Vol. 2, Issue 1, Part A (2020)

During their life time, plants always suffer from invasion of potential pathogenic microorganisms in the environment. To defend themselves against pathogen attack, plants have evolved a sophisticated immune system. Two types of innate immune responses, which are precisely regulated upon infection from different types of pathogens, have been recognized in plants so far. The first innate immune response is the pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI), which is activated by a number of PAMPs such as flagellin and chitin. The other one is the effector-triggered immunity (ETI), which is modulated by recognition of pathogen-derived avirulence effectors by plant R genes. Much progress has been made in understanding the mechanisms by which plants detect and defend themselves against microbial attack. These include the identification of components involved in the signal transduction pathways coupling pathogen recognition to the activation of defense responses and the demonstration that three endogenous plant signaling molecules, salicylic acid (SA), jasmonic acid (JA) and ethylene (ET), are involved in plant defense. In the past few years, it has become apparent that mitogen activated protein kinase (MAPK) cascades play some of the most essential roles in plant signal transduction pathways from cell division to cell death. The first reports of plant MAPKs in 1993 identified extracellular signal regulating kinase (MsERK1) in alfalfa and D5 kinase in pea. Sequential phosphorylations ensue as MAP3Ks activate downstream MAP kinase kinases (MAP2Ks; also called MKKs or MEKs) that in turn activate MAPKs. MAPKs then target various effector proteins in the cytoplasm or nucleus, which include other kinases, enzymes, or transcription factors. Li et al., 2014 identified a total of five SlMKK genes with one new member, SlMKK5 in tomato. qRT-PCR analyses revealed that expression of SlMKK2 and SlMKK4 was strongly induced by B. cinerea and by jasmonic acid and ethylene precursor 1-amino cyclopropane-1-carboxylic acid. Virus-induced gene silencing (VIGS)-based knockdown of individual SlMKKs and disease assays identified that SlMKK2 and SlMKK4 but not other three SlMKKs (SlMKK1, SlMKK3 and SlMKK5) are involved in resistance against B. cinerea