You will be presented with the abstract, introduction, and discussion section from a biomedical research article. Your task is to create a summary that a layperson can understand, capturing the essence of the research article. Keep essential scientific terms, but ensure that the language remains clear and the concepts are explained in an uncomplicated manner.

Title: Arabidopsis heterotrimeric G proteins regulate immunity by directly coupling to the FLS2 receptor}
Article:
Abstract:
The Arabidopsis immune receptor FLS2 perceives bacterial flagellin epitope flg22 to activate defenses through the central cytoplasmic kinase BIK1. The heterotrimeric G proteins composed of the non-canonical Gα protein XLG2, the Gβ protein AGB1, and the Gγ proteins AGG1 and AGG2 are required for FLS2-mediated immune responses through an unknown mechanism. Here we show that in the pre-activation state, XLG2 directly interacts with FLS2 and BIK1, and it functions together with AGB1 and AGG1/2 to attenuate proteasome-mediated degradation of BIK1, allowing optimum immune activation. Following the activation by flg22, XLG2 dissociates from AGB1 and is phosphorylated by BIK1 in the N terminus. The phosphorylated XLG2 enhances the production of reactive oxygen species( ROS) likely by modulating the NADPH oxidase RbohD. The study demonstrates that the G proteins are directly coupled to the FLS2 receptor complex and regulate immune signaling through both pre-activation and post-activation mechanisms.
Introduction:
As an intensely studied Pattern Recognition Receptor( PRR) in plants, FLS2 serves as an excellent model understanding plant innate immune signaling and receptor kinases in general( Macho and Zipfel, 2014). It forms a dynamic complex with the co-receptor BAK1 and the receptor-like cytoplasmic kinase BIK1 to perceive a conserved bacterial flagellar epitope, flg22, to activate a variety of defense responses( Chinchilla et al., 2007; Heese et al., 2007; Lu et al., 2010; Zhang et al., 2010; Sun et al., 2013). Stability of FLS2 and BIK1 is subject to regulation by ubiquitin-proteasome system and a calcium-dependent protein kinase( Lu et al., 2011; Monaghan et al., 2014). We and others previously showed that BIK1 directly phosphorylates the NADPH oxidase RbohD to prime flg22-induced reactive oxygen species( ROS; Kadota et al., 2014; Li et al., 2014). Heterotrimeric G proteins are central for signaling in animals( McCudden et al., 2005; Oldham et al., 2008), which contain hundreds of G Protein-Coupled Receptors( GPCRs). In the pre-activation state, the GDP-bound Gα interacts with the Gβγ dimer to form a heterotrimer. Upon activation by GPCR, Gα exchanges GDP for GTP, resulting in the activation of the heterotrimer. The activated Gα and Gβγ dissociate from each other to regulate downstream effectors. Plants contain canonical Gα( encoded by GPA1 in Arabidopsis), Gβ( encoded by AGB1 in Arabidopsis), Gγ proteins( encoded by AGG1, AGG2), and a non-canonical Gγ( encoded by AGG3 in Arabidopsis)( Urano and Jones, 2013). Plants additionally encode extra-large G proteins( XLGs, encoded by XLG1, XLG2, and XLG3 in Arabidopsis) that carry a variable N-terminal domain and a C-terminal Gα domain( Lee and Assman, 1999; Ding et al., 2008). Recent advances indicate that the Arabidopsis XLGs are functional Gα proteins and interact with Gβγ dimers to form heterotrimers( Zhu et al., 2009; Maruta et al., 2015; Chakravorty et al., 2015). Heterotrimeric G proteins play important roles in a variety of biological processes in plants, including cell division( Ullah et al., 2001, 2003; Chen et al., 2003), meristem maintenance( Bommert et al., 2013), root morphogenesis( Ding et al., 2008), seed development and germination( Chen et al., 2006; Pandey et al., 2006), nitrogen assimilation( Sun et al., 2014), and response to ABA( Wang et al., 2001), low temperature( Ma et al., 2015), and blue light( Warpeha et al., 2006). Accumulating evidence indicate that heterotrimeric G proteins also play an important role in plant disease resistance against diverse pathogens( Llorente et al., 2005; Trusov et al., 2006; Zhu et al, 2009; Ishikawa, 2009; Cheng et al., 2015). Recent reports indicate that