Plant resistance to plant diseases

Abstract

Plants are continuously exposed to the attack of several thousand of different strains of fungi, bacteria, viruses and other microorganisms. A plant can be attacked by thousands, hundreds and in the case of leaf spot in woody plants, even by hundreds of thousands of a single pathogen individuals. However, despite being infected, many plants manage to survive and produce relatively high yields (Agrios, 1997). This means that they are not only a passive subject of continuous microbial attack but also interact with them, which involves a broad spectrum of defense mechanisms (Jackson and Taylor, 1996; Osborne, 1996). To prevent attack of a pathogen, plants defend themselves by a combination of structure elements, which is a physical barrier to entry and spread of a pathogen (passive defense), and of biochemical reactions taking place in plant cells and tissues after the attack, when the compounds produced become either toxic to a pathogen or the conditions created inhibit its development in a plant (actively induced defense) (Agrios, 1997). A disease develops after a pathogen escapes plant passive defense and timely activation of induced defense in an attacked tissue, or overcomes induced defense by the secretion of toxins and enzymes (Alfano and Collmer, 1996; Jackson and Taylor, 1996). The characteristics of both host plant and parasite are determined by their genetic structure i.e. their DNA that is functionally organized through numerous genes. Many plants possess larger or smaller resistance whereby they tend to prevent or slow down the entry and spread of a disease causal agent. The degree of resistance or susceptibility of a host plant varies and the pathogen exhibits different degrees of virulence too. Therefore, the outcome of any disease attack is different. .

Biljke su izložene konstantnom napadu nekoliko hiljada različitih vrsta gljiva, bakterija, virusa i dragih mikroorganizama. Jednu biljku mogu da napadnu stotine, hiljade, a u slučaju lisne pegavosti drvenastih biljaka i stotine hiljada individua jednog patogena. Međutim, i pored ostvarene infekcije, mnoge parazitirane biljke uspevaju da prežive i ostvare prihvatljiv prinos (Agrios, 1997). To znači da one nisu samo pasivni predmet stalnog napada mikroorganizama, nego su sa njima u interakciji koja obuhvata i spektar odbrambenih mehanizama (Jackson and Taylor, 1996; Osborne, 1996). Od napada patogena biljke se brane kombinacijom strukturnih elemenata, koji predstavljaju fizičku barijeru za prodor i širenje patogena (pasivna odbrana) i biohemijskih reakcija koje se odigravaju u biljnim ćelijama i tkivima posle napada, pri čemu nastaju jedinjenja koja su ili toksična za patogena, ili stvaraju uslove koje inhibiraju njegov razvoj u biljci (aktivna indukovana odbrana) (Agrios, 1997). Do bolesti dolazi kada patogen zaobiđe pasivnu odbranu biljke i blagovremeno izbegne aktiviranje indukovane odbrane u napadnutom tkivu ili lučenjem toksina i enzima savlada indukovanu odbranu (Alfano and Collmer, 1996; Jackson and Taylor, 1996). Svojstva i biljke - domaćina i parazita su određene njhovim genetičkim materijalom, tj. njihovim DNA koja je funkcionalno organizovana kroz brojne gene. Mnoge biljke imaju veću ili manju otpornost kojom nastoje da zaustave ili uspore prodor i širenje prouzrokovača bolesti. Stepen otpornosti ili osetljivosti biljke domaćina je različit, a i patogen pokazuje različit stepen virulentnosti. Zbog toga je ishod svakog oboljenja različit.