In Arabidopsis thaliana, MAIGO 2 (MAG2) is involved in protein transport between the endoplasmic reticulum (ER) and the Golgi apparatus via its association with the ER-localized t-SNARE components SYP81/AtUfe1 and SEC20. To characterize the molecular machinery of MAG2-mediated protein transport, we explored MAG2-interacting proteins using transgenic A. thaliana plants expressing TAP-tagged MAG2. We identified three proteins, which were designated as MAG2-INTERACTING PROTEIN 1-3 [MIP1 (At2g32900), MIP2 (At5g24350) and MIP3 (At2g42700)]. Both MIP1 and MAG2 localized to the ER membrane. All of the mag2, mip1, mip2 and mip3 mutants exhibited a defect in storage protein maturation, and developed abnormal storage protein body (MAG body) structures in the ER of seed cells. These observations suggest that MIPs are closely associated with MAG2 and function in protein transport between the ER and Golgi apparatus. MIP1 and MIP2 contain a Zeste-White 10 (ZW10) domain and a Sec39 domain, respectively, but have low sequence identities (21% and 23%) with respective human orthologs. These results suggest that the plant MAG2-MIP1-MIP2 complex is a counterpart of the triple-subunit tethering complexes in yeast (Tip20p-Dsl1p-Sec39p) and humans (RINT1-ZW10-NAG). Surprisingly, the plant complex also contained a fourth member (MIP3) with a Sec1 domain. There have been no previous reports showing that a Sec1-containing protein is a subunit of ER-localized tethering complexes. Our results suggest that MAG2 and the three MIP proteins form a unique complex on the ER that is responsible for efficient transport of seed storage proteins.
To perceive pathogens, plants employ pattern recognition receptor (PRR) complexes, which then transmit these signals via the receptor‐like cytoplasmic kinase BIK1 to induce defense responses. How BIK1 activity and stability are controlled is still not completely understood. Here, we show that the Hippo/STE20 homolog MAP4K4 regulates BIK1‐mediated immune responses. MAP4K4 associates and phosphorylates BIK1 at Ser233, Ser236, and Thr242 to ensure BIK1 stability and activity. Furthermore, MAP4K4 phosphorylates PP2C38 at Ser77 to enable flg22‐induced BIK1 activation. Our results uncover that a Hippo/STE20 homolog, MAP4K4, maintains the homeostasis of the central immune component BIK1.
Symbiotic cnidarians such as corals and anemones form highly productive and biodiverse coral reef ecosystems in nutrient-poor ocean environments, a phenomenon known as Darwin’s paradox. Resolving this paradox requires elucidating the molecular bases of efficient nutrient distribution and recycling in the cnidarian-dinoflagellate symbiosis. Using the sea anemone Aiptasia, we show that during symbiosis, the increased availability of glucose and the presence of the algae jointly induce the coordinated up-regulation and relocalization of glucose and ammonium transporters. These molecular responses are critical to support symbiont functioning and organism-wide nitrogen assimilation through glutamine synthetase/glutamate synthase–mediated amino acid biosynthesis. Our results reveal crucial aspects of the molecular mechanisms underlying nitrogen conservation and recycling in these organisms that allow them to thrive in the nitrogen-poor ocean environments.
One-sentence summary: Arabidopsis CCC1 functions in strengthening plant structural 35 and chemical barriers and mediates plant resistance against Pseudomonas syringae. 36 37 Author contributions: B.H., Y.J., and H.H. conceived and planned the experiments; 38 B.H. and Y.J. carried out the genetic and biochemical experiments and wrote the 39 manuscript with the contribution of all authors; G.C. helped with computational RNA-40 Seq data analysis under the supervision of M.A.; J.M. assisted in LC-MS/MS data 41 acquisition and analysis under supervision of S.A.; M.R.G.R. carried out plasma 42 membrane depolarization. G.M. and J.S. performed cell wall composition analysis; Y.J. 43 and H.H. supervised the experiments. 44 45 www.plantphysiol.org on December 11, 2019 -Published by Downloaded from Abstract 46 Plasma membrane (PM) depolarization functions as an initial step in plant defense 47 signaling pathways. However, only a few ion channels/transporters have been 48 characterized in the context of plant immunity. Here, we show that the Arabidopsis 49 (Arabidopsis thaliana) Na + :K + :2Cl -(NKCC) cotransporter CCC1 has a dual function in 50 plant immunity. CCC1 functions independently of PM depolarization and negatively 51 regulates pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI). 52 However, CCC1 positively regulates plant basal and effector-triggered resistance to 53 Pseudomonas syringae pv. tomato (Pst) DC3000. In line with the compromised 54 immunity to Pst DC3000, ccc1 mutants show reduced expression of genes encoding 55 enzymes involved in the biosynthesis of antimicrobial peptides, camalexin, and 4-OH-56 ICN, as well as Pathogenesis-Related (PR) proteins. Moreover, genes involved in cell 57 wall and cuticle biosynthesis are constitutively downregulated in ccc1 mutants, and the 58 cell walls of these mutants exhibit major changes in monosaccharide composition. The 59 role of CCC1 ion transporter activity in the regulation of plant immunity is corroborated 60 by experiments using the specific NKCC inhibitor bumetanide. These results reveal a 61 function for ion transporters in immunity-related cell wall fortification and antimicrobial 62 biosynthesis. 63 64 Introduction 65 Under natural conditions, plants are invariably challenged by harmful pathogens. 66However, plants possess several lines of defense mechanisms to protect themselves 67 from disease (Hammond-Kosack and Jones, 1996; Hentschel, 2013). First, plants have 68 www.plantphysiol.org on December 11, 2019 -Published by Downloaded from
Symbiotic cnidarians such as corals and anemones form highly productive and biodiverse coral-reef ecosystems in nutrient-poor ocean environments, a phenomenon known as Darwin's Paradox. Resolving this paradox requires elucidating the molecular bases of efficient nutrient distribution and recycling in the cnidarian-dinoflagellate symbiosis. Using the sea anemone Aiptasia, we show that during symbiosis, the increased availability of glucose and the presence of the algae jointly induce the coordinated upregulation and re-localization of glucose and ammonium transporters. These molecular responses are critical to support symbiont functioning and organism-wide nitrogen assimilation through GS/GOGAT-mediated amino-acid biosynthesis. Our results reveal crucial aspects of the molecular mechanisms underlying nitrogen conservation and recycling in these organisms that allow them to thrive in the nitrogen-poor ocean environments.
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