Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/16/2008
Publication Date: 2/2/2009
Citation: Peltier, A.J., Hatfield, R.D., Grau, C.R. 2009. Soybean stem lignin concentration relates to resistance to Sclerotinia sclerotiorum. Plant Disease. 93(2):149-154. Interpretive Summary: An important disease of soybean is Sclerotinia stem rot (SSR), caused by the fungus Sclerotinia sclerotiorum (Lib.) de Bary. It is responsible for considerable economic losses in the cool humid regions of the United States and other temperate regions around the world. The seriousness of this disease is highly dependent upon weather conditions during its growing season. This creates a challenge when developing soybean varieties for resistance to this disease. The identification of a plant component linked to resistance to SSR would be useful to help select soybean plants with resistance to SSR. It has been suggested that lignin, a cell wall component, could work as a marker for SSR resistance in soybean. It is generally believed that higher levels of lignin in the soybean stems would result in greater resistance to stem rot. This work was undertaken to test this possibility and determine the connection between stem lignin levels and stem rot disease resistance. Six different lines of soybeans were grown in multiple locations in 2004 and 2006 and evaluated for resistance to SSR. Stems were harvested at flowering and tested for the level of lignin. The degree of resistance to SSR was related to the level of lignin in soybean stems. The results did not support the general belief that high lignin equals more resistance. In fact, the opposite was found: the more lignin in the stem, the more severe the disease. These results may indicate that with higher lignin levels, more energy is diverted away from the manufacture within the soybean plants of other components that would inhibit stem rot disease formation. Although not the positive link that was hoped for, it may still be a way to quickly screen soybean plants for resistance to SSR, relating low lignin with higher resistance. The lignin assay used in this study is easy to do and would allow hundreds of plants to be evaluated in a single season. Methods that would allow rapid identification of potentially SSR- resistant soybean plants would greatly speed the process of developing highly resistance soybean varieties. Enhancing the development of SSR-resistant soybeans would have a tremendous economic impact in the temperate agricultural regions.
Technical Abstract: Sclerotinia stem rot (SSR), caused by Sclerotinia sclerotiorum (Lib.) de Bary, is an economically important disease of soybean (Glycine max L. Merr.) in the north central United States and other temperate regions throughout the world. The occurrence and severity of SSR in the field is highly dependent upon prevailing environmental conditions, which can prove problematic when evaluating soybean accessions for resistance. The identification of an environmentally stable plant trait associated with resistance to S. sclerotiorum could be used to indirectly screen for resistance and would prove useful in the identification and development of resistant germplasm. Observations of the soybean - Sclerotinia sclerotiorum interaction suggest a role for preformed stem lignin in disease resistance. Despite this hypothesized direct relationship between preformed lignin, no studies to date have been conducted to specifically address the relationship between lignin concentration in soybean stems and SSR. We hypothesized that plants with low-stem lignin are more susceptible and exhibit greater SSR disease severity than plants with high lignin concentrations. A series of field studies evaluating both SSR and stem lignin concentration were conducted among six soybean accessions that varied in response to S. sclerotiorum. Soybean stems were sampled at reproductive developmental stages corresponding to specific events in both soybean plant development and the SSR disease cycle. The lignin concentration of stem component samples was quantified using the acetyl bromide procedure. Soybean accessions expressed statistically different disease phenotypes in both 2004 and 2006. Lignin concentrations differed among accessions for each stem section sampled at the R3 or beginning pod, growth stage (P values 0.0993 to < 0.0001). A positively ranked correlation was observed between accession SSR disease severity and lignin concentration for all nodes and internodes assayed. Lignin concentration of the internode between the fourth and fifth trifoliate leaves correlated best to disease severity data from each year (P = 0.005). These results indicate that metabolic demands for lignin formation in developing stems may limit reserves needed for the formation of phytoalexins as antimicrobial compounds.