The role of peroxidase in the resistance of sunflower against Orobanche cumana in Russia

Abstract

SummaryWe present the results of a histochemical study comparing seedlings of races C and D of Orobanche cumana Wallr. (syn. O. cernua Loefl.) attacking sunflower (Helianthus annuus L.) in southern Russia. Three groups of O. cumana seedlings were distinguished according to the peroxidase content of the cells in the radicles: (1) those with neither extracellular nor intracellubr peroxidase and whose radicles have a smooth apex (these were classified as non‐infective): (2) those with a high peroxidase content of the nuclei and the cytoplasm layer adjacent to the cell wall, as well as excretion of peroxidase from the apex of the radicles: (3) those with a similarly high peroxidase activity in the parasite cells, but without extracellular excretion. The apices of the radicles of the last two groups are swollen. It is suggested that these belong to O. cumana races C and D respectively. The extracellular peroxidase in O. cumana race C reacts with phenolic compounds, which are lignin precursors of the host, resulting in host resistance due to the formation of lignin layers in sunflowers possessing the Or3 gene for resistance. The absence of extracellular peroxidase in O. cumana race D prevents lignin formation and enables the parasite to attach to the host vascular system. Comparison of these data with the information on the earlier O. cumana races A and B, and older sunflower cultivars, points to a crucial role of peroxidase in the process of breeding new sunflower cultivars and the evolution of new O. cumana races.