An ancient R gene from the wild potato species Solanum bulbocastanum confers broad‐spectrum resistance to Phytophthora infestans in cultivated potato and tomato

Abstract

SummaryLate blight, caused by the oomycete pathogen Phytophthora infestans, is the most devastating disease for potato cultivation. Here, we describe the positional cloning of the Rpi-blb1 gene from the wild potato species Solanum bulbocastanum known for its high levels of resistance to late blight. The Rpi-blb1 locus, which confers full resistance to complex isolates of P. infestans and for which race speci®city has not yet been demonstrated, was mapped in an intraspeci®c S. bulbocastanum population on chromosome 8, 0.3 cM from marker CT88. Molecular analysis of a bacterial arti®cial chromosome (BAC) clone spanning the Rpi-blb1 locus identi®ed a cluster of four candidate resistance gene analogues of the coiled coil, nucleotide-binding site, leucine-rich repeat (CC-NBS-LRR) class of plant resistance (R ) genes. One of these candidate genes, designated the Rpiblb1 gene, was able to complement the susceptible phenotype in a S. tuberosum and tomato background, demonstrating the potential of interspeci®c transfer of broad-spectrum late blight resistance to cultivated Solanaceae from sexually incompatible host species. Paired comparisons of synonymous and non-synonymous nucleotide substitutions between different regions of Rpi-blb1 paralogues revealed high levels of synonymous divergence, also in the LRR region. Although amino acid diversity between Rpi-blb1 homologues is centred on the putative solvent exposed residues of the LRRs, the majority of nucleotide differences in this region have not resulted in an amino acid change, suggesting conservation of function. These data suggest that Rpi-blb1 is relatively old and may be subject to balancing selection.

Allele mining in Solanum: conserved homologues of Rpi-blb1 are identified in Solanum stoloniferum

“…Three pairs of primers (BLB1F/R, 517/1519, 1521/518, Table 2) were designed based on the Rpi-blb1 homologous sequences (van der Vossen et al 2003), aiming at speciWc ampliWcation of the Rpi-blb1 gene. Upstream and downstream of the primers BLB1F/R new primers were designed to determine whether non-ampliWcation with the BLB1F/R primer pair might be caused by a mutation in either primer site.…”

“…Examples are S. pinnatisectum (Kuhl et al 2001) and S. bulbocastanum (Naess et al 2000;Song et al 2003; Van der Vossen et al 2003 andPark et al 2005) from Mexico and S. berthaultii (Rauscher et al 2006;Ewing et al 2000), S. microdontum (Sandbrink et al 2000), S. mochiquense (Smilde et al 2005) and S. paucissectum (Villamon et al 2005) from Andean countries. In addition, the cultivated diploid S. phureja has been described to contain valuable resistance (Ghislain et al 2001;Sliwka et al 2006).…”

“…To date, four late blight R-genes have been cloned: R1 (Ballvora et al 2002), R3a (Huang et al 2005), RB or Rpiblb1 (Song et al 2003;van der Vossen et al 2003) and Rpiblb2 (van der Vossen et al 2005). The latter two genes were cloned from S. bulbocastanum and confer resistance to all P. infestans isolates tested so far.…”

“…The latter two genes were cloned from S. bulbocastanum and confer resistance to all P. infestans isolates tested so far. Rpi-blb1 is part of a resistance gene analog (RGA) cluster of four members RGA1-blb, Rpi-blb1, RGA3-blb, and RGA4-blb on chromosome VIII (van der Vossen et al 2003). Complementation analysis showed that only the genetic construct harbouring Rpi-blb1 was able to complement the susceptible phenotype.…”

“…Complementation analysis showed that only the genetic construct harbouring Rpi-blb1 was able to complement the susceptible phenotype. The sequence relationships between Rpi-blb1, RGA1-blb and RGA3-blb showed that the Rpi-blb1 gene most likely evolved from intragenic recombination between the ancestral genes of RGA3-blb and RGA1-blb (van der Vossen et al 2003). Rpi-blb2 resides in a locus harbouring at least 15 Mi gene homologues on chromosome VI, and the Rpi-blb2 protein shows 82% sequence identity to the Mi-1 protein .…”