Isolation of ethylene-insensitive soybean mutants that are altered in pathogen susceptibility and gene-for-gene disease resistance

Abstract

Plants commonly respond to pathogen infection by increasing ethylene production, but it is not clear if this ethylene does more to promote disease susceptibility or disease resistance. Ethylene production and/or responsiveness can be altered by genetic manipulation. The present study used mutagenesis to identify soybean (Glycine max L. Merr.) lines with reduced sensitivity to ethylene. Two new genetic loci were identified, Etr1 and Etr2. Mutants were compared with isogenic wild-type parents for their response to different soybean pathogens. Plant lines with reduced ethylene sensitivity developed similar or less-severe disease symptoms in response to virulent Pseudomonas syringae pv glycinea and Phytophthora sojae, but some of the mutants developed similar or more-severe symptoms in response to Septoria glycines and Rhizoctonia solani. Gene-for-gene resistance against P. syringae expressing avrRpt2 remained effective, but Rps1-k-mediated resistance against P. sojae races 4 and 7 was disrupted in the strong ethylene-insensitive etr1-1 mutant. Rps1-k-mediated resistance against P. sojae race 1 remained effective, suggesting that the Rps1-k locus may encode more than one gene for disease resistance. Overall, our results suggest that reduced ethylene sensitivity can be beneficial against some pathogens but deleterious to resistance against other pathogens.

Figure 1

Ethylene triple response of etiolated soybean seedlings. Plants were germinated for 6 d in complete darkness in sealed vessels containing air or air supplemented with ethylene to the concentrations noted. The typical triple response to ethylene includes shortening of the hypocotyl, radial thickening of the central hypocotyl, and excessive curling of the hypocotyl hook.

Figure 2

Hypocotyl elongation of etiolated soybean seedlings germinated in ethylene. Mutants are presented with their respective nonmutagenized parent line Hobbit 87 (A) or A90-312022 (B). Seedlings were germinated for 6 d in complete darkness in sealed vessels containing air supplemented with ethylene to the concentrations noted. Values are means ± se. T10N5, T10N6, and T15 are sibling lines carrying the same mutation (see text). True-breeding progeny of originally identified mutants were used in these and subsequent studies.

Figure 3

Distribution of hypocotyl-length phenotypes in parents and segregating F₂ populations germinated under etiolating conditions in 20 μL L⁻¹ ethylene. Data on a given graph are from plants tested concurrently within the same chamber. A, Hobbit 87 (broken line), Hobbit 87etr1-1 (heavy, solid line), and F₂ of Hobbit 87 × Hobbit 87etr1-1 (solid line with fill). B, A90-312022 (broken line), etr2-1 mutant T10N5 (heavy, solid line), and F₂ of A90-312022 × T10N5 (solid line with fill). C, A90-312022 (broken line), T58N5 (heavy, solid line), and F₂ of T58N5 × A90-312022 (solid line with fill).

Figure 4

Chlorophyll content in leaves from mock-inoculated plants and plants infected by virulent P. syringae pv glycinea. Leaf discs were sampled 9 d after inoculation; values are means ± se for four separate groups of 5 plants (total of 20 plants per pathogen treatment). wt, Wild type.