Abstract

         Plants infected with microorganism such as bacteria, fungi and virus result in diverse outcomes ranging from symptom-less resistance to severe disease and in cases even death. Among various factors, microbes are the major cause of agriculture losses worldwide. Undeniably, plant pathogens are controlled through application of various agrochemicals that may result in causing diseases in humans and also impact environmental and food security. While on other hand many approaches are explored to control pathogens based on molecular biology and phytochemistry to identify factors that halt an invading microbial pathogen from further invasion into or between plant cells. Plant chemical defense compounds, antimicrobial proteins and structural reinforcement of cell walls appear to slow down microbial growth or even prevent microbial penetration in resistant plants. Additionally, plants induce hypersensitive response which is a strong local immune reaction, including programmed cell death at the site of infection that stop certain biotrophic microbes from spreading into surrounding tissue. An overview of few pathogenesis of some bacterial and fungal infection in plants is discussed.

Keywords: Pathogen, Bacteria, Fungi, Plants, Pathogenesis.

Introduction

         The growing human population requires a significant increase in agricultural production. This challenge is made more difficult by the facts that changes in the climatic and environmental conditions under which crops are grown, have resulted in the appearance of new diseases. Genetic changes within the pathogen, had shown the loss of previously effective sources of resistance. Annually, large agricultural losses occur worldwide due to the susceptibility of crops to diseases caused by plant pathogens, impacting productivity and reducing the commercial value of the product. Losses are estimated upto78% in fruit crops, 54% in vegetable crops, and 32% in cereal crops due to diseases caused by pathogens. To help meet this challenge, advanced genetic and statistical methods of analysis have been used to identify new resistance genes through global screens. Studies of plant–pathogen interactions have been undertaken to uncover the mechanisms of disease resistance (Boyd et al., 2013). The informed deployment of major, race-specific and partial, racenonspecific resistance, either by conventional breeding or transgenic approaches, will enable the production of crop varieties with effective resistance without impacting on other agronomically important crop traits. Hence, the present review discusses pathogenesis of some plants pathogens.

Bacteria and plants

         Phytopathogenic organisms including bacteria, fungi, oomyctes, and animals are able to secrete effector proteins to modulate host processes to their advantage (Hogenhout et al., 2009). Effectors secreted through two major groups of phytopathogenic bacteria, Pseudomonas syringae and Xanthomonas spp. have served as primary models for these studies (Block and Alfano, 2011). Through intimate associations with plant protein and/or DNA inside the plant cell, phytopathogenic bacteria effectors alter a variety of cellular and physiological processes for pathogenesis. Some of these effectors are also recognized by plant resistance proteins and trigger immunity as a result of host adaptation to the pathogen. One such effectors is type III effectors and its mode of action is of central importance to bacterial pathogenesis and plant recognition of the pathogen. It is from this, a great deal of host targets and biochemical functions of a number of phytopathogenic bacteria is learned. Several common themes have emerged concerning how effectors promote parasitism in their host plants. Furthermore, components of these modules apparently have evolved to detect pathogen effectors and trigger effectors triggered immunity (ETI), providing exciting insight into host–pathogen co-evolution. The general pathogenesis of bacteria infection on plant leaf is shown in Figure 1.

Figure 1: Simplified diagram of the infection cycle of Pseudomonas syringae. (a) A healthy plant leaves. (b) Bacterial cells on a leaf surface, illustrating aggregation of some bacteria near a trichome. (c) Bacteria penetrating open stomate. (d) Cross-section of a leaf showing bacteria colonizing the plant apoplast. (e) Extensive multiplication of bacteria in the apoplast of a leaf. (f) Visible diseaseassociated necrosis and chlorosis.

Recognition of fungal pathogens by plants

         Plant as host recogonize fungal-derive components through specific host responses elicited during infection and reduce fungal proliferation. But fungal pathogens have made a lot of adaptations that enable them to attack plant tissues, overcome its defence mechanisms, and take possession of plant tissues for increase growth, development, and reproduction (Shuping and Eloff, 2017). For triumphant pathogenicity, fungi utilize various infection strategies to breach the host cuticle. While some enter their host through wounds and stomata while others such as viral pathogens are usually introduced directly into the plant cells by insects (Agrios, 2012). Therefore, it is important to deeply understand the mechanisms of pathogenesis of these pathogens in order to develop appropriate strategies to control or prevent post-harvest diseases. One such mechanism of action for the fungal pathogen Erysiphegraminis f. sp. hordei, an obligate parasite that infects barley leavesis shown in Figure 2.

