Development of a simple hydroponic assay to study vertical and horizontal resistance of soybean and pathotypes of phytophthora sojae
Lebreton, A., Labbé, C., De Ronne, M., Xue, A.G., Marchand, G., Bélanger, R.R. (2018). Development of a simple hydroponic assay to study vertical and horizontal resistance of soybean and pathotypes of phytophthora sojae. Plant Disease, [online] 102(1), 114-123. http://dx.doi.org/10.1094/PDIS-04-17-0586-RE
Plain language summary
Genetic resistance can be a very useful tool to manage plant diseases, as using resistant plant varieties for crop production can reduce the reliance on chemical pesticides used to protect these crops against plant pathogens. This study aimed to develop a simple assay to study different types of genetic resistance (partial and complete) of cultivated soybean to a microbial agent, Phytophthora sojae, that causes disease symptoms of seed rot, seedling damping-off and root rot on the various growth stages of soybean. Although methods for evaluating the genetic resistance of soybean to this pathogen have previously been developed, several different assays are required to evaluate the two different types of genetic resistance of soybean to this pathogen, and a single assay allowing the evaluation of both types of resistance had not been previously reported. Soybean plants were grown in a recirculating hydroponic system in the greenhouse and infected with the pathogen. This system allowed the direct observation of the live roots, since plants were grown in a clear water-based nutrient solution rather than in opaque potting mix or soil. The appearance and growth patterns of the aerial parts (stems and leaves) and the roots of the soybean plants correlated with the previously known complete resistance profile in a set of nine soybean varieties, with resistant plants showing more development and a healthier appearance of aerial parts and roots, and higher plant dry weight, when infected with two different isolates of the pathogen. The assay was also used to assess complete resistance in 10 commercial soybean varieties in development for which the researchers did not initially know the resistance profile (blind test for), and the results were later confirmed to match expectations for these varieties. The interaction between soybean plants of known complete resistance profile and the pathogen can also be used to study the genetic profile for virulence of the pathogen, in this assay and in others previously developed. Two isolates of the pathogen were tested in this fashion using this new hydroponic assay on nine soybean varieties of known complete resistance profile, and the results matched expectations based on the previously tested virulence profile of these isolates using an older method. This new assay should be a useful tool to evaluate the resistance of soybean lines to this pathogen and also the pathogen’s genetic profile and thus disease-causing potential.
Phytophthora root rot, caused by Phytophthora sojae, is one of the most damaging diseases of soybean and the introgression of Rps (Resistance to P. sojae) genes into elite soybean lines is arguably the best way to manage this disease. Current bioassays to phenotype the gene-for-gene relationship are hampered with respect to reproducibility and long-term stability of isolates, and do not accurately predict horizontal resistance individually. The aim of our study was to investigate a new way of phenotyping P. sojae isolates and vertical and horizontal resistance in soybean that relies on zoospores inoculated directly into a hydroponic system. Inoculation of P. sojae isolates against a set of eight differentials accurately and reproducibly identified pathotypes over a period of two years. When applied to test vertical resistance of soybean lines with known and unknown Rps genes, the bioassay relied on plant dry weight to correctly identify all genes. In addition, simultaneous inoculations of three P. sojae isolates, collectively carrying eight major virulence factors against 64 soybean lines with known and unknown levels of horizontal resistance, separated the plants into five distinct groups of root rot, allowing the discrimination of lines with various degrees of partial resistance. Based on those results, this bioassay offers several advantages in facilitating efforts in breeding soybean for P. sojae resistance and in identifying virulence factors in P. sojae.