Protein kinases

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    The rice Raf-like MAPKKK OsILA1 confers broad-spectrum resistance to bacterial blight by suppressing the OsMAPKK4–OsMAPK6 cascade
    Jie Chen, Lihan Wang, Zeyu Yang, Hongbo Liu, Chuanliang Chu, Zhenzhen Zhang, Qinglu Zhang, Xianghua Li, Jinghua Xiao, Shiping Wang and Meng Yuan
    J Integr Plant Biol 2021, 63 (10): 1815-1832.  
    DOI: 10.1111/jipb.13150
    Abstract (Browse 300)  |   Save
    Mitogen-activated protein kinase kinase kinase (MAPKKK) are the first components of MAPK cascades, which play pivotal roles in signaling during plant development and physiological processes. The genome of rice encodes 75 MAPKKKs, of which 43 are Raf-like MAPKKKs. The functions and action modes of most of the Raf-like MAPKKKs, whether they function as bona fide MAPKKKs and which are their downstream MAPKKs, are largely unknown. Here, we identified the osmapkkk43 mutant, which conferred broad-spectrum resistance to Xanthomonas oryzae pv. oryzae (Xoo), the destructive bacterial pathogen of rice. Oryza sativa (Os)MAPKKK43 encoding a Raf-like MAPKKK was previously known as Increased Leaf Angle 1 (OsILA1). Genetic analysis indicated that OsILA1 functioned as a negative regulator and acted upstream of the OsMAPKK4–OsMAPK6 cascade in rice–Xoo interactions. Unlike classical MAPKKKs, OsILA1 mainly phosphorylated the threonine 34 site at the N-terminal domain of OsMAPKK4, which possibly influenced the stability of OsMAPKK4. The N-terminal domain of OsILA1 is required for its homodimer formation and its full phosphorylation capacity. Taken together, our findings reveal that OsILA1 acts as a negative regulator of the OsMAPKK4–OsMAPK6 cascade and is involved in rice–Xoo interactions.
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    Reciprocal regulation between the negative regulator PP2CG1 phosphatase and the positive regulator OST1 kinase confers cold response in Arabidopsis
    Jian Lv, Jingyan Liu, Yuhang Ming, Yiting Shi, Chunpeng Song, Zhizhong Gong, Shuhua Yang and Yanglin Ding
    J Integr Plant Biol 2021, 63 (8): 1568-1587.  
    DOI: 10.1111/jipb.13100
    Abstract (Browse 588)  |   Save
    Protein phosphorylation and dephosphorylation have been reported to play important roles in plant cold responses. In addition, phospho-regulatory feedback is a conserved mechanism for biological processes and stress responses in animals and plants. However, it is less well known that a regulatory feedback loop is formed by the protein kinase and the protein phosphatase in plant responses to cold stress. Here, we report that OPEN STOMATA 1 (OST1) and PROTEIN PHOSPHATASE 2C G GROUP 1 (PP2CG1) reciprocally regulate the activity during the cold stress response. The interaction of PP2CG1 and OST1 is inhibited by cold stress, which results in the release of OST1 at the cytoplasm and nucleus from suppression by PP2CG1. Interestingly, cold-activated OST1 phosphorylates PP2CG1 to suppress its phosphatase activity, thereby amplifying cold signaling in plants. Mutations of PP2CG1 and its homolog PP2CG2 enhance freezing tolerance, whereas overexpression of PP2CG1 decreases freezing tolerance. Moreover, PP2CG1 negatively regulates protein levels of C-REPEAT BINDING FACTORs (CBFs) under cold stress. Our results uncover a phosphor/dephosphor-regulatory feedback loop mediated by PP2CG1 phosphatase and OST1 protein kinase in plant cold responses.
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    Ethylene-induced stomatal closure is mediated via MKK1/3–MPK3/6 cascade to EIN2 and EIN3
    Teng‐Yue Zhang, Zhong‐Qi Li, Yu‐Dong Zhao, Wen‐Jie Shen, Meng‐Shu Chen, Hai‐Quan Gao, Xiao‐Min Ge, Hui‐Qin Wang, Xue Li and Jun‐Min He
    J Integr Plant Biol 2021, 63 (7): 1324-1340.  
