Biochemical aspects of plant-pathogen interactions

Abstract

Plants are non-motile organisms subjected to constant attack of microorganisms, vertebrate and invertebrate animals and even other plants. This feature forced plant evolution to the development of an original defence system, much different from that found in animals. Immunological system of vertebrate animals, including human beings, involves the mobilization of specialized cells through a circulatory system to the body´s site under attack to kill or restrict the invading organism. In plant defence, each plant cell has to perform immune functions, which can be preformed and/or induced after pathogen attack. The passive defence mechanisms involve structural barriers provided by waxy cuticles or cell wall bonded polyphenols and accumulation of secondary metabolites in specific locations of plant tissues. Active or induced defence occurs in plant cells after pathogen attack and requires host metabolism to function. If pathogen is recognized after infection, plant readily initiates a local response into cells in contact or very close to the attacking organism. The first response frequently detected is an increase in active oxygen species (AOS), which can occur within less than 5 minutes in the infected site. This accumulation of AOS is also known as oxidative burst and triggers several biochemical changes in plant cells such as accumulation of phytoalexins and increase in activity of enzymes (i.e. peroxidases and phenylalanine ammonia lyase). The outcome of this primary response is often the hypersensitive response, which consists in the death of plant cells in contact and in the surroundings of the infected site. Recognition of pathogen attack is then transmitted through the whole plant body through a systemically acquired resistance (SAR). The SAR is hormonally induced and often related with an increase in salicylic and jasmonic acid levels. The following paragraphs provide general concepts and criteria for the detection of organic molecules involved in both passive and active plant defence responses. It also comprises the measurement of some enzyme activities related to active defence response.