Suppression of host resistance by fungal plant pathogens [Review]

摘要

An attempt has been made to characterize the attributes of fungal pathogens. Fungal pathogens have special characteristics that, in contrast with saprophytes, allow them to infect and colonize their hosts. These include an arsenal of enzymes and toxins that allow them to suppress host resistance responses that arise during the infection process, and to survive in the hostile environment of the infected host. Here we have reviewed the various suppression strategies employed by fungal plant pathogens. For example, virulent isolates of the powdery mildew fungi induce host cells to be accessible to colonization, and this state of accessibility is passed on to adjacent cells. Then there are the host-selective toxins, a group of diverse complex molecules that are secreted by the invading fungus to enable it to colonize the host. The enzymes of some pathogens, acting as virulence factors, can destroy or detoxify defense chemicals like the phytoalexins and phytoanticipins, so that the host becomes accessible; however, the degradation products may also suppress the hypersensitive response. Some fungal pathogens also produce supprescins, a group of nontoxic metabolites, often small glycopeptides, that suppress active defense responses; the growth hormone abscisic acid may be a part of the suppression mechanism. Arbuscular-Mycorrhizal fungi, and the various fungi that colonize orchids, also have a marked ability to suppress defense-related responses that enable these fungi to establish a successful symbiotic association. The suppression of host resistance seems to be a special characteristic that plant pathogens have acquired that sets them apart from saprophytic fungi, which live only on dead tissue. It is doubtful that pathogens could infect their hosts without such a mechanism.