XLG2, AGB1, and AGG1/2 mediate immune responses downstream of PRRs( Ishikawa, 2009; Zhu et al, 2009; Liu et al., 2013; Lorek et al., 2013; Torres et al., 2013; Maruta et al., 2015). XLG2 was first shown to play an important role in basal resistance to P. syringae( Zhu et al., 2009). A recent report showed that XLG2, but not XLG1, is required for resistance to P. syringae and flg22-induced ROS production( Maruta et al., 2015). AGB1 and AGG1/2, but not AGG3, are required for resistance to P. syringae and microbial pattern-induced ROS production( Liu et al., 2013; Lorek et al., 2013; Torres et al., 2013). Furthermore, epistatic analyses indicated that AGB1 acts in the same pathway as RbohD( Torres et al., 2013). However, it is still debated whether plants possess 7 transmembrane GPCRs( Taddese et al., 2014; Urano and Jones, 2014). One recent report suggests that GPA1, AGG1/2 can interact with BAK1 and the chitin-binding receptor kinase CERK1, but not FLS2( Aranda-Sicilia et al., 2015). However, GPA1 does not appear to play a role in flg22-induced ROS and disease resistance to P. syringae( Liu et al., 2013; Torres et al., 2013). How XLG2 and AGB1 regulate PRR-mediated immunity remains elusive. In this study, we report XLG2, AGB1, and AGG1/2 modulates flg22-triggered immunity by directly coupling to the FLS2-BIK1 receptor complex. Prior to activation by flg22, the G proteins attenuate the proteasome-dependent degradation of BIK1, ensuring optimum signaling competence. After flg22 stimulation, XLG2 dissociates from AGB1, indicating a ligand-induced dissociation of Gα from Gβγ. In addition, we provide evidence that activation by flg22 additionally leads to XLG2 phospohrylation by BIK1, and this phosphorylation positively regulates RbohD-dependent ROS production. Together the study illustrates two distinct mechanisms underlying the G protein-mediated regulation of the FLS2 signaling.
Discussion:
Heterotrimeric G proteins play important role in signal transduction in plants and animals. Paradoxically, plants do not possess functional GPCRs. The maize Gα protein CT2 interacts with the receptor kinase CLV1 to control shoot meristem development( Bommert et al., 2013). GPA1 and AGG1/2 have been shown to interact with BAK1 and CERK1 in yeast two-hybrid and Bi-Fluorescence Complementation assays, although the biological significance of this remains unknown( Aranda-Sicilia et al., 2015). We show that XLG2 directly interacts with FLS2 and BIK1 to regulate FLS2-mediated immunity. Thus, these studies collectively support that plant receptor kinases fulfil the roles of GPCRs by directly coupling to heterotrimeric G proteins. Importantly, we show that the G proteins coupled to the FLS2 receptor complex are subject to multiple regulations during FLS2 signaling. The interaction between XLG2/3 and AGB1, which leads to the formation of XLG2/3-AGB1-AGG1/2 heterotrimers( Maruta et al., 2015; Chakravorty et al., 2015), is dynamically regulated by flg22. The flg22-induced dissociation of XLG2 from AGB1 likely reflects an increase of GTP-bound form of Gα which dissociates from the Gβγ dimer upon receptor activation. Secondly, the interactions of the G proteins with FLS2 and BIK1 are also dynamically regulated by flg22. The flg22-induced dissociation of the G proteins from BIK1 coincides with the flg22-induced BIK1 phosphorylation, suggesting that the flg22-induced BIK1 phosphorylation triggers the dissociation of the G proteins. Indeed, de-phosphorylation of BIK1 strongly enhanced its interactions with both XLG2 and AGB1 in vitro. The decoupling of the G proteins from the receptor maybe necessary for the regulation of down-stream effectors. BIK1 is subject to proteasome-dependent degradation( Monaghan et al., 2014). Our genetic analyses indicated that XLG2, AGB1 and AGG1/2 are required for BIK1 stability in the pre-activation state. Loss of XLG2 and AGB1 leads to accelerated degradation of BIK1, and this is blocked by the addition of proteasome inhibitors, indicating that the G proteins attenuating the proteasome-dependent degradation of BIK1. FLS2 is also subject to proteasome-dependent degradation( Lu et al., 2011). However, FLS2 accumulation is not affected by the