Soybean Seed Sugars: A Role in the Mechanism of Resistance to Charcoal Rot and Potential Use as Biomarkers in Selection

Abstract

Charcoal rot, caused by Macrophomina phaseolina, is a major soybean disease resulting in significant yield loss and poor seed quality. Currently, no resistant soybean cultivar is available in the market and resistance mechanisms to charcoal rot are unknown, although the disease is believed to infect plants from infected soil through the roots by unknown toxin-mediated mechanisms. The objective of this research was to investigate the association between seed sugars (sucrose, raffinose, stachyose, glucose, and fructose) and their role as biomarkers in the soybean defense mechanism in the moderately resistant (MR) and susceptible (S) genotypes to charcoal rot. Seven MR and six S genotypes were grown under irrigated (IR) and non-irrigated (NIR) conditions. A two-year field experiment was conducted in 2012 and 2013 at Jackson, TN, USA. The main findings in this research were that MR genotypes generally had the ability to maintain higher seed levels of sucrose, glucose, and fructose than did S genotypes. Conversely, susceptible genotypes showed a higher level of stachyose and lower levels of sucrose, glucose, and fructose. This was observed in 6 out of 7 MR genotypes and in 4 out of 6 S genotypes in 2012; and in 5 out of 7 MR genotypes and in 5 out of 6 S genotypes in 2013. The response of S genotypes with higher levels of stachyose and lower sucrose, glucose, and fructose, compared with those of MR genotypes, may indicate the possible role of these sugars in a defense mechanism against charcoal rot. It also indicates that nutrient pathways in MR genotypes allowed for a higher influx of nutritious sugars (sucrose, glucose, and fructose) than did S genotypes, suggesting these sugars as potential biomarkers for selecting MR soybean plants after harvest. This research provides new knowledge on seed sugars and helps in understanding the impact of charcoal rot on seed sugars in moderately resistant and susceptible genotypes.

Keywords: charcoal rot; disease resistance; seed composition; seed nutrition; seed quality; seed sugars; soybean seed.

Figure 1

Charcoal rot of soybean in the field (A–F) under irrigated (A–C) and non-irrigated/drought (D–F). Notice the importance of irrigation management and the severity of charcoal rot under non-irrigation/drought conditions.

Figure 2

Soybean seed sucrose, (A); raffinose, (B); stachyose, (C); glucose, (D); and fructose, (E) across two years in moderately resistant (R) and susceptible (S) soybean genotypes under irrigated (IR) and non-irrigated (NIR). Moderately resistant genotypes (R) were seven: DS-880, DT97-4290, Dyna-Gro 36C44, Osage, R07-7232, USG 75Z38, USG Allen; and susceptible genotypes (S) were six: LS98-0358; Pharaoh; Progeny 4408; R01-581F; R02-1325; Trisoy 4788; two years were used; four replicates for each genotype were used. Letters that differ from each other in each column are significantly different at p < 0.05.

Figure 3

Distribution of soybean sugars (mg/g) across years, irrigation treatments, and genotypes. Sucrose, (A); raffinose, (B); stachyose, (C); glucose, (D); and fructose, (E). The experiment was conducted in 2012 and 2013 in Jackson, TN, USA. Gaps in x-axis in any distribution indicate there are no genotypes in that range. Frequency (y-axis) refers to the number of individual replicates of genotypes. Moderately resistant genotypes were seven: DS-880, DT97-4290, Dyna-Gro 36C44, Osage, R07-7232, USG 75Z38, USG Allen; and susceptible genotypes were six: LS98-0358; Pharaoh; Progeny 4408; R01-581F; R02-1325; Trisoy 4788; two years were used; four replicates for each genotype were used.