Marker-Assisted Selection for the Broad-Spectrum Potato Late Blight Resistance Conferred by Gene RB Derived from a Wild Potato Species

Potato (Solanum tuberosumL.) late blight, caused byPhytophthora infestans (Mont.) de Bary, is one of the most damaging diseases in any crop. Deployment of resistant varieties is the most effective way to control this disease. However, breeding for late blight resistance has been a challenge because the race-specific resistance genes introgressed from wild potato S. demissum Lindl. have been short lived and breeding for ‘‘horizontal’’ or durable resistance has achieved only moderate successes. We previously demonstrated that the highlevel late blight resistance in a wild potato relative, S. bulbocastanum Dunal subsp. bulbocastanum, is mainly controlled by a single resistance gene RB. Transgenic potato lines containing the RB gene have showed strong late blight resistance, comparable to the backcrossed progenies derived from the somatic hybrids between potato and S. bulbocastanum. Here we report the development of a polymerase chain reaction-based DNA marker for tracking the RB gene in breeding populations derived from the potato 3 S. bulbocastanum somatic hybrids. Several marker-positive breeding lines showed the expected late blight resistance in greenhouse evaluations. Our results demonstrate that marker-based selection will allow us to effectively transfer the RB gene into potato using traditional breeding methods, an alternative to deploying the RB gene through genetic transformation. LATE BLIGHT caused by P. infestans is the most serious threat to potato production worldwide (Duncan, 1999; Garelik, 2002). Late blight starts as dark, watersoaked lesions present on the leaf surface that rapidly spread over the foliage. The devastation caused by P. infestans was first noticed during the middle 19th century Irish potato famine, which resulted in one million deaths. Late blight is still the most serious problem of cultivated potatoes and may cause complete tuber loss in susceptible germplasm (Ojiambo et al., 2000). Current potato production practices use expensive fungicide applications and intensive cultural practices to control P. infestans outbreaks. However, due to the constant genetic shifts in P. infestans populations and the decrease in fungicide effectiveness, this pathogen can cause significant losses in potato (Fry and Goodwin, 1997; Garelik, 2002). The most effective and environmentally friendly way to prevent widespread devastation by late blight is to incorporate natural resistance into potato cultivars. Since the middle 19th century, there has been extensive selection and breeding for late blight resistance. Many current potato cultivars contain resistance derived from S. demissum (2n 5 6x 5 72), S. andigena Hawkes (2n 5 4x 5 48), and other wild species. Most of the resistance obtained from these wild species, mainly from S. demissum, belong to the ‘‘vertical resistance’’ type. In vertical resistance, plants containing a specific resistance gene (R gene) or virulence gene will interact with the corresponding avirulence gene found in the pathogen, which is known as the ‘‘gene-for-gene’’ concept (Flor, 1971; Hammond-Kosack and Jones, 1996; Keen, 2000; Leister, 2000). Potatoes containing these R genes are only effective in preventing the development of late blight if the invading P. infestans race contains the corresponding avirulence gene. The R gene–mediated resistance is often short lived and is rapidly overcome by new strains of the late blight pathogen. A total of 11 R genes, all from S. demissum, have been characterized in potato (Black et al., 1953; Malcolmson and Black, 1966). In the 1970s potato breeding with emphasis on vertical resistance was replaced by breeding for ‘‘horizontal resistance’’ (Wastie, 1991). Horizontal resistance is thought to be polygenic or a quantitative trait that confers resistance to multiple races of a particular pathogen (Agrios, 1997). The mechanisms of horizontal resistance have not been well understood. Horizontal resistance is believed to be much more durable than vertical resistance due to the interaction of many genes which recognize different races of the same pathogen. Durable or horizontal resistance is described as being effective during prolonged and widespread use in an environment conducive to the disease (Johnson, 1984). However, horizontal resistance is difficult to breed for due to its polygenic nature and poorly understood mechanisms of action. Many wild potato species coexist in the same habitat as the late blight pathogen and have developed mechanisms for survival along with the pathogen. Solanum bulbocastanum (2n 5 2x 5 24), a diploid species native to Mexico, has previously been characterized as possessing durable resistance against P. infestans, even under high disease pressure (Niederhauser and Millis, 1953; van Soest et al., 1984). S. bulbocastanum shows a general suppression, but not total elimination, of late blight symptoms. The resistance present in S. bulbocastanum is effective against all the known races of P. infestans. Surprisingly, the late blight resistance in the S. bulbocastanum clone PT29 was mapped to a single locus on chromosome 8 (Naess et al., 2000). This single gene, RB, has been previously cloned and transformed into Katahdin, a highly susceptible potato cultivar. Katahdin plants transformed with the RB gene showed broadspectrum resistance against a number of P. infestans Department of Horticulture, University of Wisconsin-Madison, Madison, WI 53706, USA. Received 3 Feb. 2005. *Corresponding author