Phytotoxins: Physiological and Genetic Aspects

Dear Colleagues,

Many plant pathogenic fungi and bacteria produce a wide spectrum of toxic metabolites that interfere with the metabolisms of plants. These chemicals of bacterial of fungal origin are phytotoxic to plants and have been collectively referred to as phytotoxins. In contrast, mycotoxins are used to denote fungal secondary metabolites that are toxic to humans and other animals. Numerous phytotoxins produced by plant pathogenic fungi and bacteria have been biochemically purified and structurally characterized, owing to new or improved techniques in chromatography and spectroscopy. Those phytotoxic metabolites often have diverse structures. Some are phytotoxic only to the host plant susceptible to the pathogen (host-selective toxins); others exhibit general toxicities to both host and non-host plants (non-host-selective toxins). Host-selective toxins (HST) with distinct modes of action interact with a specific target site or protein and play a profound role in pathogenicity and disease development. Compared to HST, the pathological role of non-HST for pathogenesis and for the onset of plant diseases has not yet been widely investigated. Phytotoxins could inhibit enzymatic activities, interfere with membrane integrity, disrupt mitochondrial functions or suppress defense responses. Phytotoxins have been proven to facilitate our understanding about the mechanisms of plant disease development, pathogen specificity, as well as host susceptibility and tolerance. Phytotoxins also provide a unique opportunity for investigating the normal biochemical and physiological functions of plants. Aside from their important role in the disease development, phytotoxins also have unique and intriguing biological and chemical properties. For example, the photoactivated perylenequinones, produced by a number of important fungal plant pathogens such as Alternaria, Cercospora, Cladosporium, Elsinoë, Stemphylium and many others, have pharmaceutical potential as anticancer agents through photodynamic tumor therapy. Some phytotoxins have been studied for use as agricultural pesticides or antimicrobial agents. Environmental and physiological inducers involved in the production of phytotoxins have been intensively studied as well. Recently, molecular and genetic analyses have made great strides in elucidating biosynthesis and regulation of phytotoxins, as well as signaling pathways associated with their production. The genes whose products are required for regulation and biosynthesis of phytotoxins are often clustered in the genome of pathogens and have been cloned and characterized in detail from several bacterial and fungal pathogens over the last two decades. Much has also been learned about what physiological functions of the plant cells are affected and what the primary group(s) of phytotoxins is required for their toxicity. Studies for the cellular mechanisms by which the phytotoxin-producing microorganisms protect themselves from the toxicity are equally important. Future studies may focus on how to better apply all accumulated knowledge of phytotoxins for disease management and food production.

Prof. Dr. Kuang R. Chung
Guest Editor