Plants have several defense mechanisms against insects and pathogens (fungi, bacteria, viruses, nematodes etc.). Plants defense response is based on structural, biochemical and molecular mechanisms to control insect and pathogen invasion. Hammond-Kosack and Jones (1996) suggested three reason to resist infection.

  1. The plant species is not a host for the pathogen (non – host resistance)
  2. The plant respond through structural and chemical based barriers that prevents infection
  3. Plant defense mechanisms is activated on recognition of the pathogen

Most plant disease causal organisms are unable to breach the structural barrier or withstand the antimicrobial compound of the plants. Morphological feature of the plant such as hairs, trichomes, thorn or thick leaves act as direct defense. The induction of defense responses may lead to enhanced resistance. The induced resistance is expressed after microbial attack in the form of fortification of cell wall, pathogenesis–related (PR) proteins and biosynthesis of phytoalexins.  Plant resistance is also achieved by elicitors. Elicitors are signal – inducing compounds sensed by plant innate immune system which induces defense responses (Newman et al., 2013). Elicitor compound can be derived from plant, microbe or can be synthetically made (Walters et al., 2013).

Structural and Biochemical Defense:

  • Wax, thick cuticle, lignified cork cell, formation of abscission layer, tyloses and deposition of gums act as a barrier and protect plant from pathogens.
  • Lignin deposition in the secondary walls of most of the plants provide rigidity, mechanical support and resistance against pathogen infection reason being lignin is resistant to microbial degradation. Nicholson and Hammerschmidt (1992) reviewed role of lignin in disease resistance.
  • Papilla formation. Papillae are heterogeneous materials deposited between plasma membrane and the cell wall by plants in response to pathogen and mechanical injury. Callose (polysaccharide consisting of β-1, 3-glucan) is the major component of papillae.
  • Induced structure like fortification of cell wall with lignin suberin, hydroxyproline – rich glycoprotein (HRGPs) provide resistance against pathogenic infection.
  • The chemical substances exuded by the plant serve as inhibitors against pathogen (amino acid, sugars, glycosides, organic acids, enzymes, alkaloids etc.). Phenolics, such as chlorogenic acid, caffeic acid and phytoalexin are produced by plants in response to injury caused by the pathogen.
  • Phytoalexins are fungitoxic but may possess some antibacterial activity. Some phytoalexin are ipomeamarone, orchinol, isocaumarin, pisatin and phaseolin.
  • Proteins are produced and accumulated in plant in response to infection. These plant proteins whose synthesis are induced in pathological or related situation are known as pathogenesis-related (PR) protein (Antoniw et al., 1981). PR proteins such as acidic β-1, 3-glucanases and chitinases are able to hydrolyze microbial cell wall components (Van Loon and Van Strien 1999). The significant increase in these hydrolytic enzyme is a defense reaction against the fungal pathogen.

The infection caused by a pathogen induce changes in the activity of plant cells resulting in rapid cell death around the site of invasion which may lead to induction  of resistance is known as hypersensitive response (HR) (Durrant and Dong 2004). The defense response of plant on sensing pathogen attack produces reactive oxygen intermediate (ROIs) resulting in cell death. Some susceptible plants become systemically resistant in response to localized infections, is known as acquired resistance (Do Vale et al., 2001). Plants with acquired resistance have high levels of PR proteins, salicylic acid and peroxidase. Induced resistance is also referred to acquired resistance (Ross 1961a).

Induced resistance can be divided into systemic acquired resistance (SAR) or induced systemic resistance (ISR). The systemic acquired resistance (SAR) is a mechanism of induced defense which provides the ability to plant to defend itself against various pathogens (fungi, bacteria, viruses etc.). SAR is often characterized by localized necrosis, accumulation of pathogenesis-related (PR) proteins and involves salicylic acid (SA) pathway, whereas, ISR is activated by plant growth-promoting rhizobacteria (PGPR) (Walters et al., 2013),  is not associated with necrosis and involves the jasmonate (JA) and ethylene (ET) pathways (Walters and Heil 2007; Henry et al., 2012). Hydrogen peroxide has also been proposed to have a signaling role in SAR (Alvarez et al., 1998).  SAR is induced by most pathogen that cause tissue necrosis either as a part of hypersensitive response (Ross 1961b) or as a symptom of disease (Kuc and Richmond 1977). In the SAR state, plants are primed (sensitized) to quickly and effectively activate defense responses the second time they encounter pathogen attack (Conrath 2006). SAR is a mechanism of induced defense that confers long – lasting protection against broad spectrum of microorganisms (Durrant and Dong 2004). Rhizobacterial mediated ISR is dependent on jasmonic acid (JA) and ethylene (ET) (Van Loon et al., 1998). Signals are exchanged between fungi and bacteria and plant roots. PGPR can suppress (antagonistic ability) soil borne pathogen as well as induce systemic resistance in plant against pathogen. The rhizosphere microflora (Pseudomonas, Bacillus, Trichoderma sp. etc.) play an important role in enhancing the defense.

The defense responses is activated as soon as the plant recognizes the pathogen encoded molecule called the elicitors (e.g. microbial protein, small peptide and oligosaccharide etc.) released during plant pathogen interaction. On sensing the elicitor (interaction between the pathogen elicitor and host receptor) the plant signal transduction pathway is activated which leads to:

  • Production of reactive oxygen species
  • Biosynthesis of phytoalexin
  • Cell wall rigidification as a result of callose, lignin and suberin deposition (Hammond-Kosack and Jones 1996; Yang et al., 1997)
  • Synthesis of hydrolytic enzyme (chitinases and β-1,3-glucanases)
  • Accumulation of PR protein (Yang et al., 1997; Van Loon and Strien 1999; Thakur and Sohal, 2013)

The term elicitor was used for molecules capable of inducing the production of phytoalexins, but it is now commonly used for compounds stimulating any type of plant defense (Ebel and Cosio 1994; Nurnberger 1999).

The application of compost increases biodiversity and activity of soil organisms in rhizosphere, which contribute to the increase of induced systemic resistance (ISR) (Kloepper et al., 2004; Oliveira et al., 2016). PGPR mediated ISR can be used as one of the strategy   in controlling wide range of pathogens.


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