Understanding Host-Pathogen Cross Talks and Strategies for Engineering Plants with Enhanced Resistance- a Review

Understanding Host-Pathogen Cross Talks and Strategies for Engineering Plants with Enhanced Resistance- a Review

K. A. Archana¹ , Gangaraju P² , Manjunatha S. E³ , Umesh Babu S⁴

¹Department of Genetics and Plant Breeding, University of Agricultural Sciences, Dharwad-580005, Karnataka, India

²Department of Agricultural Entomology, University of Agricultural Sciences, Dharwad-580 005, Karnataka, India

³Department of Plant Pathology, University of Agricultural Sciences, Dharwad-580005 India

⁴Department of Genetics and Plant Breeding, University of Agricultural Sciences, Raichur-584104, Karnataka, India

Corresponding Author Email: mdkrarchana@gmail.com

DOI : http://dx.doi.org/10.53709/CHE. 2020.v01i01.006

Abstract

Keywords

Download this article as:

Introduction

Co-evolution phenomenon between plants and microbes has led to substantial positive and negative effects. The effects of microbes such as positive (mutualistic), neutral (commensalistic) or deleterious (pathogenic) impact on plant fitness [1] and interactions among them are bidirectional. In general, plants are invaded by different biotic stress factors such as insect-pests, pathogens and weed parasites, which create major crop loss and can be mitigated with management strategies of crop production. There exist continuous and rapid challenges posed by different patho-types against plant system but only few gain entries into host system and cause disease symptoms [2].

Phytopathogens are broadly divided into necrotrophs (those that kill the host and feed on the nutritive materials, biotrophs (those that require a living host to continue their life cycle) and hemibiotrophs (those which require a living host initially, but kill at later stage of infection) [3]. Besides this plants have developed elegant defence systems with a wide range of constitutive and inducible defences to protect themselves from damages against different biotic factors. Plant innate immunity is capable of recognizing potential invading pathogens and mount successful defences. Every pathogen which invades the host could not cause disease, although disease outbreak is likely when the pathogens are able to suppress host defence mechanism [4].

Based on genetic, genomic and biochemical analyses, different layers of plant defence have been uncovered such as Constitutive defences includes preformed barriers such as waxy epidermal cuticles, cell walls and bark [5] and inducible defences namely production of repellents, toxic chemicals, pathogen-degrading enzymes, anti-nutritional effects and deliberate cell suicide [6] and along with these defence layers plants also posses basal and innate resistance [7].

Immune response in plants

Immune response in host against invading pathogens mainly associated with cellular response activation. In unicellular organisms such as amoeba, immune response was phagocytosis (ability to phagocyte foreign material as a part of their food uptake mechanism). Some of the examples for phagocytic cells are neutrophils, macrophages, momocytes, dendritic cells, osteoclasts and eosinophils. Acquisition of innate immune memory can be interpreted as a within-generation resistance mechanism described by Lamarck as non-Darwinian resistance process [8]. The immune response of plants grouped into two main branches namely, innate and adaptive immune responses. The innate immunity is further classified into Primary innate immunity in which basal resistance is triggered with the help of MAMPs (Microbial Associated Molecular Patterns) and PAMPs (Pathogen Associated Molecular Patterns) through recognition of general elicitors where as incase of secondary innate immunity, immune response is recognised by specific elicitors through Effector triggered immunity (ETI) mechanism. Apart from MAMP and ETI, there are other modes of plant immunity such as Systemic Acquired Resistance (SAR) and gene silencing [9].

MAMP triggered immunity (MTI)

Also known as basal or horizontal immunity which involves a class of plasma-membrane-bound extracellular or pattern recognition receptors (PRRs) for identification of microbial elicitors called microbial-associated molecular patterns (MAMPs) [10-11]. MAMPs are precisely conserved, indispensable throughout the whole class of pathogens [14]. It includes oligo galacturonides, bacterial flagellin, Pep-13, ergosterol, xylanase, cold-shock protein and lipopolysaccharides (LPS).

