Haytham Aly
M.A. in Genetics, January, 2004, Cairo University- Egypt
B.A. in Plant Pathology, June, 1997, Cairo University, Egypt

Role of Pseudomonas produced hydrogen cyanide in biological control of plant-parasitic nematodes

Abstract

Plant-parasitic nematodes are among the most devastating pests world-wide, causing extensive damage to important agriculture crops. The management of nematodes is challenging. Current methods used for nematode control rely mostly on using chemical nematicides and naturally resistant plants. Because of serious health and environmental problems associated with chemical control and over use of resistant plants resulting in resistant breaking strains of nematodes, alternative nematode-control methods are needed. Several potential nematode-biocontrol agents, including bacteria and fungi, have been studied for their antagonistic effect toward plant-parasitic nematodes. However, a concise picture about the biological agent’s mechanisms used to antagonize nematodes remains elusive.

In the present study, over 60 different isolates of PseudomonasI were identified that were lethal to the free-living nematode, Caenorhabditis elegans. All nematode-lethal isolates of Pseudomonas were genetically, phenotypically and biochemically characterized for their motility, exoprotease activity, and production of siderophores, polysaccharides and secondary metabolites (cyanide, DAPG, PCA, pyrrolnitrin and pyoluteorin). These Pseudomonas strains were also screened for biocontrol activity against plant-parasitic nematodes (Meloidogyne incognita and Heterodera glycines), plant-pathogenic bacteria (Agrobacterium rhizogenes) and fungi (Fusarium solani). From our phenotypic characterization, one of the most common traits to most of the nematode-lethal stains was production of cyanide. Cyanide production was found in 87% of the C. elegans-lethal strains and 100% of the plant-parasitic nematode lethal strains.

One of the plant-parasitic nematode-lethal strains further characterized in this study was Pseudomonas sp. strain 15G2. The phenotypic characterization revealed that this strain is motile; a producer of exoproteases, siderophores, polysaccarhides, and hydrogen cyanide (HCN); a colonizer of wheat and maize roots; and is lethal to both root-knot and soybean cyst nematodes. HCN produced by this strain is the major nematode lethal factor in in vitro and in vivo studies. From our experiments, two lines of evidence provide support for this conclusion. First, indirect exposure of nematodes to the wild-type 15G2 strain (where bacteria and nematode share the same air but do not come in contact with one another) induces nematode lethality while cyanide deficient 15G2 strains failed to kill. Second, application of wild-type 15G2 to soil produced a significant reduction in soybean cyst nematode population whereas the cyanide deficient mutant 15G2 strains had a reduced capacity for protecting plants against these plant-parasitic nematodes.

As further evidence for cyanide activity in the in vivo experiments we also demonstrate that cyanide production in soil can be monitored using cyanide-inducible gene expression in associated plant roots. The results from these studies provide significant insight into HCN as a mechanism by which Pseudomonas sp. 15G2 control plant-parasitic nematodes and highlight the potential application of this strain to broaden the scope of options for nematode management.

Defense of Dissertation Committee