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1. ERECTA family signaling constrains CLAVATA3 and WUSCHEL to the center of the shoot apical meristem

   https://doi.org/10.1242/dev.189753  

   Zhang, Liang ; DeGennaro, Daniel ; Lin, Guangzhong ; Chai, Jijie ; Shpak, Elena D. ( March 2021 , Development)

   null (Ed.)

   ABSTRACT The shoot apical meristem (SAM) is a reservoir of stem cells that gives rise to all post-embryonic above-ground plant organs. The size of the SAM remains stable over time owing to a precise balance of stem cell replenishment versus cell incorporation into organ primordia. The WUSCHEL (WUS)/CLAVATA (CLV) negative feedback loop is central to SAM size regulation. Its correct function depends on accurate spatial expression of WUS and CLV3. A signaling pathway, consisting of ERECTA family (ERf) receptors and EPIDERMAL PATTERNING FACTOR LIKE (EPFL) ligands, restricts SAM width and promotes leaf initiation. Although ERf receptors are expressed throughout the SAM, EPFL ligands are expressed in its periphery. Our genetic analysis of Arabidopsis demonstrated that ERfs and CLV3 synergistically regulate the size of the SAM, and wus is epistatic to ERf genes. Furthermore, activation of ERf signaling with exogenous EPFLs resulted in a rapid decrease of CLV3 and WUS expression. ERf-EPFL signaling inhibits expression of WUS and CLV3 in the periphery of the SAM, confining them to the center. These findings establish the molecular mechanism for stem cell positioning along the radial axis. 

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2. Ethylene signaling in plants

   https://doi.org/10.1074/jbc.REV120.010854  

   Binder, BM ( January 2020 , Journal of biological chemistry)

   Ethylene is a gaseous phytohormone and the first of this hormone class to be discovered. It is the simplest olefin gas and is biosynthesized by plants to regulate plant development, growth, and stress responses via a well-studied signaling pathway. One of the earliest reported responses to ethylene is the triple response. This response is common in eudicot seedlings grown in the dark and is characterized by reduced growth of the root and hypocotyl, an exaggerated apical hook, and a thickening of the hypocotyl. This proved a useful assay for genetic screens and enabled the identification of many components of the ethylene signaling pathway. These components include a family of ethylene receptors in the membrane of the endoplasmic reticulum (ER); a protein kinase, called constitutive triple response1 (CTR1); an ER-localized transmembrane protein of unknown biochemical activity, called ethylene insensitive2 (EIN2); and transcription factors such as EIN3, EIN3-Like (EIL), and ethylene response factors (ERFs). These studies led to a linear model, according to which in the absence of ethylene, its cognate receptors signal to CTR1, which inhibits EIN2 and prevents downstream signaling. Ethylene acts as an inverse agonist by inhibiting its receptors, resulting in lower CTR1 activity, which releases EIN2 inhibition. EIN2 alters transcription and translation, leading to most ethylene responses. Although this canonical pathway is the predominant signaling cascade, alternative pathways also affect ethylene responses. This review summarizes our current understanding of ethylene signaling, including these alternative pathways, and discusses how ethylene signaling has been manipulated for agricultural and horticultural applications. 

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3. DYNAMIN-RELATED PROTEIN DRP1A functions with DRP2B in plant growth, flg22-immune responses, and endocytosis

   https://doi.org/10.1093/plphys/kiab024  

   Ekanayake, Gayani ; Smith, John M ; Jones, Kody B ; Stiers, Hayley M ; Robinson, Samuel J ; LaMontagne, Erica D ; Kostos, Paxton H ; Cornish, Peter V ; Bednarek, Sebastian Y ; Heese, Antje ( February 2021 , Plant Physiology)

   null (Ed.)

