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Research
in the Bogdanove laboratory (updated
January 11, 2007
)
Bacterial diseases
cause significant losses in many crops, and control measures are often
limited or unavailable. Also, bacterial diseases of plants can be tractable
models for understanding plant responses to microbial pathogens generally.
Research in the Bogdanove laboratory focuses on bacterial plant pathogenesis
and plant disease resistance mechanisms. We use genomic and proteomic
approaches to gene discovery, alongside molecular biology, genetics, cell
biology and biochemical approaches to understanding gene function. Our
long-term goal is to generate knowledge and tools useful in interfering
with disease and in enhancing and extending natural plant defense for
better disease control. Please read on for more information and check
the homepage for opportunities for graduate or postdoctoral research.
Our work centers on bacterial diseases of rice and soybean.
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Rice is the fifth most valuable crop in the US, a staple worldwide,
and an important model for cereal biology for which numerous genetic
and genomic resources are in place, including a virtually complete genomic
DNA sequence. Bacterial blight and bacterial leaf streak of rice are
economically important diseases in many rice-growing regions of the
world and are representative of the two major types of disease caused
by Gram-negative pathogens in plants. Blight is a vascular disease caused
by Xanthomonas oryzae pathovar oryzae. The pathogen enters
through wounds or water pores (hydathodes) in the leaf and travels systemically
through the plant xylem. Leaf streak is a non-vascular disease caused
by Xanthomonas oryzae pv. oryzicola. This pathogen typically
enters through stomata and colonizes the intercellular spaces of the
leaf photosynthetic tissue. Because the bacteria are members of the
same species (they are greater than 90% similar by DNA hybridization
studies) and the host is a model cereal, together these diseases constitute
a uniquely valuable system for understanding the pathogen and host traits
that allow microbes to exploit different plant tissues.
To identify rice genes that affect susceptibility to the two pathogens
we are screening rice mutants for altered susceptibility and we are
comparing global transcription profiles of plants infected with one
or the other pathogen. To identify key bacterial determinants, we are
carrying out comparative genomic and proteomic analyses of the two pathovars.
Experiments to functionally characterize relevant host and pathogen
genes as they are identified include mutagenesis, heterologous expression,
and biochemical assays.
Some
Project Specific Links:
>
Functional genomics of rice susceptibility to
bacterial diseases: project website at www.public.iastate.edu/~ajbog/nsf.
>
Xanthomonas pathovars enabling a broad comparative genomics
approach to understanding vascular and non-vascular bacterail diseases
of dicots and monocots: project related database, Xanthomonas Genomics
Resource, at www.plantpath.iastate.edu/xgr.
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 As in
most crops, yields in soybean can be seriously affected by numerous different
diseases, caused by a variety of pathogenic fungi, bacteria, viruses,
and nematodes. Bacterial blight of soybean, caused by Pseudomonas
syringae pv glycinea, is the most common bacterial disease of soybean.
It occurs worldwide, and is widespread in soybean-growing regions of the
United States. Yield losses can be as high as 15-20% under conditions
favorable to the disease. Typically, however, the disease is prevalent
only early in the growing season, during periods of cool, wet weather,
and is not a major threat to most production farmers. Bacterial blight
is an important concern of growers in the seed industry because it is
seed-transmitted and subject to quarantine in many areas.
Our efforts in soybean are directed toward understanding the role of bacterial
effector proteins in disease and the elicitation of defense in in this
plant. To establish conditions favorable for colonization of plants, gram-negative
phytopathogenic bacteria depend on the type III secretion system to deliver
suites of effector proteins into host cells. Collectively, effectors are
required for pathogenesis, but their individual functions are only beginning
to be understood. Many effectors were first identified as "avirulence"
(avr) proteins by virtue of their ability to trigger plant defense in
host varieties expressing corresponding resistance proteins. The effector
avrB of P. syringae pv. glycinea governs race-cultivar-specific
resistance to bacterial blight of soybean in conjunction with its corresponding
resistance (R) gene Rpg-1b (Ashfield et al., 2004; Tamaki
et al., 1988). In soybean cultivars lacking Rpg1-b, avrB
contributes to pathogen virulence (Ashfield et al., 1995). avrPto
of P. syringae pv. tomato corresponds to the Pto gene
for resistance to bacterial speck of tomato (Ronald et al., 1992). avrPto
was reported to trigger resistance in several soybean cultivars when expressed
in P. syringae pv. glycinea (Lorang et al., 1994; Ronald et al.,
1992). In addition to its avirulence function, AvrPto was observed to
enhance the virulence of P. syringae pv. tomato in tomato plants
lacking the Pto gene (Shan et al., 2000a), and more recently
shown to suppress cell wall-based defense responses in Arabidopsis
(Hauck et al., 2003).
In
collaboration with John Hill's lab here at Iowa State, we have used Soybean
mosaic virus as a transient expression vector to examine the function
of avrB and avrPto in soybean. We have discovered that
avrB but not avrPto triggers resistance that is effective
against the virus. We have also found that both avrB and avrPto
can contribute to virus-associated symptoms in susceptible varieties and
that avrPto, like avrB, can contribute to virulence
in bacterial blight. We are continuing to pursue expression of avrB
and avrPto in soybean in isolation from other bacterial proteins,
combined with mutagenesis and genetic analyses of both the effectors and
the plant, to further elucidate effector activity and identification of
targets toward a better understanding of soybean susceptibility and resistance
to disease.
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