ETI (Effector Triggered Immunity)

It involves recognition of type III secreted effectors (TTSEs) into host cells. It triggers immunity mechanisms by (1) acting as transcription factors that directly activate transcription in host cells, (2) affecting histone packing and chromatin configuration and/or (3) directly targeting host transcription factor activity and ultimately promoting the release of nutrients required for the survival of pathogen [15]. R genes present in plant hosts are precisely capable of recognizing effectors (Avr proteins) produced by the pathogens. Plant-pathogen interaction is controlled by diverse R genes in hosts against numerous pathogens [16].

Systemic acquired resistance (SAR)

The defence mechanism elicited to avoid infection of unharmed tissues at distal parts often referred as SAR [17]. The mode of action mainly depends on endogenous salicylic acid (SA) for activation of pathogenicity related (PR) proteins leading to resistance. There have been some reports in which methyl-SA was believed to be the long-distance signal in SAR because of its presence reported in the phloem sap and exudates collected from SAR-induced cucumber [18], but some scientist found that Me-SA is not essential for induction of SAR. Elicitor BAR11 directly acts upon catalase enzyme in host and trigger various defense mechanisms via ROS, nitrous oxide and plant hormones. ROS induction leads to higher deposition of callose and Hydrogen peroxide (H₂O₂) production. It has triggered early defence mechanism against plant pathogens in Arabidopsis thaliana and Induced Systemic Resistance (ISR) against Pseudomonas syringaepv tomato (Pst) DC3000 [19].

Gene Silencing (GS)

Majorly there are two distinct ways of gene silencing mechanisms in plants viz., transcriptional gene silencing (TGS) and post-transcriptional gene silencing (PTGS) [20-22]. The best way of defense to combat viral infection is by degradation of viral RNA and this can be achieved with an aid of RNA interference (RNAi). In spite of extensive defence response by plants against different microbes there are successful pathogens evolved by effector proteins that can suppress immune response of host. There is always an R gene in the host for the avr gene in the pathogen-this has become a motto when the study aims to research on the host plant resistance.

Pathogen entry and recognition

Pathogen entry is mediated by various components in different patho-systems. In bacteria structural components such as flagelin is a signal exerted by pathogen to which Plasma membrane bound FLS2 plant receptors interact to detect the foreign agent and activate various defence mechanisms. Bacterial pilus plays a vital role in translocation of bacterial effectors and provided direct evidence through several immunological studies [23]. YopT is a bacterial effector (Pseudomonas syringae) having conserved residues that comprise a cysteine protease catalytic activity which is required for autocatalytic processing of an AvrP phB precursor and for eliciting the plant’s hypersensitive response. In animal cells YopJ inhibits MAPK and NF-kB signaling, presumably by disruption of SUMO-1 modification of these regulatory targets [24]. The mutational studies in Arabidopsis showed that a possible plant target CC-NB-LRR protein RPS5 acting as PBS1 protein kinase required specifically forrecognition of AvrPphB mediated by R gene resistance mechanism to Pseudomonassyringaepv Phaseolicola [25]. In case of fungi (oomycetes), the pathogen entry is via various structural components, degradation of plant polymers, Overcoming plant defences, secreted toxins, secreted peptides and hormones. In rice, Magnoportheoryzae was known to directly penetrate the cell membranes acting on cell wall chitin with a high turgor pressure, and growth of mycelia results in subsequent destruction of a living cell [25]. The virus reaches the plant system through specialized channels such as wounds and natural openings like stomata and hydathodes. Plants can recognize the foreign particle i.e., either DNA or RNA produced by viruses in the host cell during replication [26]. Virus particles directly enter into vascular tissues in the plant and disseminating throughout via vector mediated viral infection by sap sucking insects. Plants can sense the attack by various path types. During the process of plant pathogen interaction, pathogen elicitors interact with host receptors, resulting in the activation of numerous signaling pathways including ROS (Reactive oxygen species), MAPKs (Mitogen activated phosphate kinases), ion channel fluxes etc., and in turn conferring resistance to wide range of pathogens.

Phytohormones in plant defence

Phytohormones also play a crucial role in regulating plant growth and development. The three most reported phytohormones against pathogens include salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) [27]. 

