Presence of defeated qualitative resistance genes frequently has major impact on quantitative resistance to Melampsora larici-populina leaf rust in P. × interamericana hybrid poplars

Abstract

Qualitative resistance to Melampsora larici-populina leaf rust inherited from North American species Populus deltoides did not allow for durable control of this pathogen in interspecific hybrid cultivars. Despite significant levels of strain-specificity, quantitative resistance would exert lower selection pressures on the pathogen populations, and hence could be more durable. Previous studies restricted to a large P.×interamericana (i.e., P. deltoides×Populus trichocarpa) F 1 family revealed that the presence of R 1 , a segregating defeated qualitative resistance gene inherited from P. deltoides, had major beneficial effects on quantitative resistance. The present study was based on 14 F 1 families from a 4×5 P. deltoides×P. trichocarpa factorial mating design where at least four defeated qualitative resistances segregate 1:1. Even though quantitative resistance assessments were conducted in the laboratory with a M. laricipopulina strain able to overcome these qualitative resistances, their presence had a significant effect on the mean level and on the genetic variability for quantitative resistance. One unprecedented result is the identification of a defeated qualitative resistance which presence is associated with lower levels of quantitative resistance. Possible inferences on the nature of the genetic relationship between both resistance types are discussed.

Positioning of the major locus for Puccinia psidii rust resistance (Ppr1) on the Eucalyptus reference map and its validation across unrelated pedigrees

“…In all cases a major locus conferring resistance to the rust disease was found and its presence validated across unrelated pedigrees. In a comprehensive study carried out in Poplar with 14 F1 families of Populus deltoides×Populus trichocarpa, where at least four qualitative resistance factors were segregating, a complex structure of interactions was found between qualitative resistance genes and the levels of quantitative resistance to M. larici-populina, highlighting the importance of such investigations both from the evolutionary and breeding standpoints (Dowkiw and Bastien 2007). Validation of QTLs mapped in Eucalyptus pedigrees is still a poorly explored topic.…”

Nonhost and basal resistance: how to explain specificity?

“…Another R‐ gene‐related hypothesis proposes that some R genes have a residual effect on virulent isolates (Li et al. 1999; Dowkiw & Bastien, 2007), and that such defeated R genes may be perceived as QTLs for basal resistance. A third R ‐gene‐related hypothesis, often confused with the previous one, is that quantitative resistance genes are small‐effect allelic variants of R genes.…”

Discovery of genes affecting resistance of barley to adapted and non-adapted powdery mildew fungi

“…The introgression of single major R genes usually confers strong protection against specific adapted pathogen races carrying the matching avirulence (Avr) effector genes, but the trait is often overcome by rapidly evolving new pathogen races with mutated Avr effectors acting in concert with other functionally redundant effectors. In principle, QR could also be mediated by partially functional (defeated) major R-genes weakly recognizing ubiquitous Avr effectors such as ECP1 or ECP2 [12], but molecular evidence for this type of interactions is scarce [13,14].…”