agb1 mutation, indicating that the G proteins do not generally regulate proteasome-mediated protein degradation. These observations raise an interesting question as to whether the G proteins specifically impede the ubiquitination of BIK1 or loading of BIK1 to proteasome. Importantly, introduction of a transgenic copy of BIK1 largely restored flg22-triggered immune responses and disease resistance in xlg2 and agb1 mutants, demonstrating that a major function of the heterotrimeric G protein is to maintain the signaling competence of the FLS2-BIK1 complex in the pre-activation state. In addition to the ligand-induced dissociation of XLG2 from AGB1, flg22 induces the phosphorylation of XLG2 in the N terminus. This phosphorylation is likely caused by the BIK1 family kinases, as two of the major phosphor-sites induced by flg22 are also phosphorylated by BIK1 in vitro. Non-phosphorylatable XLG2 mutants are compromised in flg22-induced ROS production and disease resistance to Pst, indicating that the phosphorylation plays a positive role. However, mutations in the phospho-sites did not affect XLG2-BIK1 interaction nor BIK1 stability, suggesting that the phosphorylated XLG2 regulates components other than BIK1. Indeed, XLG2 constitutively interacts with RbohD, suggesting that the phosphorylated XLG2 further enhances ROS production by modulating RbohD. RbohD is known to be subject to multiple regulations including phosphorylation by CPK5 and BIK1( Dubiella et al., 2013; Kadota et al., 2014; Li et al., 2014) and calcium binding to EF-hand( Ogasawara et al., 2008). The results presented here show that RbohD is additionally regulated by a phosphorylated XLG2, although the underlying mechanism remains unknown. The Arabidopsis heterotrimeric G proteins reported here are analogous to the animal counterpart in that, upon activation by receptors, dissociate from each other and activate downstream effectors. Unlike the animal heterotrimeric G proteins, however, the Arabidopsis heterotrimeric G proteins additionally regulate signaling competence of the FLS2-BIK1 complex prior to receptor activation. Taken together, our results highlight remarkable similarities and striking differences in heterotrimeric G protein-coupled receptor signaling in animals and plants as a result of independent evolution.
Living cells need to be able to detect changes in their environment and respond accordingly . This ability involves signals from outside of the cell triggering changes to the activity inside the cell . Heterotrimeric G proteins are important for this kind of signaling in a wide range of organisms . In animals and fungi , these proteins directly work with a specific class of receptor proteins called G protein-coupled receptors ( or GPCRs for short ) . Plants also have heterotrimeric G proteins , but it remains unclear whether they similiarly work with GPCRs . Plants detect invading microbes by using receptors that are completely different from GPCRs . For example , a receptor called FLS2 from the model plant Arabidopsis senses a telltale protein produced by bacteria , and then passes the signal to another protein called BIK1 to activate the plant’s defenses . Heterotrimeric G proteins are required for this process , but the underlying mechanisms remain unknown . Liang , Ding et al . now show that heterotrimeric G proteins regulate FLS2-controlled defenses by directly interacting with FLS2 and BIK1 . Heterotrimeric G proteins also enhance defenses in at least two different ways . Firstly , in the absence of an infection , heterotrimeric G proteins stabilize the BIK1 protein to ensure that it is ready to respond . Secondly , if FLS2 does detect the telltale bacterial protein , BIK1 marks one of the heterotrimeric G proteins with a phosphate group . This then allows the G protein to boost the activity of another plant enzyme that is vital for defense signaling . In the future , it will be important to work out how activation of FLS2 leads to the activation of heterotrimeric G proteins . Furthermore , heterotrimeric G proteins are likely to regulate additional plant proteins when defenses are activated , and further studies are needed to identify these proteins .