Horizontal
resistance
When a pathogen attacks a host plant, the genes of the
pathogen are activated, produce and release all their weapons of attack
(enzymes, toxins, etc.) against the plants that they try to infect. With the
help of different combinations of pre-existing or induced toxic chemical
substances or defence structures, most plants manage to defend themselves
partially or nearly completely. Such plants show sufficient resistance that
allows them to survive the pathogen attacks and to produce a satisfactory
yield. This type of defence or resistance is known as polygenic, general, or
quantitative resistance because it depends on many genes for the presence or formation
of the various defence structures and for pre-existing or induced production of
many substances toxic to the pathogen. This type of resistance is present at
different levels against different pathogens in absolutely all plants and is
also known as partial, quantitative, horizontal, multigenic, field, durable, or
minor gene resistance.
Vertical resistance
In many plant pathogen combinations, specially those involving
biotrophic oomycetes (downy mildews), fungi(powdery mildews, rusts), and many
other fungi e.g. Cochliobolus, Magnaporthe, Cladosporium, many bacteria,
nematodes and viruses, defence(resistance) of host plant against many of its
pathogens is through the matching pairs of juxtaposed genes for disease in the
host plant and the pathogen. The host plant carries one or few resistance genes
(R) per pathogen capable of attacking it while each pathogen carries matching
genes for avirulence (A) for each of the matching R genes of the host plant.
The avirulence gene of the pathogen serves to trigger the host R gene into the
action. This then sets into the motion a series of defence reaction that
neutralize and eliminate the specific pathogen that carries the corresponding
gene fro avirulence(A) while the attacked and a few surrounding cells die. This
type of defence or resistance is known as race specific, hypersensitive
response (HR), major gene, R gene, or vertical resistance. However some R genes
eg. Xa21 of rice do not induce a visible HR.
Tags
Plant Pathology