SA usually induces resistance mechanisms which are active against bio trophic and hemi-bio trophic pathogens. It also interacts with other phytohormones either synergistically or antagonistically. There is an obvious cross-talk between JA and SA signaling pathways in pepper to control foot rot (Phytophthoracapsici) by differential expression of  PR genes viz. β-1,3-glucanase (PR-2), osmotin (PR-5) and cytosolic ascorbate peroxidase (cAPX, PR-9) in  susceptible (Sreekara) and resistant (04-P24) black pepper lines. Among these cAPX gene expressed in higher level may play a significant role in resistance of 04-P24 to P. capsici [28]. The addition of SA to Arabidopsis seedlings promotes movement of NPR1 (non-express or of PR1), to the nucleus, where it is able to bind several TGA (TGACG DNA motif) class transcription factors, conferring a possible direct route to defence gene induction [29]. foliar application of salicylic acid (7 mM) at 45 days after sowing significantly reduced the late leaf spot incidence and increased the pod yield [30].

JA induces resistance against necrotrophic pathogens. Two branches are distinguished in JA-dependent signaling: (i) MYC2 is the master regulator of the MYC branch,which is Co-regulated by JA and ABA, activating downstream marker genes VSP2 and LOX2, while (ii) EIN3,EIL1 and ERF transcription factors like ERF1 and ORA59 regulate the ERF branch, which is co-regulated by JA and ET, activating the downstream marker gene PDF1.2 [31].

Mutagenised population of arabidopsis ecotype Columbiaglabrous (Colg11) seeds homozygousfor the VSP1 (Vegetative Storage Protein 1) with expression of luciferase reporter transgene for JA responsive promoter were screened for constitutive expression of

VSP1. One of the mutant, cev1 produced plants that were smaller than wild type, had stunted roots with long root hairs accumulated anthocyanin, had constitutive expression of the defense-related genes and had enhanced resistance to powdery mildew diseases [32].

Approaches for discovery of plant-pathogen communication

Several approaches have been used to know and understand plant-pathogen communications and can be broadly classified as sequence acquisition, transcriptome analysis, reverse genetic approaches and transient gene function evaluation.

Comparative analysis to know number of genes involved in different pathways in perennial herb Impatienta. The results revealed that 3491 genes with pathway annotations have been involved in plant pathogen interaction and a total of 164 unigenes were identified through functional annotation to be potentially involved in disease resistance when the host is infected by Plasmo paraobducens causing downy mildew [30].

Transcriptome profiling using In vitro seed colonization technique was carried out to know host-pathogen crosstalk and mechanisms involved with resistance to Aspergillusflavus infection in groundnut by [32] revealed that pathogen require 7 days for sporulation. From differential expression analysis, a total of 4445 DEGs (Differential Expressed Genes) were identified throughout the infection phase. The highest number of DEGs were observed during 1DAI (1,283), followed by 7DAI (1,141), 3DAI (1,061) and 2DAI (960). Among all the DEGs, most abundant genes expressed included senescence-associated proteins, resveratrol synthase, seed linoleate 9s lipoxygenases (9s-LOX), Mlp like protein 43, pathogenesis-related (PR) proteins, peroxidases, glutathione-S-transferase, chalcone synthase, defensins, chitinases, etc.  Among the PR proteins, PR-10 was found to be in abundant during Aspergillus infection and expression was more at 3DAI.

Modes of molecular resistance breeding

Resistance breeding is a remarkable breeding technique followed from time immortal and there have been setbacks in these breeding methods. Basic resistance breeding methods includes selection (Kufri red potato is disease resistant selection from Darjeeling Red Round), Introduction (Ridely Wheat introduced from Australia had been useful as a rust resistant varieties) and

While breeding for molecular resistance, breeder should aware of different ways to achieve it such as R gene based breeding, PR gene based resistance, over-expression of master switches and transcription factors involved in defence various mechanisms. Before choosing any method for resistance breeding, breeder should consider the objectives.

Discussion

In the recent past, both human and plants are affected by climate change such as rise in temperature, chilling weathers, uncertainty of rainfall etc., this associated temperature rises shown to increase geographic distribution and reproductive potential of pathogens. A vast data has been generated by researchers about interaction between biotic stress and their associated receptors, signaling pathways and genes. Better understanding of these interactions along with molecular approaches has led a foundation for engineering of plants for enhanced resistance. Whenever pathogen attacks host certain defence mechanisms and cascades gets activated either to confer resistance or susceptibility. Plants activate innate and acquired immunity through expression of certain specific receptor and enzyme activity. Hence understanding its nature of communication between host and pathogen is helpful in breeding for resistance for different race of patho-types.

