Thursday, August 19, 2010

Mechanism of antifungal action

I was writing on Trichoderma as Biological Control Agent, during last fortnight was going through some literature on biological control agents which says that
--> agriculture in developed countries undergoes continuous change often based on new introductions, some based on customer preferences and some based on ethical considerations. This is particularly true of crop protection. The global consensus to reduce inputs of chemical pesticide which are perceived as being hazardous by some consumers has provided opportunities for the development of novel, benign, sustainable crop protection strategies. A great many chemical pesticides have been or are being phased out (e.g. organochlorine insecticides, methyl bromide) either because of potential human health risks, environmental pollution, effects on non-target organisms or the development of pest resistance. Today’s new chemical pesticides are significantly more benign than yesterday’s products, but will these products ever receive registrations on small area crops or in subsistence agriculture? Are there any alternatives to chemical crop protection? There is no doubt that there is a need to develop alternative control systems in the new future and these must be implemented to replace or complement conventional pesticide usage. Therefore this becomes more important to understand about various sustainable methods of crop production/ protection. Biological control agents are one among those. Trichoderma a magical plant health manager is one which has been studied vastly. How it works is very important.
Today I shall be writing about this....

Against fungal pathogens, Trichoderma species rely on three major mechanisms viz. mycoparasitism/ hyperparasitism, antibiosis and competition.
i. Mycoparasitism/ Hyperparasitism
One of the most salient characters of the genus Trichoderma is its ability to
parasitize other fungi. Weindling in 1932 for the first time described the biocontrol of R. sol
ani (causing citrus seedling disease) by Trichoderma lignorum to mycoparasitism. Mycoparasitism is a complex process involving tropic growth of the biocontrol agent towards the target
organism, coiling and finally dissolution of the target organism’s cell wall/cell membrane by the activity of enzymes. (Rather than coiling, hyphae of Trichoderma may grow attached
with hyphae of R. solani, form haustoria, which may penetrate host fungal cell to draw nutrients.

In coming posts I shall be further discussing about the

i) Mycoparasitism/ Hyperparasitism ii) Enzymes iii) Antibiosis
iv) Competition and rhizosphere competence v) Siderophore production

vi) Signal Transduction

Monday, August 2, 2010

Trichoderma as Biological Control Agent

Last time I started writing on Trichoderma a Magical Plant Health Manager. This post is in continuation to the above Topic. Today we shall be discussing here the Trichoderma as Biological Control Agent (BCA).

Weindling in 1932, for the first time implicated the role of Trichoderma lignorum in the biological control of citrus seedling disease caused by Rhizoctonia solani. Since this pioneering work, several reports on successful biocontrol by Trichoderma spp. have accumulated. T. harzianum, T. viride and T. virens are the most widely used/cited for biological control. They are reported effective in controlling root rots /wilt complexes and foliar diseases in several crops (Singh et al., 2004, 2005, 2006; Zaidi and Singh, 2004) and are reported to inhibit a number of soil borne fungi like Rhizoctonia, Pythium, Sclerotinia, Sclerotium, Fusarium spp., Macrophomina etc. and recently root knot nematode, Meloidogyne spp.

Biocontrol activity of Trichoderma is due to combination of its ability to serve as antagonist, plant growth promoter, plant defense inducer, rhizosphere colonizer and neutralizer of pathogen’s activity favouring infection.

Trichoderma as a fungal antagonist

an antagonist, Trichoderma may directly kill the pathogen by mycoparasitism and/or antibiosis. Also, it may adversely affect the growth of the pathogen either by antibiosis or by competing for the nutrient,oxygen or space. Indirectly, it may promoting plant growth which manifests itself as root and shoot growth. resistance to biotic and abiotic stresses and changes in the nutritional status of the plant.

In coming posts I shall be mainly concentrating on the Mechanisms of anti fungal action with following subheadings

i) Mycoparasitism/ Hyperparasitism
ii) Enzymes
iii) Antibiosis

iv) Competition and rhizosphere competence

v) Siderophore production

vi) Signal Transduction