More studies need to be conducted to conclude whether T3SS of Salmonella acts as “recognizable” surface structure similar to flagellum and/or as a conduit to deliver effectors in plant tissues and trigger ETI. It is worth mentioning that T3SS and effectors of the phytopathogen P. syringae pv. syringae have functions on ETI as well as bacterial fitness on plant surface and the filamentous T3SS protein EspA is required for E. coli O157:H7 attachment to arugula leaves . The invA structural mutant, that is defective in all T3SS-1 system-associated phenotypes, induces high ROS and extracellular alkalinizing in tobacco BY-2 cell suspension and hypersensitive reaction in tobacco leaves as compared to the wild type strain suggesting that T3SS is important for this suppression of immunity. However, Shirron and Yaron also reported that plant response to the regulatory mutant phoP that modulates the expression of many effector proteins and membrane components , is no different to that of the wild type bacterium. These findings raised the question whether the phenotypes observed are due to the T3SS structure itself or due to the translocated effectors.