Reactive Oxygen Species and Mitogen Activated Protein Kinase cascade crosstalk in Plants: An overview
Author: Dhammaprakash P. Wankhede
ICAR-National Bureau of Plant Genetic Resources, Pusa campus, New Delhi
Reactive oxygen species (ROS) are partially reduced or activated derivatives of oxygen and are highly reactive by-products of aerobic metabolism. ROS are generated from various biochemical reactions can cause oxidative damage to cells. Plants possess a sophisticated ROS network, comprising anti-oxidative enzymes, antioxidants, and ROS producing enzymes, which allow them to keep ROS levels under tight control. Studies have shown that plants have developed efficient strategies for targeted production of ROS. ROS play a role in programmed cell death (PCD), development, and stress response. MAPK cascades are key players in ROS signalling. Several studies have shown ROS mediated induction of MAPK cascades as well as regulation ROS production per se by MAPK cascade (Pitzschke and Hirt, 2009).
One strategy that plants use to overcome oxidative stress is the production of scavenger enzymes such as catalases which decompose H2O 2. For instance, A. thaliana CAT1 is regulated by ABA and MAPK specific inhibitor PD98059 hindered ABA-mediated CAT1 expression (Xing et al., 2008). In addition, the A. thaliana mkk1 and mpk6 mutants showed altered responses to ABA and desiccation stress. These findings along with absence of ABA induced activated AtMPK6 in mkk1 mutant line, clearly suggest role of MKK1 and MPK6 in regulating H2O2 metabolism through CAT1 (Xing et al., 2008). Additionally, CAT1 and CAT2 expression seems to be regulated by MEKK1 and MPK4 (Pitzschke and Hirt, 2009) which are involved in plant defence and SA accumulation. The MEKK1-MPK4 cascade is also an essential component in ROS metabolism (Nakagami et al., 2006). Altered antioxidant enzymes activities such as superoxide dismutase, catalase and glutathione peroxidase have been reported in two cultivars of Brassica juncea in response to arsenite stress (Gupta et al., 2009). Arsenite stress also led to ROS generation and activation of OsMPK3 and OsMPK4 in rice (Rao et al., 2011). These findings suggest MAPK mediated oxidative stress response and that MAP kinase cascades are not only induced by ROS but also influence catalase activity and thereby regulate ROS levels. It is further important to note that ROS homeostasis is a convergence point showing stress status of plant as oxidative stress is a common response shared by both biotic and abiotic stress (Suarez-Rodriguez et al., 2010).
Similar to apoptosis in animal cells, a number of stress stimuli trigger the cell death pathway. The PCD, a hypersensitive response of pathogen attack in plants is characterized with generation of ROS, activation of specific proteases, and DNA fragmentation. ROS can also induce lignin and callose deposition and thereby reinforce cell walls around infection sites (Pontier et al., 1998). The hypersensitive response thus helps to restrict pathogen growth and spread. Role of AtMKK3 has been shown in biotic stress signalling involving H2O2 (Doczi et al., 2007). The growth of pathogen P. syringae was shown to increase in mkk3 knockout Arabidopsis plants whereas it is decreased in MKK3-overexpressing plants. MEKK1 has been implicated in mediating flagellin (flg22) signalling (Asai et al., 2002). Arabidopsis mekk1 homozygous knockout plants show a severe dwarf phenotype (Nakagami et al., 2006). MEKK1 is essential for H2O2 mediated activation of MPK4. These observations point to a role of MEKK1 and its downstream target MPK4 in the maintenance of ROS homeostasis.
Increased expression of several MPK/MKK genes have been observed in microarray analyses of plants after short and prolonged exposure to various biotic and abiotic stresses (Kreps et al., 2002; De Vos et al., 2005; Gust et al., 2007). These microarrays gave cumulative evidence that transcriptional control of MAPK signalling components was involved in stress signalling. Some MPK/MKK genes respond to different types of stresses e.g. MKK9 is induced both by wounding and by bacterial elicitors (Navarro et al., 2004; Walley et al., 2007). It is however not clear whether the same or different MAPK gene promoter elements are involved in these stress responses.
Sudden activation of MPK3 upon a series of challenging conditions (Nakagami et al., 2005; Djamei et al., 2007) indicates that in initial period already existing MPK3 protein is being used for signal transduction. The quick transcriptional up-regulation of MPK3 post stress exposure (Walley et al., 2007) might be indicative of the requirement of a continuous supply of MPK3 enzyme which would then contribute amplify the stress signal. However, it is not known till date why enhanced levels of some MPK/MKK transcripts persist long even after MPK/MKK activity has declined.
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About Author / Additional Info:
Scientist, Division of Genomic Resources, ICAR-National Bureau of Plant Genetics Resources, New Delhi , India