Post-haustorial resistance based in an increase of phenolic compounds provides a powerful tool to control the parasitic weed broomrape in sunflower

Abstract

Sunflower is a major source of vegetable oil in the world. The most important areas for sunflower crop are Russia, Ukraine and European Union, and the parasitic weed broomrape (Orobanche cumana Wallr.) is one of the most important limiting factor of production there. The knowledge of the race structure of the parasite and the availability of resistance sources in the host are critical factors to provide effective control of the parasite by genetic resistance. In this study we perform the phenotypic characterization of a new resistance source in sunflower and the genetic mapping of the trait. The sunflower inbred P1 was observed to be resistant to broomrape in several field tests. Subsequently, it was infested artificially in pots with 10 different broomrape populations from different countries of race E, F, G and upper independently. P1 was resistant to all of them. A susceptible inbred line (P2) without resistance genes and a commercial hybrid with pre-haustorial resistance until race G were used as controls. The phenotype of the resistance was characterized by the formation of broomrape nodules which remain underground and they did not emerge. In order to get insights into the mechanisms involved in the resistance, a histological study was developed using different techniques. Accumulation of phenolic compounds was observed under epi-fluorescence (340–380 nm) in P1 line while this phenotype was not observed in the other two genotypes. Subsequently, the genetic basis of the resistance was studied in an F2:3 mapping population created by crossing P1 and P2 inbred lines. A total of 150 F2 plants were genotyped with 157 SNP markers and 12 plants of each F3 progenies evaluated for resistance to broomrape in pot tests artificially infested with a race G broomrape, and in the field with race F. On the basis of a Chi-square test, the observed ratio did not differ from the 1:2:1 ratio, indicating that resistance from P1 inbred line is controlled by a single dominant gene. This gene was mapped in chromosome 4 at an absolute position of 29cM from the upper end of linkage group 4. Since no other broomrape resistance genes have been mapped in that chromosome, this gene was new and designated OrSII. Molecular markers for MAS of this gene are under the patent US 7872170 B2. Dominance of the resistance in several hybrids and combination of this gene with pre-haustorial resistance will be discussed as a way of developing durable genetic resistance.