    DOI: 10.1111/jipb.13083
    Abstract (Browse 346)  |   Save
    Mitogen-activated protein kinases (MPKs) play essential roles in guard cell signaling, but whether MPK cascades participate in guard cell ethylene signaling and interact with hydrogen peroxide (H2O2), nitric oxide (NO), and ethylene-signaling components remain unclear. Here, we report that ethylene activated MPK3 and MPK6 in the leaves of wild-type Arabidopsis thaliana as well as ethylene insensitive2 (ein2), ein3, nitrate reductase1 (nia1), and nia2 mutants, but this effect was impaired in ethylene response1 (etr1), nicotinamide adenine dinucleotide phosphate oxidase AtrbohF, mpk kinase1 (mkk1), and mkk3 mutants. By contrast, the constitutive triple response1 (ctr1) mutant had constitutively active MPK3 and MPK6. Yeast two-hybrid, bimolecular fluorescence complementation, and pull-down assays indicated that MPK3 and MPK6 physically interacted with MKK1, MKK3, and the C-terminal region of EIN2 (EIN2 CEND). mkk1, mkk3, mpk3, and mpk6 mutants had typical levels of ethylene-induced H2O2 generation but impaired ethylene-induced EIN2 CEND cleavage and nuclear translocation, EIN3 protein accumulation, NO production in guard cells, and stomatal closure. These results show that the MKK1/3–MPK3/6 cascade mediates ethylene-induced stomatal closure by functioning downstream of ETR1, CTR1, and H2O2 to interact with EIN2, thereby promoting EIN3 accumulation and EIN3-dependent NO production in guard cells.
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    TMK4 receptor kinase negatively modulates ABA signaling by phosphorylating ABI2 and enhancing its activity
    Lan Li, Bin Li, Sirui Zhu, Long Wang, Limei Song, Jia Chen, Zhenhua Ming, Xuanming Liu, Xiushan Li and Feng Yu
    J Integr Plant Biol 2021, 63 (6): 1161-1178.  
    doi: 10.1111/jipb.13096
    Abstract (Browse 377)  |   Save
    In plants, clade A type 2C protein phosphatases (PP2CAs) have emerged as major players in abscisic acid (ABA)-regulated stress responses by inhibiting protein kinase activity. However, how different internal and external environmental signals modulate the activity of PP2CAs are not well known. The transmembrane kinase (TMK) protein 4 (TMK4), one member of a previously identified receptor kinase subfamily on the plasma membrane that plays vital roles in plant cell growth, directly interacts with PP2CAs member (ABA-Insensitive 2, ABI2). tmk4 mutant is hypersensitive to ABA in both ABA-inhibited seed germination and primary root growth, indicating that TMK4 is a negative regulator in ABA signaling pathway. Further analyses indicate that TMK4 phosphorylates ABI2 at three conserved Ser residues, thus enhancing the activity of ABI2. The phosphorylation-mimic ABI2S139DS140DS266D can complement but non-phosphorylated form ABI2S139AS140AS266A cannot complement ABA hypersensitive phenotype of the loss-of-function mutant abi1-2abi2-2. This study provides a previously unidentified mechanism for positively regulating ABI2 by a plasma membrane protein kinase.
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    The novel peptide NbPPI1 identified from Nicotiana benthamiana triggers immune responses and enhances resistance against Phytophthora pathogens
    Qujiang Wen, Manli Sun, Xianglan Kong, Yang Yang, Qiang Zhang, Guiyan Huang, Wenqin Lu, Wanyue Li, Yuling Meng and Weixing Shan
    J Integr Plant Biol 2021, 63 (5): 961-976.  