However, a look backward for the research and views made by various eminent scientists towards the knowledge about host pathogen interactions will be helpful in future. A thorough understanding of future challenges, available resistance genes, avr proteins and their interaction can be very much helpful to break the hurdles of susceptibility reaction showed by host system on attack by numerous lists of pathogens. In this regard we have to work for resistance breeding for ensuring break point for the challenges posed by various stresses in this field in mere future.

References:

[1].         Bari, R. and Jones, J. D., 2009, Role of plant hormones in plant defence responses. Plant Mol. Biol., 69:473–488.

[2].         Beck, M., Heard, W., Mbengue, M. and Robatzek, S., 2012, The Ins andOUTs of pattern recognition receptors at the cell surface. Curr.Opin. Plant Biol., 15: 367–374.

[3].         Christine, E. and John, G.  T., 2001, Thearabidopsis mutant cev1 has constitutively activejasmonate and ethylene signal pathways and enhanced resistance to pathogens. The Plant Cell, 13: 1025–1033.

[4].         Conti, G., Rodriguez, M. C., Venturuzzi, A. L. and Asurmendi, S., 2017, Modulation of host plant immunity by Tobamo virus proteins. Ann. Bot., 119(5): 737–747.

[5].         Dodds, P. N. and Rathjen, J. P., 2010, Plant Immunity: towards an integratedview of plant-pathogen interactions. Nat., 11: 539–548.

[6].         Durrant, W. E. and Dong, X., 2004, Systemic acquired resistance. Annu.Rev. Phytopathol., 42: 185–209.

[7].         Fan, W. H. and Dong, X. N., 2002, In vivo interaction between NPR1 and transcription factor TGA2 leads to salicylic acid-mediated gene activation in Arabidopsis. Plant Cell, 14:1377-1389.

[8].         Felix, G.,Regenass, M. and Boller, T., 1993, Specific perception ofsubnanomolar concentrations of chitin fragments by tomatocells. Induction of extracellular alkalinization, changes in proteinphosphorylation, and establishment of a refractory state. Plant J., 4: 307-316.

[9].         Feng, F. and Zhou, J. M., 2012, Plant-bacterial interactions mediated bytype III effectors. Curr.Opin. Plant Biol., 15: 469-476.

[10].       Ghosh, P., Anjanabha, B and Bharat, C., 2017, Manipulating disease and pest resistance pathways in plants for enhanced crop improvement. Biosci.Biotech. Res. Comm., 10(4): 631-644.

[11].       Gourbal, B., Pinaud, S., GeroldJ. M. B., Jos W. M. V.  D. M., Conrath, U and Mihai G. N., 2018, Innate immune memory: An evolutionary perspective. Immunol Rev., 283: 21–40.

[12].       Hammond-Kossack, K. and Jones, J. D. G., 2000, Responses to PlantPathogens.Biochemistry and molecular biology of plants(eds), 1102–1146.

[13].       Kim, E.H.  K. and Jane E. P., 2003,Deciphering plant–pathogen communication: fresh perspectives for molecular resistance breeding. Curr.Opin.Biotech.,14:177–193.

[14].       Krishna, B., Weining, W., Zhe, C. and Zhanao, D., 2018, Comparative analysis of Impatiens leaf transcriptomes reveal candidate genes for resistance to downy mildew caused by Plasmoparaobducens. Int. J. Mol. Sci., 19: 2057.

[15].       Li, C. M., Brown, I., Mansfield, J., Stevens, C., Boureau, T., Romantschuk, M and Taira, S., 2002, The Hrppilus of Pseudomonas syringae elongates from its tip and acts as a conduit for translocation of the effector protein Hrp Z.  Embo. J., 21: 1909-1915.

[16].       Lu, R., Martin-Hernandez, A. M., Peart, J. R., Malcuit, I. and Baulcombe, D. C., 2003, Virus-induced gene silencing in plants.Methods. 30: 296–303.

