Histology and physiology of compatibility and incompatibility between lettuce and the downy mildew fungus, Bremia lactucae Regel

Abstract

Three races of Bremia lactucae Regel and five cultivars of lettuce were investigated to determine early events which distinguish between compatible and incompatible interactions during the infection process. The time-course of the infection process was followed histologically and by the leakage of electrolytes. Infection types were graded 1 (low = incompatible), 2 and 3 (intermediate) and 4 (high = fully compatible). Low infection types were characterized by the death of host cells during penetration or 2 to 5 h after penetration, depending upon the host gene for resistance and fungal gene for avirulence. Host cells adjacent to penetrated cells were not killed. The fungus grew out of the dead, penetrated cell, and eventually died within 12 to 24 h of penetration. Intermediate infection types were characterized by large penetration pores, a delay in onset of nuclear division in the fungus, and eventual necrosis of host cells one or more days after penetration, although the fungus continued growth and sporulated sparsely. High infection types were typical of a biotrophic relationship; the fungus grew rapidly through the living host tissue and, prior to sporulation, showed little or no tendency to kill penetrated cells of the host.The cause and significance of the rapid death of host cells observed in low infection types are discussed in relation to the cause of resistance.