    DOI: 10.1111/jipb.13033
    Abstract (Browse 317)  |   Save
    In plants, recognition of small secreted peptides, such as damage/danger‐associated molecular patterns (DAMPs), regulates diverse processes, including stress and immune responses. Here, we identified an SGPS (Ser‐Gly‐Pro‐Ser) motif‐containing peptide, Nicotiana tabacum NtPROPPI, and its two homologs in Nicotiana benthamiana, NbPROPPI1 and NbPROPPI2. Phytophthora parasitica infection and salicylic acid (SA) treatment induced NbPROPPI1/2 expression. Moreover, SignalP predicted that the 89‐amino acid NtPROPPI includes a 24‐amino acid N‐terminal signal peptide and NbPROPPI1/2‐GFP fusion proteins were mainly localized to the periplasm. Transient expression of NbPROPPI1/2 inhibited P. parasitica colonization, and NbPROPPI1/2 knockdown rendered plants more susceptible to P. parasitica. An eight‐amino‐acid segment in the NbPROPPI1 C‐terminus was essential for its immune function and a synthetic 20‐residue peptide, NbPPI1, derived from the C‐terminus of NbPROPPI1 provoked significant immune responses in N. benthamiana. These responses led to enhanced accumulation of reactive oxygen species, activation of mitogen‐activated protein kinases, and up‐regulation of the defense genes Flg22‐induced receptor‐like kinase (FRK) and WRKY DNA‐binding protein 33 (WRKY33). The NbPPI1‐induced defense responses require Brassinosteroid insensitive 1‐associated receptor kinase 1 (BAK1). These results suggest that NbPPI1 functions as a DAMP in N. benthamiana; this novel DAMP provides a potentially useful target for improving plant resistance to Pytophthora pathogens.
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    The Arabidopsis phosphatase PP2C49 negatively regulates salt tolerance through inhibition of AtHKT1;1
    Moli Chu , Pengwang Chen, Sufang Meng, Peng Xu and Wenzhi Lan
    J Integr Plant Biol 2021, 63 (3): 528-542.  
    doi: 10.1111/jipb.13008
    Abstract (Browse 522)  |   Save
    Type 2C protein phosphatases (PP2Cs) are the largest protein phosphatase family. PP2Cs dephosphorylate substrates for signaling in Arabidopsis, but the functions of most PP2Cs remain unknown. Here, we characterized PP2C49 (AT3G62260, a Group G PP2C), which regulates Na+ distribution under salt stress and is localized to the cytoplasm and nucleus. PP2C49 was highly expressed in root vascular tissues and its disruption enhanced plant tolerance to salt stress. Compared with wild type, the pp2c49 mutant contained more Na+ in roots but less Na+ in shoots and xylem sap, suggesting that PP2C49 regulates shoot Na+ extrusion. Reciprocal grafting revealed a root‐based mechanism underlying the salt tolerance of pp2c49. Systemic Na+ distribution largely depends on AtHKT1;1 and loss of function of AtHKT1;1 in the pp2c49 background overrode the salt tolerance of pp2c49, resulting in salt sensitivity. Furthermore, compared with plants overexpressing PP2C49 in the wild‐type background, plants overexpressing PP2C49 in the athtk1;1 mutant background were sensitive to salt, like the athtk1;1 mutants. Moreover, protein–protein interaction and two‐voltage clamping assays demonstrated that PP2C49 physically interacts with AtHKT1;1 and inhibits the Na+ permeability of AtHKT1;1. This study reveals that PP2C49 negatively regulates AtHKT1;1 activity and thus determines systemic Na+ allocation during salt stress.
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    RING finger protein RGLG1 and RGLG2 negatively modulate MAPKKK18 mediated drought stress tolerance in Arabidopsis
    Jiayi Yu, Lu Kang, Yuanyuan Li, Changai Wu, Chengchao Zheng, Pei Liu and Jinguang Huang
    J Integr Plant Biol 2021, 63 (3): 484-493.  
    doi: 10.1111/jipb.13019
    Abstract (Browse 540)  |   Save
    Mitogen activated protein kinase kinase kinase 18 (MAPKKK18) mediated signaling cascade plays important roles in Arabidopsis drought stress tolerance. However, the post‐translational modulation patterns of MAPKKK18 are not characterized. In this study, we found that the protein level of MAPKKK18 was tightly controlled by the 26S proteasome. Ubiquitin ligases RGLG1 and RGLG2 ubiquitinated MAPKKK18 at lysine residue K32 and K154, and promoted its degradation. Deletion of RGLG1 and RGLG2 stabilized MAPKKK18 and further enhanced the drought stress tolerance of MAPKKK18‐overexpression plants. Our data demonstrate that RGLG1 and RGLG2 negatively regulate MAPKKK18‐mediated drought stress tolerance in Arabidopsis.
