G.K. Mutua

The Pochonia chlamydosporia Serine Protease Gene vcp1 Is Subject to Regulation by Carbon, Nitrogen and pH: Implications for Nematode Biocontrol

The alkaline serine protease VCP1 of the fungus Pochonia chlamydosporia belongs to a family of subtilisin-like enzymes that are involved in infection of nematode and insect hosts. It is involved early in the infection process, removing the outer proteinaceous vitelline membrane of nematode eggs. Little is known about the regulation of this gene, even though an understanding of how nutrients and other factors affect its expression is critical for ensuring its efficacy as a biocontrol agent. This paper provides new information on the regulation of vcp1 expression. Sequence analysis of the upstream regulatory region of this gene in 30 isolates revealed that it was highly conserved and contained sequence motifs characteristic of genes that are subject to carbon, nitrogen and pH-regulation. Expression studies, monitoring enzyme activity and mRNA, confirmed that these factors affect VCP1 production. As expected, glucose reduced VCP1 expression and for a few hours so did ammonium chloride. Surprisingly, however, by 24 h VCP1 levels were increased in the presence of ammonium chloride for most isolates. Ambient pH also regulated VCP1 expression, with most isolates producing more VCP1 under alkaline conditions. There were some differences in the response of one isolate with a distinctive upstream sequence including a variant regulatory-motif profile. Cryo-scanning electron microscopy studies indicated that the presence of nematode eggs stimulates VCP1 production by P. chlamydosporia, but only where the two are in close contact. Overall, the results indicate that readily-metabolisable carbon sources and unfavourable pH in the rhizosphere/egg-mass environment may compromise nematode parasitism by P. chlamydosporia. However, contrary to previous indications using other nematophagous and entomopathogenic fungi, ammonium nitrate (e.g. from fertilizers) may enhance biocontrol potential in some circumstances.

Evaluating the effect of Bacillus and Rhizobium. bi-inoculant on nodulation and nematode control in Phaseolus vulgaris L

The study was undertaken to evaluate the effect of inoculating beans (Phaseolus vulgaris L.) with a bio-inoculant containing Bacillus spp. and Rhizobium spp. isolates on root damage by root-knot nematodes, nodulation and their interactions. Twenty Bacillus isolates from a previous experiment were selected based on their ability to reduce root-knot nematode populations and damage on beans. These isolates were screened to determine their compatibility with Rhizobium strains CIAT 899 and USDA 2674 in vitro. Seven Bacillus isolates were found to be compatible with the rhizobia strains. A greenhouse experiment to assess the effect of the dual inoculation of Bacillus and Rhizobium spp. on root damage by root-knot nematodes using sterile Leonard jar assembly revealed significant differences between the various treatment combinations. Damage by root-knot nematodes was generally lower in plants that were inoculated with both Bacillus and Rhizobium inoculation as compared to those that received rhizobia alone. Nodulation was significantly (P ⩽ 0.05) enhanced on bean plants even those that were treated with Bacillus alone. The seven isolates were then screened in a soil experiment in a glasshouse. Two Bacillus isolates caused an increase in nodulation, while the rest suppressed nodulation. Two isolates (K194 and K89) were found to have health promoting effects while the rest had growth promoting effects

Chapter 5:Agrobiodiversity and Potential Use for Enhancing Soil Health in Tropical Soils of Africa

a Department of Land Resource Management and Agricultural Technology, University of Nairobi, P.O Box 29053-00625, Nairobi, Kenya; b Department of Agricultural Economics, University of Nairobi, P.O Box 30197-00100, Nairobi, Kenya; c School of Biological Sciences, University of Nairobi, P.O Box 30197-00100, Nairobi, Kenya; d Department of Plant Sciences, University of Nairobi, P.O Box 30197-00100, Nairobi, Kenya; e World Agroforestry Centre, P.O Box 30677, Nairobi, Kenya*Corresponding author *E-mail: fredrick.ayuke@yahoo.com