Associate Professor of Plant Pathology and USDA/ARS Plant Pathologist
2416 Gardner Hall-Box 7616
Raleigh, NC 27695-7616
Tel: (919) 515-6996
FAX: (919) 515-7716
B.S. (Biological Sciences)
M.S. (Microbiology), North Carolina State University
Ph.D (Microbiology), North Carolina State University
Soybean responses to pathogen stress: the interaction of fatty acid composition and defense gene expression. Cercospora kikuchii is the causal agent of Cercospora leaf blight (CLB) and purple seed stain (PSS) and Diaporthe phaseolorum var. meriodinales is the causal agent of southern stem canker (SSC) and Phomopsis seed decay (PSD) in soybean. Though resistance in soybean to CLB, PSS, SSC, and PSD has been identified and deployed, the molecular responses of soybean to C. kikuchii and D. phaseolorum have not been characterized. Given the periodic reemergence of these diseases in U.S. soybean growing areas, research aimed at a molecular characterization of the host responses to pathogen infection is justified and may assist breeders in identifying improved sources of resistance for incorporation into adapted germplasm. We developed procedures for quantifying pathogen growth in infected soybean tissues and the expression of several soybean defense genes representing three biochemical/functional classes. Our initial studies have shown that in leaves of a common southern cultivar inoculated with C. kikuchii or Diaporthe phaseolorum, the expression of several defense genes were activated by both pathogens, some more highly by one or the other and pathogen growth declined during a 48 hour post inoculation period (hpi). In seeds inoculated with either pathogen, different defense genes were more highly activated, but pathogen growth increased over a 48 hpi period. Results suggested that activation of certain defense genes may be associated with growth inhibition of C. kikuchii and D. phaseolorum in leaves, but no such association could be made for pathogen inoculated seeds. Research is now focused on comparing defense gene expression profiles and pathogen growth in the leaves and seeds of susceptible and resistant soybean varieties, and on an analysis of the effects of seed fatty acid composition on the activation of defense gene expression and pathogen growth.
Soybean mid-oleic trait stability and soybean response to day-length. Soybean oil containing a higher content of oleic acid (mid-oleic, 50-60% oleic acid) is desirable because the oil is more healthful for human consumption and has improved properties for industrial uses. Seeds of soybean mutant M23 accumulate mid-oleic acid levels because of a deletion of GmFAD2-1A, an allele of the microsomal omega-6 oleate desaturase gene. In other less well characterized soybean varieties that are phenotypically mid-oleic, little is known about the expression levels of omega-6 desaturase GmFAD2 genes and other candidate genes that may determine seed oleic acid content. Efforts to characterize fatty acid metabolism genes in these varieties may expand our understanding of the basis of the mid-oleic trait and provide new germplasm for breeders to manipulate. When we compared the steady-state transcript abundance during seed development of the oleate-ACP thioesterase (GmFATB1a), delta-9 stearoyl acyl carrier protein desaturase (GmSACPD) and the omega-6 fatty acid desaturase (GmFAD2) genes in five natural mid-oleic varieties and mutant M23 to gene expression data for the non mid-oleic cultivar Dare, we found that relative to Dare, there were instances where lower expression of GmFATB1a, GmFAD2-1A,GmFAD2-1B, GmFAD2-2, and GmFAD2-3 and higher expression of the GmSACPD-C were associated with the mid-oleic seed phenotype. This suggests that soybean genomic loci known or suspected to be involved in oleic acid phenotype are likely to contain genes involved in regulation of expression of the oleate biosynthetic genes. Recently we found that under extreme short day-length conditions, some, but not all mid-oleic varieties had significant variability in seed yield and oleic trait stability. We plan to characterize this phenomenon by examining the effect of day-length on the expression of fatty acid metabolism genes during seed development and we will also evaluate the feasibility of day-length “stress” as a tool to assess yield stability and mid-oleic trait stability in soybean lines of interest.
Byfield, G.E, Xue, H., and Upchurch, R.G. 2006. Two soluble Δ9 stearoyl-ACP desaturase genes from soybean. Crop Sci. 46:840-846.
Xue, H.Q., Upchurch, R.G. and Kwanyuen, P. 2006. Ergosterol as a quantifiable biomass marker for Diaporthe phaseolorum and Cercospora kikuchii. Plant Dis. 90: 1395-1398.
Byfield, G.E. and Upchurch, R.G. 2007. Effect of temperature on delta-9 stearoyl-ACP and microsomal omega-6 desaturase gene expression and fatty acid content in developing soybean seeds. Crop Sci. 47:1698-1704.
Byfield, G.E. and Upchurch, R.G. 2007. Effect of temperature on microsomal omega-3 linoleate desaturase gene expression and linolenic acid content in developing soybean seeds. Crop Sci. 47:2445-2452.
Upchurch, R.G. 2008. Fatty acid unsaturation, mobilization, and regulation in the response of plants to stress. Biotechnol. Lett. 30:967-977.
Xue, H.Q., Upchurch, R.G. and Kwanyuen, P. 2008. Relationships between oleic and linoleic acid content and seed colonization by Cercospora kikuchii and Diaporthe phaseolorum. Plant Dis. 92:1038-1042.
Zhang, P., Burton, J.W., Upchurch, R.G., Whittle, E., Shanklin, J., and Dewey, R.E. 2008. Mutations in a Δ9-stearoyl-ACP-desaturase gene are associated with enhanced stearic acid levels in soybean seeds. Crop Sci. 48:235-2313.