Characterizing Resistance to Phakopsora pachyrhizi in Soybean

Abstract

Resistance in soybean to Phakopsora pachyrhizi, the cause of soybean rust, is characterized by either reddish-brown (RB) lesions or an immune response. The RB type of resistance can be incomplete, as evidenced by the presence of sporulating uredinia within lesions. Susceptibility, on the other hand, is exemplified by tan-colored (TAN) lesions, and can be expressed in gradations of susceptibility or partial resistance that are less well defined. This study evaluated traits associated with incomplete or partial resistance to P. pachyrhizi in soybean by comparing 34 soybean accessions inoculated with four P. pachyrhizi isolates. Six accessions produced RB lesions to all four isolates, while 19 accessions produced TAN lesions, including plant introduction (PI) 200492 (Rpp1) and the susceptible check ‘Williams’. Williams had among the largest area under the disease progress curve (AUDPC) values and area under the sporulating uredinia progress curve (AUSUPC) values, while eight accessions had lower AUSUPC values. Of the known sources of single-gene resistance, only PI 230970 (Rpp2), PI 459025B (Rpp4), and PI 594538A (Rpp1b) had lower AUDPC and AUSUPC values than Williams. PI 594538A and PI 561356 had RB lesions and had the lowest AUDPC and AUSUPC values. Of the known sources of single-gene resistance, only PI 230970 (Rpp2) and PI 594538A (Rpp1b) produced fewer and smaller-diameter uredinia than Williams. This study characterized reactions to P. pachyrhizi in 34 accessions based on lesion type and sporulation, and defined incomplete resistance and partial resistance in the soybean–P. pachyrhizi interaction.

Evaluation of Soybean Germplasm Accessions for Resistance to Phakopsora pachyrhizi Populations in the Southeastern United States, 2009–2012

“…Because of the wide range of variation in infection types (i.e., RB or TAN lesions) with regard to color and intensity of sporulation from pustules, infection type was not used as a major rating determinant in this study. Furthermore, Miles et al (2011) concluded that lesion type was not associated with disease severity in a group of 34 accessions inoculated with four P. pachyrizi isolates in the greenhouse. Bromfield (1984) himself later acknowledged that three infection type descriptions (including the symptomless immune response) were inadequate for describing the full range of lesion appearances observed, and other researchers have reached a similar conclusion (Bonde et al, 2006;Kato and Yorinori, 2008;Morel et al, 2008).…”

Characterization and Quantification of Fungal Colonization of Phakopsora pachyrhizi in Soybean Genotypes

“…The responses described most frequently include the (i) immune reaction (IM) without visible lesions, indicating an incompatible host-parasite interaction; (ii) RB lesions without sporulation (complete resistance); (iii) RB lesions with varying levels of sporulation (incomplete resistance); (iv) tan-colored lesions (TAN) with limited sporulation (partial resistance); and (v) TAN lesions with abundant urediniospore production (susceptible reaction) (3,25,30). Based on genetic analyses of phenotypes within segregating populations developed from crossing parents having IM or RB resistant phenotypes with TAN susceptible parents, six soybean rust resistance loci (Rpp1-Rpp5, some multi-allelic) have been identified and mapped in the soybean genome, and the interaction of these loci with P. pachyrhizi pathotypes has been summarized (9).…”

Asian Soybean Rust Resistance: An Overview

“…According to studies from Miles et al [49], the rust severity was not related to the type of lesions. Among the varieties with RB lesions, PI 561356 and PI 594538A showed lower levels of severity than PI 230970, PI 423972 and PI 459025B, who were the varieties with higher rates of severity.…”

“…According Parlevliet [47] and Ribeiro Do Vale [48], incomplete resistance allows some growth or reproduction of the pathogen in the host tissues. Finally, the tan colored lesions (TAN), indicative of susceptibility [49]. Either IM or RB reaction are initiated with the early perception of the pathogen avirulence proteins by plant R proteins, according the classical gene-for-gene resistance theory [50].…”

“…Besides the influence of the host genotype and levels of pathogen sporulation, the color of the lesions varies with their age, especially in the field where the infection events are continuous. In the work of Miles et al [49], the IM and RB reactions were considered the unique forms of resistance expression. However, partial resistance also occurred in the interaction between P. pachyrhizi and soybeans, since differences were observed between the lines with TAN lesions.…”