Stress and innate immune responses in plants
Control of Cell Death by Proteases and Serpins
We showed that PCD in plants can be controlled by sequestered proteases and protective serpins: In animals, protease inhibitors (serine protease inhibitors) of the serpin family are associated with many physiological processes, including blood coagulation and innate immunity. The serpins feature a reactive center polypeptide loop (RCL), which displays a protease target sequence as bait. In plants serpins belong to a well conserved gene family but were of unknown function (Fluhr et al., 2011). Cleavage of the cleavage results in an irreversible, covalent serpin-protease complex (Roberts et al., 2011). As shown below the X-ray structure of Arabdopsis AtSerpin1 shows unique plant specific attributes together with conservation of animal type serpin features (Lampl et al., 2010).
Reactive Oxygen Species in Plant Stress Responses
AtSerpin1 forms stable protein complexes with the RD21-death cysteine protease that is sequestered in the vacuole or in ER bodies. As shown below when plant cells embark on PCD, RD21-RFP (tagged red) is released from the ER bodies and mixes with AtSerpin1-GFP (tagged green) present in the cytoplasm (yellow region; Lampl et al 2013).
When sufficient serpin is present in the cytoplasm cell death is prevented. Importantly, these molecules are highly conserved throughout the plant kingdom and are hypothesized to work as a pair in the control of PCD as shown below. The scheme below shows the relationship between proteases, serpins and PCD. (A) Death-inducing proteases reside in the vacuole (yellow Pac Man). When pathogens are present, plant cells are induced to die (Alkan et al 2012). In a pro-death scenario, leakage from the vacuole leads to release of the protease and PCD. The extent of PCD will depend on the level of serpin (arrows). A high serpin to protease ratio results in a pro-life scenario. A low serpin to protease ratio results in a pro-death scenario (Lampl et al., 2010; 2013; Cohen et al., 2015). (B) Alternatively, serpins and proteases can be in the same compartment. An inactive pro-protease is activated during PCD. Again, the ratio of serpin to active protease will determine cell death. This may function in the case of metacaspases localized to the cytoplasm that are activated in PCD. These scenario and control of PCD in plant pathogen and environmental interactions are part of the labs ongoing research.
Successful defense against abiotic stress and pathogens requires perception of the environment followed by the execution of a preplanned defense posture. There are commonalties in the way plants react to abiotic stress and pathogens among them the production of reactive oxygen species (ROS). These are versatile molecules that play an indispensable role in mediating a diversity of cellular responses in plant cells, including necrotic reactions, programmed cell death (PCD), development, gravitropism, and hormonal signaling.
Pathogens, chemical elicitors and calcium (Kaplan et al., 2006; Alkan et al., 2012, 2015) rapidly induce ROS by activating a gene family of respiratory burst oxidase homologs, Rboh (Fluhr, 2009). Creation of mutant tomato plants with diminished Rboh activity demonstrated developmental changes and reduced wound-induced ROS formation and gene expression (Sagi et al., 2004, Sagi and Fluhr, 2006). ROS plays a role in leaf senescence as established by transcriptome and analysis of redox-sensitive probes (Mor et al 2014; Rosenwasser et al., 2011; Volodarsky et al., 2009). Our current efforts are to understand organellar intracellular redox interactions and further refine transcriptome tools to understand ROS signaling specificity. The recent discovery of a role for singlet oxygen used techniques for rapid spatial imaging of this ROS as shown in the figure below (Mor et al., 2014).
