Mechanism of seed transmission and seed infection in major agricultural crops in India

Abstract

The mechanism involved in transmission and infection of seed-borne pathogens in agricultural crops is an important aspect. Healthy tissues of seeds and vegetative propagative materials act as reservoir of major nutrients, viz. carbohydrates, protein and minerals. The seed-associated fungi and bacteria survive and proliferate by using these nutrients and, therefore, subsequently become pathogenic on seedlings and growing plants. However, seeds are also constituted with defence molecules, phenolics, lectins and many more anti-pathogenic proteins in addition to physical barriers. The successful pathogenesis depends on their aggressiveness to overcome such barriers to infect seeds. To counteract such barriers, most of the necrotrophic microbes elaborate suitable biomolecules as compared to obligate parasites of seeds. Plant viruses are exclusive biotrophic pathogens, responsible for yield and quality losses to the crop plants. The horticultural crops, viz. temperate fruits, vegetables and few plantations, are infected by such viruses. Comparatively, cereals are free from virus diseases but few localized and systemic viruses are mostly transmitted by insect vectors and are economically important. Seeds are unique as a carrier of infective virus particles. Available moisture content is normally less in well-maintained dry seeds as compared to fresh seeds from mature plants. Nutrient content is also high in seeds than foliage. These factors could favour better survival and transmission similar to seed-borne fungal mycoflora. Some of the seed-borne viruses are systemically transmitted through seeds to seedlings and adult plants. Such viruses infect male and female gametophytes at the time of seed formation. These processes could favour consistent association of infective viruses. Similarly, seed-borne nematodes also play an important role in seed health. However, there is less understanding about their survival, infection and mechanism of transmission. Various aspects, i.e. nature of pathogens, their mode of entry and survival, transmission and infection, are discussed in this chapter.