[17].       Martin, G. B., Bogdanove, A. J. and Sessa, G., 2003, Understanding thefunctions of plant disease resistance proteins. Annu. Rev. PlantBiol., 54: 23–61.

[18].       Muthamilarasan, M. and Prasad, M., 2013, Plant innate immunity: An updated insight into defense mechanism.J. Biosci., 38(2): 433–449.

[19].       Nayak, N. S., Gaurav, A., Manish, K. P., Hari, K. S., Ashwin, S. J., Shilp, P., Aarthi, D., Liyun, W., Baozhu, G., Boshou, L. and Rajeev, K. V., 2017, Aspergillusflavusinfection triggered immune responses and host-pathogen cross-talks in groundnut during in-vitro seedcolonization. Nat.Scientific Rep., 7:9659.

[20].       Onaga, G. and Kerstin, W., 2017, Advances in plant tolerance to biotic stresses. Intech plant genomics, 10:57-67.

[21].       Orth, K., Xu, Z., Mudgett, M. B., Bao, Z. Q., Palmer, L. E., Bliska, J. B., Mangel, W. F., Staskawicz, B. and Dixon, J. E., 2002, Disruption of signaling by Yersiniaeffector YopJ, a ubiquitin-like protein protease. Sci., 290:1594-1597.

[22].       Padmanabhan, C., Zhang, X. and Jin, H., 2009, Host small RNAs are bigcontributors to plant innate immunity. Curr.Opin. Plant Biol., 12: 465–472.

[23].       Rasmussen, J. B., Hammer, S. R. and Zook, N., 1991, Systemicinduction of salicylic acid accumulation in cucumber after inoculation with Pseudomonas syringaepv.syringae. Plant Physiol., 97: 1342–1347.

[24].       Sahu, P. P., Puranik, S., Khan, M. and Prasad, M., 2012a, Recent advancesin tomato functional genomics: utilization of VIGS. Protoplasma, 09:1007

[25].       Swiderski, M. R. and Innes, R. W., 2002, The Arabidopsis PBS1 resistance geneencodes a member of a novel protein kinase subfamily.Plant J.,26:101-112.

[26].       Shifa, H., Gopalakrishnan, C. and Velazhahan, R., 2018, Management of late leaf spot (Phaeoisariopsispersonata) and root rot (Macrophominaphaseolina) diseases of groundnut(Arachishypogaea L.) with plant growth-promoting rhizobacteria, systemic acquired resistance inducers and plant extracts. Phytoparasitica, 46:19-30.

[27].       Thrall, P. H., Hochberg, M, E., Burdon, J. J. and Bever, J. D., 2007, Coevolution of symbiotic mutualists and parasites in a community context. Trends Ecol. Evol., 22:120-126.

[28].       Vandana, V. V. and Suseela, B. R., 2018, Differential expression of PR genes in response to Phytophthoracapsici inoculation in resistant and susceptible black pepper (Piper nigrum L.) lines. Eur. J. Plant Pathol., 150:713–724.

[29].       Vos, I. A., Verhage, A., Schuurink, R. C., Watt, L. G., Pieterse, C. M .J. and Van Wees, S.C.M., 2013b, Onset of herbivore-induced resistance in systemic tissue primedforjasmonate-dependentdefencesisactivatedbyabscisicacid. Front. Plant Sci., 4:539.

[30].       Wasternack,C.and Hause, B., 2013, Jasmonates: biosynthesis, perception, signal transductionandactioninplantstressresponse,growthanddevelopment. Ann. Rev. Bot.,111: 1021-1058.

[31].       Yulin, J., Guangjie, L., Stefano, C., Seonghee, L. and Yuntao, D., 2009, Current progress on genetic interactions of rice with rice blast and sheath blight fungi, Front. Agric., 3(3): 231–239.

[32].       Zhang, Y., Yan, X., Guo, H., Zhao, F. and Huang, L., 2018, A novel protein elicitor BAR11 from SaccharothrixyanglingensisHhs.015 improves plant resistance to pathogens and interacts with catalases as targets. Front. Microbiol., 9:700.