Test: Biogas, Biocontrol Agents & Biofertilisers: Microbes - NEET MCQ

Heterocysts or heterocytes are specialized nitrogen-fixing cells formed during nitrogen starvation by some filamentous cyanobacteria, such as Nostoc punctiforme, Cylindrospermum stagnale, and Anabaena sphaerica.

Thus gobar (excreta/dung) of cattle is rich in these bacteria. Dung can be used for generation of biogas, commonly called gobar gas.
The residue left after methane production from cattle dung is used as manure.

VAM contains symbiotic fungi as VAM (Vesicular Arbuscular Mycorrhizae) grows luxuriently in less irrigated lands and association of VAM helps in conversion of less productive fields into more productive fields.

Endomycorrhizal fungi (more commonly referred to as endomycorrhizae) is one of the major types of known mycorrhizae which differs from the another type of mycorrhizae, ectomycorrhizae, in structure. Unlike ectomycorrhizae which form a system of hyphae that grow around the cells of the root, the hyphae of the endomycorrhizae not only grow inside the root of the plant but penetrate the root cell walls and become enclosed in the cell membrane as well. This makes for a more invasive symbiotic relationship between the fungi and the plant. The penetrating hyphae create a greater contact surface area between the hyphae of the fungi and the plant.
Glomus is a genus of endomycorrhizal fungi, and all species form symbiotic relationships (mycorrhizas) with plant roots. 

A biological control being developed for use in the treatment of plant disease is the fungus Trichoderma.