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    Abscisic acid signaling negatively regulates nitrate uptake via phosphorylation of NRT1.1 by SnRK2s in Arabidopsis
    Hang Su, Tian Wang, Chuanfeng Ju, Jinping Deng, Tianqi Zhang, Mengjiao Li, Hui Tian and Cun Wang
    J Integr Plant Biol 2021, 63 (3): 597-610.  
    doi: 10.1111/jipb.13057
    Abstract (Browse 550)  |   Save
    Nitrogen (N) is a limiting nutrient for plant growth and productivity. The phytohormone abscisic acid (ABA) has been suggested to play a vital role in nitrate uptake in fluctuating N environments. However, the molecular mechanisms underlying the involvement of ABA in N deficiency responses are largely unknown. In this study, we demonstrated that ABA signaling components, particularly the three subclass III SUCROSE NON‐FERMENTING1 (SNF1)‐RELATED PROTEIN KINASE 2S (SnRK2) proteins, function in root foraging and uptake of nitrate under N deficiency in Arabidopsis thaliana. The snrk2.2snrk2.3snrk2.6 triple mutant grew a longer primary root and had a higher rate of nitrate influx and accumulation compared with wild‐type plants under nitrate deficiency. Strikingly, SnRK2.2/2.3/2.6 proteins interacted with and phosphorylated the nitrate transceptor NITRATE TRANSPORTER1.1 (NRT1.1) in vitro and in vivo. The phosphorylation of NRT1.1 by SnRK2s resulted in a significant decrease of nitrate uptake and impairment of root growth. Moreover, we identified NRT1.1Ser585 as a previously unknown functional site: the phosphomimetic NRT1.1S585D was impaired in both low‐ and high‐affinity transport activities. Taken together, our findings provide new insight into how plants fine‐tune growth via ABA signaling under N deficiency.
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    Arabidopsis U‐box E3 ubiquitin ligase PUB11 negatively regulates drought tolerance by degrading the receptor‐like protein kinases LRR1 and KIN7
    Xuexue Chen, Tingting Wang, Amin Ur Rehman, Yu Wang, Junsheng Qi, Zhen Li, Chunpeng Song, Baoshan Wang, Shuhua Yang and Zhizhong Gong
    J Integr Plant Biol 2021, 63 (3): 494-509.  
    doi: 10.1111/jipb.13058
    Abstract (Browse 718)  |   Save
    Both plant receptor‐like protein kinases (RLKs) and ubiquitin‐mediated proteolysis play crucial roles in plant responses to drought stress. However, the mechanism by which E3 ubiquitin ligases modulate RLKs is poorly understood. In this study, we showed that Arabidopsis PLANT U‐BOX PROTEIN 11 (PUB11), an E3 ubiquitin ligase, negatively regulates abscisic acid (ABA)‐mediated drought responses. PUB11 interacts with and ubiquitinates two receptor‐like protein kinases, LEUCINE RICH REPEAT PROTEIN 1 (LRR1) and KINASE 7 (KIN7), and mediates their degradation during plant responses to drought stress in vitro and in vivo. pub11 mutants were more tolerant, whereas lrr1 and kin7 mutants were more sensitive, to drought stress than the wild type. Genetic analyses show that the pub11 lrr1 kin7 triple mutant exhibited similar drought sensitivity as the lrr1 kin7 double mutant, placing PUB11 upstream of the two RLKs. Abscisic acid and drought treatment promoted the accumulation of PUB11, which likely accelerates LRR1 and KIN7 degradation. Together, our results reveal that PUB11 negatively regulates plant responses to drought stress by destabilizing the LRR1 and KIN7 RLKs.
