Host Plant Responses During Compatible and Incompatible
Plant-Nematode Interactions
Nematodes
induce multifaceted changes in plant cellular metabolism and gene
expression during the infection process that ultimately gives rise
to specialized feeding cells (syncytia) within host plant roots. The
underlying molecular mechanisms controlling these processes are largely
unknown. We have used laser capture microdissection (LCM) to specifically
isolate the contents of nematode-induced feeding cells over a time-course
of their development in soybean roots infected with soybean cyst nematode
and coupled this with microarray analysis to develop the most comprehensive
profile of syncytia-expressed genes to date. We are currently characterizing
the function of genes up and down regulated in developing syncytia
to assess for direct roles in syncytium induction, development and
maintenance. This approach may also prove successful in identifying
host targets for engineered resistance. Little is known regarding
the molecular mechanisms of soybean resistance to soybean cyst nematode.
In soybean resistant to SCN, feeding cell formation is compromised
and nematode development is impeded. Using LCM and microarrays, we
have also directly compared gene expression profiles in developing
syncytia of resistant and susceptible soybean differing at major loci
controlling SCN resistance to identify components of the soybean resistance
response to SCN. At present, we are characterizing the function of
these genes to determine their role in resistance. In addition, we
are collaborating with other groups to confirm the identity and function
of candidate SCN
resistance genes. Our goal is to identify both upstream
and downstream components of the resistance gene signaling pathways
during the SCN-soybean interaction using functional genomic and reverse
genetic approaches.
Identification and Functional Analysis of Nematode
Esophageal Gland Secretions
With regard
to the nematode, we have been focusing on the identification and functional
analysis of nematode genes encoding esophageal gland secretions (i.e.
nematode parasitism genes) as part of a Molecular Nematology collaboration
with the labs of Dr.
Eric Davis (NCSU), Dr.
Dick Hussey (UGA), Dr.
Thomas Baum (ISU), and Dr.
Xiaohong Wang (Cornell). Our group is interested in elucidating
the underlying mechanisms of cyst nematode parasitism, in particular
how cyst nematodes utilize esophageal gland secretions to modify plant
cells during the formation of a complex feeding site (syncytium) within
the host root, which is required for their growth and development.
It is unclear how feeding sites are induced by the nematode and the
nature and origin of the stimulus
required to elicit the formation of feeding sites has not been identified.
However, evidence suggests that nematode esophageal gland secretions
are key molecules involved in initiating the interaction and modifying
plant cells for parasitism. Notable progress has been made to determine
the identity and nature of the molecules involved in establishing
the parasitic interaction. Previously, nematode esophageal gland cell-specific
cDNA libraries were constructed from microaspirated gland cell mRNA
using PCR-based approaches and subjected to extensive EST sequence
analysis. My laboratory is using molecular genetic approaches to conduct
functional analyses of some of these parasitism gene products to determine
their role in plant parasitism. Approaches include RNA interference,
ectopic expression in plants, and protein-protein interaction studies.
Of particular interest is a class of Heterodera genes encoding
secreted CLAVATA3/ESR-like (CLE) peptides and we
are currently conducting detailed functional studies to assess the
role of ligand mimicry in plant parasitism. We have also identified
differences in the molecular structure of parasitism gene products
among H. glycines genotypes that correlate with virulence
on resistant soybean and are examining a potential role for these
proteins in eliciting, suppressing or evading host plant resistance
mechanisms.
The Role of Phytohormones in Plant-Nematode Interactions
Phytohormones have been known for decades to modulate plant development;
however, the molecular mechanisms involved are only beginning to be
discovered. Although not well understood, several lines of evidence
suggest considerable interplay and crosstalk among various phytohormones
for the modulation of plant growth. Morphological and biochemical
evidence have shown that local phytohormone levels and hormone response
pathways are altered in nematode-infected roots and may play a significant
role in nematode feeding site (NFS) formation. We have characterized
a tobacco endo-ß-1,4-glucanase gene promoter (NtCel7)
that it is upregulated within nematode feeding sites during the early
stages of their formation and is responsive to auxin. Phytohormones
imbalances induced by nematodes likely result in altered expression
of cell wall modifying enzymes with a central role in the controlled
cell wall architechural modifications observed during feeding cell
development. Several other hormone-responsive plant gene promoters
have been shown to be upregulated in NFS and both auxin and ethylene-insensitive
mutants are less susceptible to cyst nematodes due to impairments
in feeding cell development. It is not entirely clear whether the
nematode produces plant hormones for secretion into plant cells, or
modulates the level of host phytohormone levels by affecting transport
or redirecting normal plant biosynthetic and signaling pathways. We
are using the model plant, Arabidopsis thaliana, as a parallel
system to dissect the complex plant-nematode interaction. We are interested
in elucidating how the nematode alters the complex plant hormone biosynthetic
and signaling networks for the development of nematode feeding sites
in plant roots. This involves studying the expression and function
of genes encoding biosynthetic and catabolic enzymes, hormone signaling
pathway components, and response genes in both model plants and soybean.
