A Critical Review on Defense Mechanisms of Plants against Bacterial Pathogens: From Morphological to Molecular Levels

Better understanding of plant defense mechanism is crucial for improving crop health and yield. Plant defense against bacterial pathogens results from a complex combination of structural plant characteristics and induced biochemical reactions. In addition to the constitutive defense, plants may perceive directly or indirectly the presence of a bacterium and subsequently induce plant defense responses. These inducible biochemical reactions tend to create protective physiological conditions to limit bacterial growth and invasion in the host tissues. The inducible plant defense starts when a particular bacterial molecule or its structural feature is recognized by trans-membrane protein recognition receptors (PRRs) on plant cell surface. The recognition is based on conserved features of molecules of bacterial origin, namely pathogen associated molecular patterns (PAMPs). This induces PAMP-triggered immunity (PTI) and the expression of defense genes, what prevents pathogenesis. However, some pathogens may release effector molecules and surpass PTI what leads to effector-triggered susceptibility (ETS). Subsequently, plants possess resistance (R) proteins usually containing nucleotide-binding (NB) and leucine-rich repeat (LRR) domains which trigger signaling cascade by recognizing specific effectors. This leads to the activation of downstream genes in order to create a robust and fast defense response preventing the spread of bacteria. Generally, these actions against invading bacterial pathogen are controlled directly or indirectly by genetic materials (gene) of the host plants. Therefore, the objective of this review is to discuss and summarize how the receptors are thought to activate defenses, how bacterial pathogens surpass this basal defense system and how plants have evolved a second defense layer, with an emphasis on the future research priorities.