Plant elicitor peptides (Peps) are widely distributed among angiosperms, and have been shown to amplify immune responses in multiple plant families. Here, we characterize three Peps from soybean (Glycine max) and describe their effects on plant defences against two damaging agricultural pests, the root-knot nematode (Meloidogyne incognita) and the soybean cyst nematode (Heterodera glycines). Seed treatments with exogenous GmPep1, GmPep2 or GmPep3 significantly reduced the reproduction of both nematodes. Pep treatment also protected plants from the inhibitory effects of root-knot nematodes on above-ground growth, and up-regulated basal expression levels of nematode-responsive defence genes. GmPep1 induced the expression of its propeptide precursor (GmPROPEP1), a nucleotide-binding site leucine-rich repeat protein (NBS-LRR), a pectin methylesterase inhibitor (PMEI), Respiratory Burst Oxidase Protein D (RBOHD) and the accumulation of reactive oxygen species (ROS) in leaves. In addition, GmPep2 and GmPep3 seed treatments up-regulated RBOHD expression and ROS accumulation in roots and leaves. These results suggest that GmPeps activate plant defences through systemic transcriptional reprogramming and ROS signalling, and that Pep seed treatments represent a potential strategy for nematode management.

The reniform nematode, Rotylenchulus reniformis, is a serious threat to cotton (Gossypium hirsutum) production in the United States, causing an annual loss of about $80 million. The objective of this study was to isolate fungi from eggs of R. reniformis and select potential biocontrol agents for R. reniformis on cotton. We focused on the fungus Pochonia chlamydosporia because it suppresses root-knot and cyst nematodes and because preliminary data indicated that it was present in Arkansas cotton fields. Soil samples were collected from six cotton fields in Jefferson County, Arkansas. A total of 117 isolates of the nematophagous fungus P. chlamydosporia were obtained. In an in vitro test, 105 of the 117 isolates parasitized fewer than 15% of R. reniformis eggs, but 12 isolates parasitized between 16 and 35% of the eggs. These 12 isolates produced from 6.8 x 10(4) to 6.9 x 10(5) chlamydospores per gram of medium in vitro, and chlamydospore production was similar on rice grain and corn grain media. In two greenhouse experiments, a single application of isolate 37 (5,000 chlamydospores per gram of soil) significantly reduced the numbers of R. reniformis on cotton roots and in soil. The three isolates (37, 26, and 14) that parasitized the most eggs in vitro were also the most effective in suppressing numbers of R. reniformis and in increasing cotton growth in the greenhouse.

The sterile nematophagous fungus ARF (Arkansas fungus) had been separated into two phenotypes based on morphological differences: those with compact sclerotium-like-structures (SLS) (ARF-C), and those with loose SLS (ARF-L). The ARF-L isolate TN14 had been more effective than the ARF-C isolate BG2 in suppressing nematode numbers in glasshouse studies, and the BG2 isolate had been more virulent in in vitro tests. No tests had been conducted on the effect of depth in the soil on the level of parasitism by ARF. The objectives of this study were to determine: i) which isolate is more competitive in field microplots by comparing isolation frequency of the two fungal strains from parasitised eggs of Heterodera glycines; and ii) whether parasitism by ARF varies with soil depth (0-10, 10-20, 20-30 cm deep). TN14 was isolated more frequently than BG2, irrespective of whether the original mixture of BG2 and TN14 was 50% TN14 + 50% BG2, 75% TN14 + 25% BG2, or 25% TN14 + 75% BG2. The percentage of parasitised eggs of H. glycines by ARF was different for the 0-10 cm soil layer vs the 10-20 cm soil layer, or the 0-10 cm soil layer vs the 20-30 cm soil layer. Parasitism by ARF was greatest at a soil depth of 10-20 cm. This would correspond with the level where the root concentration and numbers of cysts should be greatest.

Isolates of a sterile fungus designated ARF (Arkansas fungus) can be separated into two groups, ARF-C and ARF-L, that differ morphologically and in their ability to suppress numbers of Heterodera glycines on soybean. Our objectives were to determine if the two ARF groups differed in their ability to parasitize juveniles, females, and eggs in the rhizo-sphere of soybean and to proliferate in soil. The experiments were conducted in a greenhouse using soil infested with homogenized ARF mycelium. The ARF-L isolates parasitized more juveniles and young females than did the ARF-C isolates. Suppression of these stages was 67% for ARF-L and 12% for ARF-C isolates 14 days after nematode inoculation. When soybean plants containing gravid females were transplanted into fungus-infested soil, ARF-L isolates parasitized 55 to 98% of nematode eggs, whereas ARF-C isolates parasitized 0 to 22%. In both heat-treated and nonheated soil, the biomass of mycelial mats, a measure of relative proliferation, tended to be greater for ARF-L than for ARF-C isolates. The ability of ARF-L isolates to parasitize a large percentage of both pre-reproductive stages and eggs of H. glycines may contribute to its effectiveness as a biological control agent.

Plant parasitic nematodes are a major yield-limiting factor of soybean in the United States and Canada. It has been indicated that soybean cyst nematode (SCN, Heterodera glycines Ichinohe) and reniform nematode (RN, Rotylenchulus reniformis Linford and Oliveira) resistance could be genetically related. For many years fragmentary data has shown this relationship. This report evaluates RN reproduction on 418 plant introductions (PIs) selected from the USDA Soybean Germplasm Collection with reported SCN resistance. The germplasm was divided into two tests of 214 PIs reported as resistant, and 204 PIs moderately resistant to SCN. The defining and reporting of RN resistance changed several times in the last 30 years, causing inconsistencies in RN resistance classification among multiple experiments. Comparison of four RN resistance classification methods was performed: (1) ≤10% as compared to the susceptible check, (2) using normalized reproduction index (RI) values, and transformed data (3) log10 (x) and (4) log10 (x+1), in an optimal univariate k-means clustering analysis. The method of transformed data log10 (x) was selected as the most accurate for classification of RN resistance. Among 418 PIs with reported SCN resistance, the log10 (x) method grouped 59 PIs (15%) as resistant, and 130 PIs (31%) as moderately resistant to RN. Genotyping of a subset of the most resistant PIs to both nematode species revealed their strong correlation with rhg1-a allele. This research identified genotypes with resistance to two nematode species and potential new sources of RN resistance that could be valuable to breeders in developing resistant cultivars.