Nematodes
induce multifaceted changes in plant cellular metabolism and gene
expression during the infection process that ultimately gives rise
to specialized feeding cells (syncytia) within host plant roots. The
underlying molecular mechanisms controlling these processes are largely
unknown. We have used laser capture microdissection (LCM) to specifically
isolate the contents of nematode-induced feeding cells over a time-course
of their development in soybean roots infected with soybean cyst nematode
and coupled this with microarray analysis to develop the most comprehensive
profile of syncytia-expressed genes to date. We are currently characterizing
the function of genes up and down regulated in developing syncytia
to assess for direct roles in syncytium induction, development and
maintenance. This approach may also prove successful in identifying
host targets for engineered resistance. Little is known regarding
the molecular mechanisms of soybean resistance to soybean cyst nematode.
In soybean resistant to SCN, feeding cell formation is compromised
and nematode development is impeded. Using LCM and microarrays, we
have also directly compared gene expression profiles in developing
syncytia of resistant and susceptible soybean differing at major loci
controlling SCN resistance to identify components of the soybean resistance
response to SCN. At present, we are characterizing the function of
these genes to determine their role in resistance. In addition, we
are collaborating with other groups to confirm the identity and function
of candidate 
required to elicit the formation of feeding sites has not been identified.
However, evidence suggests that nematode esophageal gland secretions
are key molecules involved in initiating the interaction and modifying
plant cells for parasitism. Notable progress has been made to determine
the identity and nature of the molecules involved in establishing
the parasitic interaction. Previously, nematode esophageal gland cell-specific
cDNA libraries were constructed from microaspirated gland cell mRNA
using PCR-based approaches and subjected to extensive EST sequence
analysis. My laboratory is using molecular genetic approaches to conduct
functional analyses of some of these parasitism gene products to determine
their role in plant parasitism. Approaches include RNA interference,
ectopic expression in plants, and protein-protein interaction studies.
Of particular interest is a class of Heterodera genes encoding
secreted CLAVATA3/ESR-like (CLE) peptides and we
are currently conducting detailed functional studies to assess the
role of ligand mimicry in plant parasitism. We have also identified
differences in the molecular structure of parasitism gene products
among H. glycines genotypes that correlate with virulence
on resistant soybean and are examining a potential role for these
proteins in eliciting, suppressing or evading host plant resistance
mechanisms. 
Phytohormones have been known for decades to modulate plant development;
however, the molecular mechanisms involved are only beginning to be
discovered. Although not well understood, several lines of evidence
suggest considerable interplay and crosstalk among various phytohormones
for the modulation of plant growth. Morphological and biochemical
evidence have shown that local phytohormone levels and hormone response
pathways are altered in nematode-infected roots and may play a significant
role in nematode feeding site (NFS) formation. We have characterized
a tobacco endo-ß-1,4-glucanase gene promoter (NtCel7)
that it is upregulated within nematode feeding sites during the early
stages of their formation and is responsive to auxin. Phytohormones
imbalances induced by nematodes likely result in altered expression
of cell wall modifying enzymes with a central role in the controlled
cell wall architechural modifications observed during feeding cell
development. Several other hormone-responsive plant gene promoters
have been shown to be upregulated in NFS and both auxin and ethylene-insensitive
mutants are less susceptible to cyst nematodes due to impairments
in feeding cell development. It is not entirely clear whether the
nematode produces plant hormones for secretion into plant cells, or
modulates the level of host phytohormone levels by affecting transport
or redirecting normal plant biosynthetic and signaling pathways. We
are using the model plant, Arabidopsis thaliana, as a parallel
system to dissect the complex plant-nematode interaction. We are interested
in elucidating how the nematode alters the complex plant hormone biosynthetic
and signaling networks for the development of nematode feeding sites
in plant roots. This involves studying the expression and function
of genes encoding biosynthetic and catabolic enzymes, hormone signaling
pathway components, and response genes in both model plants and soybean.