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    Arabidopsis E3 ligase KEG associates with and ubiquitinates MKK4 and MKK5 to regulate plant immunity
    Chenyang Gao, Pengwei Sun, Wei Wang and Dingzhong Tang
    J Integr Plant Biol 2021, 63 (2): 327-339.  
    doi: 10.1111/jipb.13007
    Abstract (Browse 706)  |   Save
    Mitogen‐activated protein kinase (MAPK) cascades are highly conserved signaling modules that regulate plant immune responses. The Arabidopsis thaliana Raf‐like MAPK kinase kinase ENHANCED DISEASE RESISTANCE1 (EDR1) is a key negative regulator of plant immunity that affects the protein levels of MKK4 and MKK5, two important MAPK cascade members, but the underlying mechanism is poorly understood. Here, genome‐wide phosphorylation analysis demonstrated that the E3 ligase KEEP ON GOING (KEG) is phosphorylated in the edr1 mutant but not the wild type, suggesting that EDR1 negatively affects KEG phosphorylation. The identified phosphorylation sites in KEG appear to be important for its accumulation. The keg‐4 mutant, a previously identified edr1 suppressor, enhances susceptibility to the powdery mildew pathogen Golovinomyces cichoracearum. In addition, MKK4 and MKK5 protein levels are reduced in the keg‐4 mutant. Furthermore, we demonstrate that MKK4 and MKK5 associate with full‐length KEG, but not with truncated KEG‐RK or KEG‐RKA, and that KEG ubiquitinates and mediates the degradation of MKK4 and MKK5. Taken together, these results indicate that MKK4 and MKK5 protein levels are regulated by KEG via ubiquitination, uncovering a mechanism by which plants fine‐tune immune responses by regulating the homeostasis of key MAPK cascade members via ubiquitination and degradation.
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    Comprehensive identification of lysine 2‐hydroxyisobutyrylated proteins in Ustilaginoidea virens reveals the involvement of lysine 2‐hydroxyisobutyrylation in fungal virulence
    Xiaoyang Chen, Xiabing Li, Pingping Li, Xiaolin Chen, Hao Liu, Junbin Huang, Chaoxi Luo, Tom Hsiang and Lu Zheng
    J Integr Plant Biol 2021, 63 (2): 409-425.  
    doi: 10.1111/jipb.13066
    Abstract (Browse 458)  |   Save
    Lysine 2‐hydroxyisobutyrylation (Khib) is a newly identified post‐translational modification (PTM) that plays important roles in transcription and cell proliferation in eukaryotes. However, its function remains unknown in phytopathogenic fungi. Here, we performed a comprehensive assessment of Khib in the rice false smut fungus Ustilaginoidea virens, using Tandem Mass Tag (TMT)‐based quantitative proteomics approach. A total of 3 426 Khib sites were identified in 977 proteins, suggesting that Khib is a common and complex PTM in U. virens. Our data demonstrated that the 2‐hydroxyisobutyrylated proteins are involved in diverse biological processes. Network analysis of the modified proteins revealed a highly interconnected protein network that included many well‐studied virulence factors. We confirmed that the Zn‐binding reduced potassium dependency3‐type histone deacetylase (UvRpd3) is a major enzyme that removes 2‐hydroxyisobutyrylation and acetylation in U. virens. Notably, mutations of Khib sites in the mitogen‐activated protein kinase (MAPK) UvSlt2 significantly reduced fungal virulence and decreased the enzymatic activity of UvSlt2. Molecular dynamics simulations demonstrated that 2‐hydroxyisobutyrylation in UvSlt2 increased the hydrophobic solvent‐accessible surface area and thereby affected binding between the UvSlt2 enzyme and its substrates. Our findings thus establish Khib as a major post‐translational modification in U. virens and point to an important role for Khib in the virulence of this phytopathogenic fungus.
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    Protein kinases in plant responses to drought, salt, and cold stress
    Xuexue Chen, Yanglin Ding, Yongqing Yang, Chunpeng Song, Baoshan Wang, Shuhua Yang, Yan Guo and Zhizhong Gong
    J Integr Plant Biol 2021, 63 (1): 53-78.  
    doi: 10.1111/jipb.13061
    Abstract (Browse 1183)  |   Save
    Protein kinases are major players in various signal transduction pathways. Understanding the molecular mechanisms behind plant responses to biotic and abiotic stresses has become critical for developing and breeding climate‐resilient crops. In this review, we summarize recent progress on understanding plant drought, salt, and cold stress responses, with a focus on signal perception and transduction by different protein kinases, especially sucrose nonfermenting1 (SNF1)‐related protein kinases (SnRKs), mitogen‐activated protein kinase (MAPK) cascades, calcium‐dependent protein kinases (CDPKs/CPKs), and receptor‐like kinases (RLKs). We also discuss future challenges in these research fields.
