Plants interact with a large number of microoganisms which have a major
impact on their growth either by establishing mutually beneficial
symbiotic relationships or by developing as pathogens at the expense of
the plant with deleterious effects. These microorganisms differ greatly
not only in their nature (viruses, phytoplasmas, bacteria, fungi,
nematodes, ... ) but also in the way they contact, penetrate and invade
their host. Histology and cytology have brought an essential
contribution to our knowledge of these phenomena. They have told us for
instance, how specialized structures of the pathogen are often involved
in the adhesion and penetration into the plant, how the interface
between both organisms is finely arranged at the cellular level, or what
structural alterations affect the infected tissues. They have thus set
the stage for the investigations of the underlying molecular mechanisms
could be undertaken. Such investigations have been remarkably successful
in the recent years, expanding considerably our understanding of
plant-microorganism interactions in terms of biochemical changes, rapid
modifications of enzymatic activities, coordinated gene activation,
signal reception and transduction. Biochemistry, molecular biology and
cellular physiology have taken precedence in the phytopathologist's set
of methods.