Effects of the rhg1 gene on resistance to Heterodera glycines in soybean (Glycine max)

Heterodera glycines is a major yield-suppressing pathogen of soybean. Effects of the rhg1 gene on nematode penetration, development, and population development were investigated using near-isogenic lines (NILs) that were previously identified from a cross between PI 209332 (resistant) and Evans (susceptible) and differed at the rhg1 locus. The presence of the rhg1 gene did not stop SCN from penetrating, however, it slowly suppressed SCN growth, development and fecundity. A significant difference between the NIL-R (carrying the PI 209332 rhg1 allele) and NIL-S (carrying the Evans rhg1 allele) lines was consistently observed at 16 and 21 days after inoculation (DAI) in the number of females and eggs per plant. Significantly fewer eggs were produced per female from the NIL-R line than from the NIL-S line at 21 DAI. The females on the NIL-R line were also significantly smaller than those from the NIL-S line at 28 DAI. The relationships between initial population density (Pi) and final population density (Pf = females or eggs/plant) or reproduction factor (Rf = final egg density/Pi) on both NILs were adequately described by quadratic models with R2 ranged from 0.90 to 0.97. The rhg1 gene suppressed Pf and Rf at all Pi of an H. glycines race 3 population; equilibrium population density (E) and maximum Rf were higher on the NIL-S line than on the NIL-R line. After two generations of culture of the race 3 population on the NIL-R line, the population selected by the rhg1 gene had higher Pf and Rf on the NIL-R line than the population cultured on the NIL-S line. RNA blot analysis revealed that transcript abundance of the gene encoding proline-rich cell wall protein increased in both NILs upon nematode inoculation at 1 and 4 DAI, but the increase was greater in the NIL-S line than in the NIL-R line. An increase in mRNA level of the gene encoding formylglycinamidine ribonucleotide synthase in response to nematode infection was only detected in the NIL-S line. The mRNA level of the other two genes assayed, encoding ethylene-responsive element-binding protein and chalcone synthase, did not differ dramatically between the infected and noninfected NILs.