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    Cell polarity: Regulators and mechanisms in plants
    Kezhen Yang, Lu Wang, Jie Le and Juan Dong
    J Integr Plant Biol 2020, 62 (1): 132-147.  
    doi: 10.1111/jipb.12904
    Abstract (Browse 278)  |   Save

    Cell polarity plays an important role in a wide range of biological processes in plant growth and development. Cell polarity is manifested as the asymmetric distribution of molecules, for example, proteins and lipids, at the plasma membrane and/or inside of a cell. Here, we summarize a few polarized proteins that have been characterized in plants and we review recent advances towards understanding the molecular mechanism for them to polarize at the plasma membrane. Multiple mechanisms, including membrane trafficking, cytoskeletal activities, and protein phosphorylation, and so forth define the polarized plasma membrane domains. Recent discoveries suggest that the polar positioning of the proteo‐lipid membrane domain may instruct the formation of polarity complexes in plants. In this review, we highlight the factors and regulators for their functions in establishing the membrane asymmetries in plant development. Furthermore, we discuss a few outstanding questions to be addressed to better understand the mechanisms by which cell polarity is regulated in plants.

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    Engineering plants to secrete affinity-tagged pathogen elicitors for deciphering immune receptor complex or inducing enhanced immunity
    Shuang Miao, Jiuer Liu, Jianhang Guo and Jian‐Feng Li
    J Integr Plant Biol 2020, 62 (6): 761-776.  
    doi: 10.1111/jipb.12859
    Abstract (Browse 405)  |   Save

    Plant cells mount plenty of pattern‐recognition receptors (PRRs) to detect the microbe‐associated molecular patterns (MAMPs) from potential microbial pathogens. MAMPs are overrepresented by proteinaneous patterns, such as the flg22 peptide from bacterial flagellin. Identification of PRR receptor complex components by forward or reverse genetics can be time/labor‐consuming, and be confounded by functional redundancies. Here, we present a strategy for identifying PRR complex components by engineering plants to inducibly secrete affinity‐tagged proteinaneous MAMPs to the apoplast. The PRR protein complexes bound to self‐secreted MAMPs are enriched through affinity purification and dissected by mass spectrometry. As a proof of principle, we could capture the flg22 receptor FLS2 and co‐receptor BAK1 using Arabidopsis plants secreting FLAG‐tagged flg22 under estradiol induction. Moreover, we identified receptor‐like kinases LIK1 and PEPR1/PEPR2 as potential components in the FLS2 receptor complex, which were further validated by protein–protein interaction assays and the reverse genetics approach. Our study showcases a simple way to biochemically identify endogenous PRR complex components without overexpressing the PRR or using chemical cross‐linkers, and suggests a possible crosstalk between different immune receptors in plants. A modest dose of estradiol can also be applied to inducing enhanced immunity in engineered plants to both bacterial and fungal pathogens.

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    The juxtamembrane domains of Arabidopsis CERK1, BAK1, and FLS2 play a conserved role in chitin‐induced signaling
    Qi Zhou, Jun Liu, Jingyi Wang, Sufen Chen, Lijuan Chen, Jinfa Wang, Hong-Bin Wang and Bing Liu
    J Integr Plant Biol 2020, 62 (5): 556-562.  
    doi: 10.1111/jipb.12847
    Abstract (Browse 397)  |   Save

    Arabidopsis thaliana CERK1 is an essential receptor‐like kinase in the chitin signal transduction pathway. The juxtamembrane (JM) domain of CERK1 regulates the kinase activity of this receptor. Here we demonstrate that the JM domains of LysM‐RLKs, CERK1, and OsCERK1 play a functionally conserved role in the activation of chitin signaling in Arabidopsis. The C‐termini of the JM domains of both CERK1 and OsCERK1 are indispensable for their function. Moreover, after replacing the JM domain of CERK1 with that of the nonhomologous RLK, BAK1 (CJBa) or FLS2 (CJFl), the chimeric CERK1 receptors maintained their ability to activate chitin signaling in Arabidopsis. Interestingly, the heterologous expression of CJBa and CJFl did not induce cell death in Nicotiana benthamiana leaves. These results suggest that the JM domains of CERK1, BAK1, and FLS2 play a conserved role in chitin signaling via a mechanism not related to sequence homology.

