Plants are non motile but they constantly encounters both abiotic and biotic stress. There is a constant war between the pathogenic microbes and the host plants-the outcome of which determines resistance or disease.

I am very interested in plant bacterial pathogenesis and the mechanisms by which they utilize host resources for their own growth and survival. Genome sequence and resources for many plant pathogenic bacteria are now available like- Xanthomonas, Pseudomonas, Ralstonia including many of their model plant host system such as rice, Arabidopsis. I want to identify both host and pathogen factors that promote and suppress pathogenesis.

a) Extracellular cell-cell communication system in plant pathogenic bacteria

Several plant pathogenic bacteria make extracellular signaling molecules in a density dependent manner to co-ordinate regulation of virulence factor synthesis. The phenomena called as quorum sensing (Fig. 1) is an important trait in the pathogenesis of many plant pathogenic bacteria like –Xanthomonas, Pseudomonas.

Xanthomonas makes an extracellular fatty acid like signaling molecule called as Diffusible Signaling Molecule (DSF), which appears to differ in structure as well as mode of action in regulating virulence functions across closely related Xanthomonas. We will use both forward and reverse genetics approach to identify role of DSF like signaling molecules in Xanhomonas like- Rice pathogen’s= Xanthomonas oryzae pv. oryzae, Xanthomonas oryzae pv. oryzaecola., crucifer pathogen= Xanthomonas campestris pv. campestris (Xcc) and the tomato pathogen=Xanthomonas campestris pv. vesicatoria (XCV).

Fig. 1. A simplified model of plant-microbe interaction. Plant pathogenic bacteria secrete different host modulating components by various secretion systems which include toxins, extracellular enzymes and effectors. Plants recognize these effectors including microbial surface components like flagella and mount a defense response. Bacteria secrete quorum sensing molecules and regulate production of virulence functions. Integration of signals from the plant and the extracellular environment is coupled with two component signal transduction systems

1. Role of DSF in virulence of these pathogen-How DSF regulate virulence? Mechanism of DSF mediated signal transduction in regulating virulence factor synthesis.
2. What are the environmental conditions inside the plants which effect DSF production and sensing?
3. Mode of DSF production inside the plant
4. Mechanism of DSF mediated biofilm formation both inside the plant (in planta) and in vitro

Recently there is elucidation of genome sequence of several microbes. It appears that several plant and soil associated bacteria (which do not cause disease in plants but are associated with them (as endophytes or epiphytes) encodes potential DSF like quorum sensing system, which rises interesting questions- Whether there is cross talk with DSF like signaling molecules between these pathogenic and non pathogenic bacteria ?. To what extent these cross talks influence the out come of the plant-microbe interaction (Disease or Resistance)?

b) Role of di-Cylcic GMP as intracellular signaling molecule in plant pathogens

Recent advances in plant-pathogen interaction as elucidated the role of cyclic di-GMP as an intracellular signaling molecule which couples extracellular quorum sensing signal and is involved in variety of process like-Biofilm formation, motility, quorum sensing and regulation of virulence gene expression (Fig. 2). Three protein domains-HD-GYP, GGDEF and EAL have been implicated in the synthesis and degradation of di-Cyclic GMP (Fig. 3). Genome analysis indicates that various Xanthomonas have proteins with potential di-Cyclic GMP modulating activity. It will be interesting to determine the role of these proteins in pathogenesis and role of di-cyclic GMP in modulating behavior of the pathogens inside the host plant.

Interestingly several two component signal transduction systems have the di-cyclic GMP modulating domains, role of these in virulence and adaptation inside the plant will give us understanding of how pathogens couples different extracellular signals with physiological adaptation to different environment (Fig. 3). 

Fig. 2. di-Cyclic GMP

Fig. 3. Two component signal transduction system with di-Cyclic GMP modulating domains influence quorum sensing and interaction of pathogen with plant.

c) Role of secretion of virulence factors and modulation of plant environment

Plants have an innate response to pathogen by which they can recognize a pathogen or a beneficial microbe. Some bacterial derived components like LPS (lipopolysaccharide), flagella etc. (Fig. 1), is recognized by plants and they can mount a localized defense response. These components are called as PAMP or MAMP (Pathogen associated molecular pattern; microbe associated molecular pattern). The defense response in plants is mediated by the synthesis of defense proteins, reactive oxygen species and cross linking of cell wall, so that to restrict the spread of the pathogen (Fig. 1).

Pathogens are clever, as they have evolved several types of secretion systems which can either directly deliver proteins inside the plant cell or in the near vicinity. These proteins can suppress the plant defense response, and hence the pathogen can multiply inside the plant and can cause disease.

We will be interested in determining role of PAMP and MAMP’s in virulence and host response. The mechanism of action of secreted bacterial effectors and how they are able to suppress the plant defense response. Using genetic screens for protein delivery system inside the plant cell and Agrobacterium mediated transient transformation, we will be interested in identifying novel effectors as well as to visualize the localization of these secreted effectors inside the plant cell. Based on these genetic studies, we hope to finally address the actual biochemical roles of these effectors inside the plant cell.

To address the components of plant which are involved in defense response, we will use mutants in different defense signaling pathways as well as transient loss of function studies using VIGS (Virus Induced Gene Silencing).

d) Alternative secretion systems of plant pathogens and their role in plant colonization

Xanthomonas group of plant pathogens are interesting as they have a very wide host range as well as they exhibit variability in the mode of entry inside the plants. Genome sequence of several Xanthomonas plant pathogens have now been completed and has elucidated some very exciting features. There are several different kind of secretion system like-Type III (Fig. 1; syringe like apparatus which can directly deliver bacterial effectors molecules inside the plant cell) and Type I and TypeII, which can deliver effectors proteins in the vicinity of plant cell and can modulate pathogenesis. We will be interested in determining the role of these effectors secretion systems in virulence and role in fitness of these pathogens. Several alternative secretion systems like Type IV and TypeV are present in Xanthomonas and pseudomonas group of plant pathogens. It will be also interesting to understand the role of these system in virulence as well as effectors delivered by these alternative secretion systems

e) Biology of Xanthomonas citri

Xanthomonas citri is an important member of the Xanthomonas group of plant pathogen. It causes an economically important disease called as Citrus Canker. In India, it causes severe economic loss in the citrus production. Very little information is known about the biology of this pathogen, its interaction with host plant. Large genetic variation and lack of well defined resistance citrus plants makes it a challenging pathosystem to work with. We will explore the biology of this pathogen by molecular genetics studies which will elucidate new virulence functions in this pathogen