Research

Most research in our lab is on basic and applied aspects of bacterial plant diseases, with emphasis on bacterial fruit blotch of cucurbits caused by Acidovorax citrulli and plant diseases caused by Xanthomonas species. With that said, we are also interested in plant-associated beneficial bacteria that may contribute to plant health by promoting growth and/or inhibiting pathogens (biological control).

Bacterial fruit blotch of cucurbits

Acidovorax citrulli is the causal agent of bacterial fruit blotch (BFB), a serious disease of cucurbits, and mainly of watermelon and melon. The unavailability of effective tools for managing BFB, the lack of reliable sources of BFB resistance, and the disease's high destructive potential, place BFB as one of the major threats to the cucurbit industry. We are cooperating with others to develop more efficient tools for BFB management.

We are also conducting basic research to increase our knowledge on BFB pathogenesis. Despite the economic importance of BFB, little is known about basic aspects of this disease. We optimized molecular techniques and inoculation assays, and are screening for bacterial genes that contribute to the virulence of A. citrulli. We demonstrated that polar flagella and type IV pili (T4P) play important roles in virulence of this bacterium. Furthermore, we characterized phenotypic variation in A. citrulli and found that this phenomenon is associated with the loss of the ability to synthesize T4P.

As many Gram-negative pathogenic bacteria, A. citrulli requires a functional type III secretion system (T3SS) for pathogenicity. Recently, we sequenced the genome of strain M6, which was isolated in Israel from a symptomatic melon fruit in 2002, and is the model strain in our lab for basic investigation of A. citrulli-host interactions. We found that A. citrulli strains possess a large arsenal of type III-secreted effectors (T3Es), including some that seem to occur only in plant-pathogenic Acidovorax spp. We are currently investigating the mode of action of selected T3Es as well as their contribution to virulence and host preferential association. In addition, complete assembly of the A. citrulli M6 genome revealed the occurrence of a low-copy number plasmid in this bacterium, which we named pACM6. We found this or highly similar plasmids occur in other (but not all) A. citrulli strains. We are currently investigating the role of pACM6 in virulence and fitness of this pathogen.

Our collaborators in some A. citrulli-projects: Prof. Ron Walcott (University of Georgia, USA), Prof. Bingyu Zhao (Virginia Tech, USA), Prof. Gregory Welbaum (Virginia Tech, USA), Prof. Guido Sessa (Tel Aviv University), Prof. Tal Pupko (Tel Aviv University) and Dr. Shulamit Manulis-Sasson (Volcani Center).

Phenotypic variation in Acidovorax citrulli. See more in Kumar-Shrestha et al. (PLoS One 2013).

Figure taken from Burdman and Walcott (Mol. Plant Pathol. 2012).

Complete assembly of the A. citrulli M6 genome revealed a low-copy number plasmid, pACM6, in this bacterium (Yang et al., Front. Microbiol. 2019).

The A. citrulli M6 APS58_1448 type III effector localizes to the plant cell nucleus. This effector seems to occur only in pant-pathogenic Acidovorax spp. See more in Jimenez-Guerrero et al. (Mol. Plant Pathol. 2020).

Plant-pathogenic Xanthomonas species

The Xanthomonas genus contains hundreds of species and pathovars that infect many crops with agricultural importance. In terms of basic research, we have been investigating Xanthomonas species that cause disease on tomato (bacterial spot disease, BS). At least four Xanthomonas lineages are able to cause BS on tomato and/or pepper: X. euvesicatoria pv. euvesicatoria, X. euvesicatoria pv. perforans, X. hortorum pv. gardneri and X. vesicatoria. BS has great economic importance, especially in places with high temperature and relative humidity that are conducive to the disease. The tomato-Xanthomonas pathosystem is also considered a model for investigation of plant-pathogen interactions. Despite the large amount of knowledge accumulated on plant-pathogen interactions, many virulence determinants are largely unknown, and how these determinants function to promote disease is still unclear. We used a genetic approach (In Vivo Expression Technology, IVET) to identify novel X. euvesicatoria pv. perforans genes that contribute to bacterial virulence and fitness (see 'Publications'). We are also collaborating with Prof. Zvi Hayouka (Institute of Biochemistry, Food Science and Nutrition, our Faculty) to develop novel antimicrobial compounds for control of Xanthomonas diseases and other bacterial plant diseases.

A citH gene, encoding a citrate transporter in X. euvesicatoria pv. perforans, was one of the genes detected by IVET. When mutated, this gene leads to reduced virulence of the bacterium on tomato. Left: symptom development on a tomato leaflet, 9 days after syringe infiltration with a citH mutant strain (left side) and with the wild type strain (right side). Right: in planta growth of the citH mutant strain (pink) compared to the wild type strain (blue) following dip inoculation of tomato plants. See more in Tamir-Ariel et al. (J. Bacteriol. 2007) and Tamir-Ariel et al. (Mol. Plant Pathol. 2011).

In collaboration with Prof. Zvi Hayouka, we demonstrated the potential of random peptide mixtures (RPMs) and short lipo-RPMs to control Xanthomonas spp. Read more in Topman et al. (Microb. Biotechnol. 2018) and Topman-Rakover et al. (Chem. Comm. 2020).

Plant beneficial bacteria

Azospirillum brasilense is one of the most investigated plant growth promoting rhizobacterial (PGPR) species. These are free-living, nitrogen-fixing, soil bacteria that commonly colonize the rhizosphere of plants from diverse families. Azospirilla are able to promote plant growth through improvement of root development and efficiency of water and mineral uptake. Secretion of phytohormones like auxin and gibberellin, as well as nitric oxide by the bacteria, are key components of plant growth promotion effects. The beneficial effects often result in increased yield in many plants of agronomic importance, mainly under sub-optimal levels of fertilization and water supply or under abiotic stress. Plant growth promotion by A. brasilense has been mainly reported in cereals and legumes, and commercial inoculants of A. brasilense strains are commonly used in some parts of the world. In the frame of the "Root of the Matter" consortium, and with additional support from ICA in Israel, we are collaborating with Dr. Yael Helman (our Department) to assess the potential of A. brasilense in combination with other bacterial strains to promote growth and yields of various crops. We are also testing combinations of beneficial bacterial strains to promote plant health via biological control.

A. brasilense (strain Cd) cells observed under scanning electron microscopy. Modified from Burdman et al. (Microbiology 1998).

Phenotypic variation in A. brasilense: emerging of a pink, carotenoid-overproducing variant of strain Sp7 (Reem-Brenholtz et al., Res. Microbiol. 2017).

Funding agencies/projects