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    CLE25 peptide regulates phloem initiation in Arabidopsis through a CLERK-CLV2 receptor complex
    Shi-Chao Ren, Xiu-Fen Song, Wen-Qiang Chen, Ran Lu, William J. Lucas and Chun-Ming Liu
    J Integr Plant Biol 2019, 61 (10): 1043-1061.  
    doi: 10.1111/jipb.12846
    Abstract (Browse 750)  |   Save
    The phloem, located within the vascular system, is critical for delivery of nutrients and signaling molecules throughout the plant body. Although the morphological process and several factors regulating phloem differentiation have been reported, the molecular mechanism underlying its initiation remains largely unknown. Here, we report that the small peptide‐coding gene, CLAVATA 3 (CLV3)/EMBEYO SURROUNDING REGION 25 (CLE25), the expression of which begins in provascular initial cells of 64‐cell‐staged embryos, and continues in sieve element‐procambium stem cells and phloem lineage cells, during post‐embryonic root development, facilitates phloem initiation in Arabidopsis. Knockout of CLE25 led to delayed protophloem formation, and in situ expression of an antagonistic CLE25G6T peptide compromised the fate‐determining periclinal division of the sieve element precursor cell and the continuity of the phloem in roots. In stems of CLE25G6T plants the phloem formation was also compromised, and procambial cells were over‐accumulated. Genetic and biochemical analyses indicated that a complex, consisting of the CLE‐RESISTANT RECEPTOR KINASE (CLERK) leucine‐rich repeat (LRR) receptor kinase and the CLV2 LRR receptor‐like protein, is involved in perceiving the CLE25 peptide. Similar to CLE25, CLERK was also expressed during early embryogenesis. Taken together, our findings suggest that CLE25 regulates phloem initiation in Arabidopsis through a CLERK‐CLV2 receptor complex.
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    BAK1‐mediated phosphorylation of canonical G protein alpha during flagellin signaling in Arabidopsis
    Jiao Xue, Ben-Qiang Gong, Xinran Yao, Xiangjuan Huang and Jian-Feng Li
    J Integr Plant Biol 2020, 62 (5): 690-701.  
    DOI: 10.1111/jipb.12824
    Abstract (Browse 289)  |   Save

    Heterotrimeric G proteins consisting of Gα, Gβ and Gγ are conserved signaling hubs in eukaryotes. Without analogs to canonical animal G protein‐coupled receptors, plant cells are thought to use RGS1 and a yet unknown mechanism to regulate the activity of Gα. Meanwhile, the exact role of canonical Gα in plant innate immunity remains controversial. Here, we report multiple immune deficiencies in the null allele of Arabidopsis Gα (GPA1) in response to bacterial flg22 elicitor, clarifying a positive regulatory role of GPA1 in flg22 signaling. We also detect overall increased phosphorylation of GPA1 but reduced phosphorylation at Thr19 upon flg22 elicitation. Interestingly, flg22 could not induce phosphorylation of GPA1T19A and GPA1T19D, suggesting that the dynamic Thr19 phosphorylation is required for GPA1 to respond to flg22. Moreover, flg22‐induced GPA1 phosphorylation is largely abolished in the absence of BAK1 in vivo, and BAK1 could phosphorylate GPA1 but not GPA1T19A in vitro at the phosphorylation sites identified in vivo, suggesting BAK1 is likely the kinase for GPA1 phosphorylation in response to flg22. Furthermore, the T19A mutation could promote flg22‐induced association, rather than dissociation, between GPA1 and RGS1. Taken together, our findings shed new insights into the function and regulation of GPA1 in Arabidopsis defense signaling.

