ABSTRACT
Pathogens contain various elicitors to act as primary signal molecules triggering host defense
genes. These elicitors include general elicitors such as oligosaccharides, proteins, glyco-
proteins, and lipids (Halim et al., 2004; Ning et al., 2004; Hu et al., 2005; Vidhyasekaran,
2007) and race-specific elicitors including several avirulence (avr) gene products (Dodds et al.,
2004; Schu¨rch et al., 2004; Shan et al., 2004; Rowland et al., 2005). Plants are endowed with
several hundreds of disease resistance genes (Zhu et al., 2002; Eckardt et al., 2003; Ling et al.,
2003). The disease resistance genes offer two types of resistance. The first type of resistance is
called qualitative resistance, whereas the second type of resistance is called quantitative
resistance (Toojinda et al., 2000). The qualitative resistance is conferred by major genes
(dominant or recessive), whereas quantitative resistance is conferred by minor genes (Kousik
and Ritchie, 1999; Ovesna´ et al., 2000). Major genes have distinct phenotypic expressions
showing clear Mendelian segregation, whereas minor genes have small effects on the expres-
sion of the phenotype for resistance showing quantitative segregation (Vidhyasekaran, 2007).
Two types of major genes, such as dominant and recessive genes, contribute for qualitative
resistance. Recessive gene is phenotypically manifested in the homozygous state but is masked
in the presence of its dominant allele (dominant gene). Usually the dominant gene produces a
functional product, whereas its recessive gene does not (Vidhyasekaran, 2007).