         The fungal hyphae develops appressorium, a special cell for attachment to host leaf involves series of stages, which includes; i.) nuclear mitosis, ii.) first septum formation, iii.) emergence of germling, iv.) tip swelling, and v.) second septum formation (Matrose, et al., 2020). The first nuclear division occurs after surface attachment and a nucleus from the second round of cell division migrates into the enthusiastic cell during tip bulge and before septum formation. A mature appressorium usually contain a single nucleus. Inside the appressorium turgor pressure increases and a fine growing point, called the penetration hyphae develops that passes through the host cuticle and cell wall (Li et al., 2019).The penetration hyphae are crucial for the germ tube attachment and appressorium formation (Xu et al.,2018). If any of process in this mechanism could be interrupted then the plant could successfully evade this pathogen. Hence, most of the studies are made in understating the biology of appressorium development in plant pathogenic fungi.

Applications

         Various advantages evinced by the synthetic communities also include its potential to co-exist with communities that can never be achieved in the natural environment. Such combinatorial benefits enhance the metabolic activity there by leading to a better or entirely new function that cannot be achieved by naturally prevailing communities. This pragmatic approach is highly effective in biodiesel production and synthesis of bio active compounds (Bader et al., 2010). The enhanced metabolic activities of the combinatorial synthetic communities have been successfully exploited for the production of various resources that include hydrogen (Asada et al., 2006), acetic acid (Collet et al., 2005; Kondo and Kondo 1996; Talabardon et al., 2000) and lactic acid (Taniguchi et al., 2004; Roble et al., 2003). Degradation of polycyclic aromatic hydrocarbons (Boonchan et al., 2000) and cellulose (Haruta et al., 2002; Poszytek et al., 2016) has also been successfully carried out by this approach. Treatment of textile effluent has been made easy by this combinatorial approach whereby the researchers have combined three different species which can successfully degrade the textile effluents thereby improving the water quality (Ayed et al., 2010).

         In addition to interactions, stomatal regulation has also been observed in some plant-fungal interactions. For example, the fungal toxin fusicoccin has long been known to promote stomatal opening and to antagonize ABA-induced stomatal closure through activation of a plasma membrane H+ ATPase; it has been widely used as a tool for the study of many aspects of plant biology.

Figure 2: Diagram showing infection of barley by the fungal pathogen Erysiphegraminis f. sp. hordei, an obligate parasite that infects barley leaves using a specialized cell called an appressorium which penetrates the leaf cuticle. The fungus is an extremely parasite of the living plant, and produces a specialized feeding structure, the haustorium, that allows it to subsist in leaf epidermal cells (Talbot and Hamer, 2000).

         On contrary, treating the microbial consortia on the whole as a single unit has also been proven effective. This technique is specifically used for consortium with proven emergent functions. The marine consortium used above-mentioned approach to develop synthetic combinatorial community that can effectively fix CO₂ in the marine environment (Hu et al., 2014; 91. Hu et al., 2016). This approach has also successfully improved the lignocelluloytic enzyme activity by designing a synthetic consortium from a preexisting consortium with cellulolytic activity (Poszytek et al., 2016). Apart from reducing the number of community members in the consortium this technique also allows to include new species in the consortium that cannot be isolated. These techniques allow the research community to effectively evaluate and improve the functional abilities of a pre-existing synthetic consortium without drastically altering the combinatorial evaluations.

Conclusion

         The infection of plants and subsequent losses of fruits, vegetables, cereals, etc. due to microbial pathogens remain a global challenge. Majority of the existing disease management strategies are heavily dependent on the use of synthetic bactericide, virucide, fungicide which are associated with health, safety and economic implications. Therefore, the need to develop more efficient and economically sustainable disease management strategies is crucial. In front of a crisis, the probably worst option to choose is not to choose. Thus, looking for compounds that could potentially increase natural plant immunity or affect the pathogenesis of microbial pathogens through biological and eco-friendly approach is must. Hence, understand the pathogenesis of microbial infections and plant interactions thoroughly would be best for exploitation to develop effective and environment friendly compounds.

References

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