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Bioassays to
characterize and dissect mechanisms of resistance to striga
C. Grenier,
A. Mohammed, P.J. Rich, T. Housley & G. Ejeta
We have developed three in vitro techniques that reveal critical stages in host-parasite
interaction during infection of host plants by Striga (Striga ssp.). We routinely
employ these bioassays to evaluate host plants for resistance to Striga
and to characterize the specific mechanisms involved in the defense
reaction of host genotypes. The agar gel assay (AGA) and the extended
agar gel assay (EAGA) involve placing conditioned Striga seeds in an
agar layer and measuring Striga germination and haustorial production in
response to stimuli exuded by host genotypes. The paper roll assay (PRA)
involves growing sorghum seedlings with their roots between rolled
layers of germination paper and allows observation beyond germination
and haustorial development associated with early attachment and
penetration. Host responses to Striga parasitism using these assays
reveal the potential existence of at least the following four separate
mechanisms of Striga resistance: 1) low production of Striga seed
germination stimulants and evidence of germination inhibitors; 2) low
production of the signal required for haustoria initiation; 3) a
hypersensitive response reaction characterized by a distinct necrotic
area on the host root at the attachment site that discourages parasitic
establishment; and, 4) an incompatibility response where parasite
development is arrested with no apparent necrosis on the host root. In
addition to breaking down resistance mechanisms, these assays offer
several additional advantages to a plant breeding program including: a)
rapid and cost effective screening of host germplasm to identify useful
genetic variants; b) cataloguing host of germplasm on the basis of gene
sources for different mechanisms of resistance; c) introgressing and
pyramiding multiple resistance genes into a desired cultivar; d) serve
in mapping of genomic regions associated with specific resistance
mechanisms for marker assisted introgression; and, e) targeting
isolation and cloning of different Striga resistance genes.
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