The inhibition of infection thread development in the cultivar-specific interaction of Rhizobium and subterranean clover is not caused by a hypersensitive response

The cultivar specific interaction of Trifolium subterranean cv. Woogenellup and Rhizobium leguminosarum bv. trifolii strain ANU 794 was examined to establish the basis for nodulation failure on this cultivar. Infections were initiated by strain ANU 794 on cv. Woogenellup. Root hair curling, the initiation of infection threads, and cortical cell divisions were evident on the tap root and appeared normal after microscopic observation. However, in most cases, the infection threads stayed confined to the root hairs. No evidence was found for a hypersensitive response by the plant. The progress of infections on the tap roots was different from that on the lateral roots. This was confirmed by the differential tap and lateral root nodulation patterns of the mutants derived from strain ANU 794, which show enhanced nodulation on cv. Woogenellup. On the lateral roots, cortical cell divisions progressed further than those on the tap root and formed macroscopically visible swellings, which could be divided into two morphological classes. In some cases infection threads developed into these primordia but successful nodules were not established. The inhibition of infection appeared to be manifested at two levels: first, on the tap roots in the root hairs, where many of the infection threads are contained and secondly, in the primordia induced on the lateral roots, where the infection threads sometimes penetrate further than the root hair cell but stop in the primordial cells. It appears that an essential factor or trigger in the communication between plant and bacteria is missing or altered, resulting in an array of primordia-structures, which cease to develop.