Plant that contain all the components necessary to sustain life are extreme
ly attractive to a vast array of fungi, bacteria, viruses and nematodes, Th
e tissues of plants are resistant to invaders because of the presence of di
fferent defense Systems. Resistance of vascular plants to diverse microorga
nisms is based upon an arsenal of innate defense reactions that preexist in
the plant and on inducible resistance mechanisms that are prosed;by tissue
s of an infected plant. Some Of them are broadly effective against a wide r
ange of invaders whereas others are specific for individual infectious agen
ts. The innate mechanisms represented by anatomical structures and biochemi
cal resistance mechanisms act to exclude invaders, Physical barriers, altho
ugh very helpful, are not entirely effective in themselves. Other protectiv
e mechanisms, the local defenses? such as lignification, hypersensitivity r
esponse, papillae formation and Vascular occlusion restrict the invaders wi
thin the infected tissues. The inducible protective mechanisms, mostly phyt
oalexins and pathogenesis related proteins that act deep within the plant,
destroy microorganisms that succeed in avoiding the innate immune defenses.
Genetic manipulations to increase resistance of crop plants create an attr
active prospect for food production.