ABSTRACT
The intact, healthy plant is a community of cells built in a fortress-like fashion. Plant cells consist of cell wall contains the nucleus and various organelles and all the substances for which the pathogens attack them. The cytoplasm and the organelles it contains are separated from each other by membranes that carry various types of proteins embedded in them (Fig. 5-2). The plant surfaces that come in contact with the environment either consist of cellulose, as in the epidermal cells of roots and in the intercellular spaces of leaf parenchyma cells, or consist of a cuticle that covers the epidermal cell walls, as is the case in the aerial parts of plants. Often an additional layer, consisting of waxes, is deposited outside the cuticle, especially on younger parts of plants Pathogens attack plants because during their evolutionary development they have acquired the ability to live off the substances manufactured by the host plants, and some of the pathogens depend on these substances for survival. Many substances are contained in the protoplast of the plant cells, however, and if pathogens are to gain access to them they must first penetrate the outer barriers formed by the cuticle and/or cell walls. Even after the outer cell wall has been penetrated, further invasion of the plant by the pathogen necessitates the penetration of more cell walls. Furthermore, the plant cell contents are not always found in forms immediately utilizable by the pathogen and must be broken down to units that the pathogen can absorb and assimilate. Moreover, the plant, reacting to the presence and activities of the pathogen, produces structures and chemical substances that interfere with the advance or the existence of the pathogen; if the pathogen is to survive and to continue living off the plant, it must be able to overcome such obstacles. Therefore, for a pathogen to infect a plant it must be able to make its way into and through the plant, obtain nutrients from the plant, and neutral

ABSTRACT
One such novel technology is nanotechnology, which has been revolutionized in health care, textile, materials, information and communication technology, and energy sectors too. There are many ways depicted in various literatures to synthesize silver nanoparticles. These include physical, chemical, and biological methods. The physical and chemical methods are numerous in number, and many of these methods are expensive or use toxic substances which are major factors that make them “not so favored” methods of synthesis. An alternate, feasible method to synthesize silver nanoparticles is to employ biological methods of using microbes and plants. Biotechnology has considered a safe agricultural tool to enhance crop protection, subsequently to produce more agricultural products, improve food process, nutritional value, and better flavor. Side by side it has detrimental ecological consequences such as spreading genetically engineered genes to indigenous plants, elevated toxicity, which may transmit through food chain, disrupting nature’s system of pest control, developing new weeds or virus strains, biodiversity loss, and insecticidal resistance (Goswami et al., 2010). Hence, it is necessary to bring forth new innovative technology/methods to overcome the above mentioned problems. The potential application of biogenic nanoparticles as antimicrobial agents will be also reviewed. The mechanism of action of nanoparticles as bactericidal and antifungal agents will be highlighted in this chapter.

ABSTRACT
The Pseudomonas fluorescens isolate FP 7 was found to protect the tomato plants against soil borne fungus, Pythium aphanidermatum. The ability to induce defense proteins viz. -1,3 glucanase, peroxidase (PO), polyphenol oxidase (PPO) and phenylalanine ammonia lyase (PAL) by this P. fluorescens isolate FP 7 against P. aphanidermatum fungus was further studied. Increased activity of PO, PPO and -1,3 glucanase were observed in plants pretreated with FP 7 isolate. Native PAGE (polyacrylamide gel electrophoresis) of PO showed the single isoform in all the treatment including the control and difference is by the increased intensity of the band in the inoculated control and FP 7 treatment in the tomato plant. Isoform analysis of the PPO showed the induction of the PPO in the P. fluorescens treated plants challenged with P. aphanidermatum. -1,3 glucanase in tomato cultivar, Co 3 with and without challenge inoculation of P. aphanidermatum, revealed changes in the isoform pattern after staining the gel with 2,3,5-triphenyl tetrazolium chloride. Moreover, higher accumulation of phenolics was noticed in plants pretreated with P. fluorescens isolate FP 7 challenged with P. aphanidermatum. The relevant function of Thaumatin like defense proteins and secondary metabolites involved in the phenylpropanoid pathway collectively contributing to enhanced resistance is discussed.