Annals of Botany

Exhausted invader meets solid resistance

Parasitic plants, especially those that rely totally on their hosts for carbon and other key nutrients, are a serious problem in some agricultural systems where they can cause significant reductions in crop yield. Amongst these obligate parasites are the broomrapes (Orobanche spp.) and it is important to establish what enables some plants to resist attack while others cannot. A truly international team have been addressing this leading question (Pérez-de-Luque et al., pp. 935–942) by examining Orobanche crenata, a species that parasitizes a range of legume species. Within these species, resistant varieties exist that make possible the direct comparison of successful and unsuccessful attack by the parasite. Because of the visual appearance of the host tissue around the infection site, it had been thought previously that resistance was based on a hypersensitive response. This is a well-known mechanism of resistance to pathogens in which host cells round the infection site die, preventing further spread of the pathogen. However, in this careful study it is shown that tissue darkening in resistant hosts is caused not by cell death but by a secretion from the invading parasite haustorium. Indeed, it is likely that this is an excess production of the parasite’s normal secretion, which contains, amongst other substances, tissue-softening enzymes. This excessive secretion appears to be triggered by an early reaction of the host, namely the lignification of endodermis and pericycle, which impedes the parasite’s progress. However, the parasite’s response does not help further penetration because its secretion results in local blockage of the host’s vessels and prevents nutrient transfer around the infection site. The result of all this is that, even if the parasite develops as far as forming tubercles, it cannot establish a functional relationship with the host and cannot take up any carbon or other nutrients. The invading tissues of parasite are thus contained and undergo necrotic death.