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    Specifying the role of BAK1-interacting receptor-like kinase 3 in brassinosteroid signaling
    Ruth Großeholz, Anna Feldman-Salit, Friederike Wanke, Sarina Schulze, Nina Glöckner, Birgit Kemmerling, Klaus Harter and Ursula Kummer
    J Integr Plant Biol 2020, 62 (4): 456-469.  
    DOI: 10.1111/jipb.12803
    Abstract (Browse 253)  |   Save

    Brassinosteroids (BR) are involved in the control of several developmental processes ranging from root elongation to senescence and adaptation to environmental cues. Thus, BR perception and signaling have to be precisely regulated. One regulator is BRI1‐associated kinase 1 (BAK1)‐interacting receptor‐like kinase 3 (BIR3). In the absence of BR, BIR3 forms complexes with BR insensitive 1 (BRI1) and BAK1. However, the biophysical and energetic requirements for complex formation in the absence of the ligand have yet to be determined. Using computational modeling, we simulated the potential complexes between the cytoplasmic domains of BAK1, BRI1 and BIR3. Our calculations and experimental data confirm the interaction of BIR3 with BAK1 and BRI1, with the BAK1 BIR3 interaction clearly favored. Furthermore, we demonstrate that BIR3 and BRI1 share the same interaction site with BAK1. This suggests a competition between BIR3 and BRI1 for binding to BAK1, which results in preferential binding of BIR3 to BAK1 in the absence of the ligand thereby preventing the active participation of BAK1 in BR signaling. Our model also suggests that BAK1 and BRI1 can interact even while BAK1 is in complex with BIR3 at an additional binding site of BAK1 that does not allow active BR signaling.

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    Control of grain size by G protein signaling in rice
    Ran Xu, Na Li and Yunhai Li
    J Integr Plant Biol 2019, 61 (5): 533-540.  
    doi: 10.1111/jipb.12769
    Abstract (Browse 876)  |   Save
    Heterotrimeric G proteins are involved in multiple cellular processes in eukaryotes by sensing and transducing various signals. G protein signaling in plants is quite different from that in animals, and the mechanisms of plant G protein signaling are still largely unknown. Several recent studies have provided new insights into the mechanisms of G protein signaling in rice grain size and yield control. In this review, we summarize recent advances on the function of G proteins in rice grain size control and discuss the potential genetic and molecular mechanisms of plant G protein signaling.
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    SUMO E3 Ligase AtMMS21 Regulates Drought Tolerance in Arabidopsis thaliana
    Shengchun Zhang, Yanli Qi, Ming Liu and Chengwei Yang
    J Integr Plant Biol 2013, 55 (1): 83-95.  
    DOI: 10.1111/jipb.12024
    Abstract (Browse 1225)  |   Save

    Post-translational modifications of proteins by small ubiquitin-like modifiers (SUMOs) play crucial roles in plant growth and development, and in stress responses. The MMS21 is a newly-identified Arabidopsis thaliana L. SUMO E3 ligase gene aside from the SIZ1, and its function requires further elucidation. Here, we show that MMS21 deficient plants display improved drought tolerance, and constitutive expression of MMS21 reduces drought tolerance. The expression of MMS21 was reduced by abscisic acid (ABA), polyethylene glycol (PEG) or drought stress. Under drought conditions, mms21 mutants showed the highest survival rate and the slowest water loss, and accumulated a higher level of free proline compared to wild-type (WT) and MMS21 over-expression plants. Stomatal aperture, seed germination and cotyledon greening analysis indicated that mms21 was hypersensitive to ABA. Molecular genetic analysis revealed that MMS21 deficiency led to elevated expression of a series of ABA-mediated stress-responsive genes, including COR15A, RD22, and P5CS1 The ABA and drought-induced stress-responsive genes, including RAB18, RD29A and RD29B, were inhibited by constitutive expression of MMS21. Moreover, ABA-induced accumulation of SUMO-protein conjugates was blocked in the mms21 mutant. We thus conclude that MMS21 plays a role in the drought stress response, likely through regulation of gene expression in an ABA-dependent pathway.

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