   Abstract Ligand-induced endocytosis of the immune receptor FLAGELLIN SENSING2 (FLS2) is critical for maintaining its proper abundance in the plasma membrane (PM) to initiate and subsequently down regulate cellular immune responses to bacterial flagellin or flg22-peptide. The molecular components governing PM abundance of FLS2, however, remain mostly unknown. Here, we identified Arabidopsis (Arabidopsis thaliana) DYNAMIN-RELATED PROTEIN1A (DRP1A), a member of a plant-specific family of large dynamin GTPases, as a critical contributor to ligand-induced endocytosis of FLS2 and its physiological roles in flg22-signaling and immunity against Pseudomonas syringae pv. tomato DC3000 bacteria in leaves. Notably, drp1a single mutants displayed similar flg22-defects as those previously reported for mutants in another dynamin-related protein, DRP2B, that was previously shown to colocalize with DRP1A. Our study also uncovered synergistic roles of DRP1A and DRP2B in plant growth and development as drp1a drp2b double mutants exhibited severely stunted roots and cotyledons, as well as defective cell shape, cytokinesis, and seedling lethality. Furthermore, drp1a drp2b double mutants hyperaccumulated FLS2 in the PM prior to flg22-treatment and exhibited a block in ligand-induced endocytosis of FLS2, indicating combinatorial roles for DRP1A and DRP1B in governing PM abundance of FLS2. However, the increased steady-state PM accumulation of FLS2 in drp1a drp2b double mutants did not result in increased flg22 responses. We propose that DRP1A and DRP2B are important for the regulation of PM-associated levels of FLS2 necessary to attain signaling competency to initiate distinct flg22 responses, potentially through modulating the lipid environment in defined PM domains. 

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4. The NIN-LIKE PROTEIN 7 transcription factor modulates auxin pathways to regulate root cap development in Arabidopsis

   https://doi.org/10.1093/jxb/erad058  

   Kumar, Narender ; Caldwell, ,. Chloe ; Iyer-Pascuzzi, Anjali S. ; Gifford, ed., Miriam ( February 2023 , Journal of Experimental Botany)

   The root cap is a small tissue located at the tip of the root with critical functions for root growth. Present in nearly all vascular plants, the root cap protects the root meristem, influences soil penetration, and perceives and transmits environmental signals that are critical for root branching patterns. To perform these functions, the root cap must remain relatively stable in size and must integrate endogenous developmental pathways with environmental signals, yet the mechanism is not clear. We previously showed that low pH conditions altered root cap development, and these changes are mediated by the NIN LIKE PROTEIN 7 (NLP7) transcription factor, a master regulator of nitrate signaling. Here we show that in Arabidopsis NLP7 integrates nitrate signaling with auxin pathways to regulate root cap development. We found that low nitrate conditions promote aberrant release of root cap cells. Nitrate deficiency impacts auxin pathways in the last layer of the root cap, and this is mediated in part by NLP7. Mutations in NLP7 abolish the auxin minimum in the last layer of the root cap and alter root cap expression of the auxin carriers PIN-LIKES 3 (PILS3) and PIN-FORMED 7 (PIN7) as well as transcription factors that regulate PIN expression. Together, our data reveal NLP7 as a link between endogenous auxin pathways and nitrate signaling in the root cap.

    

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5. Generation of a fertile ask1 mutant uncovers a comprehensive set of SCF‐mediated intracellular functions

   https://doi.org/10.1111/tpj.14939  

   Yapa, Madhura M. ; Yu, Peifeng ; Liao, Fanglei ; Moore, Abigail G. ; Hua, Zhihua ( August 2020 , The Plant Journal)

   Many eukaryotic intracellular processes employ protein ubiquitylation by ubiquitin E3 ligases for functional regulation or protein quality control. In plants, the multi‐subunit Skp1–Cullin1–F‐box (SCF) complexes compose the largest group of E3 ligases whose specificity is determined by a diverse array of F‐box proteins. Although both sequence divergence and polymorphism ofF‐boxgenes well support a broad spectrum of SCF functions, experimental evidence is scarce due to the low number of identified SCF substrates. Taking advantage of the bridge role of Skp1 between F‐box and Cullin1 in the complex, we systematically analyzed the functional influence of a well‐characterizedArabidopsis Skp1‐Like1(ASK1)Dsinsertion allele,ask1, in different Arabidopsis accessions. Through 10 generations of backcrossing with Columbia‐0 (Col‐0), we partially rescued the fertility of this otherwise sterileask1allele in Landsbergerecta, thus providing experimental evidence showing the polymorphic roles of SCF complexes. Thisask1mutant produces twisted rosette leaves, a reduced number of petals, fewer viable pollen grains, and larger embryos and seeds compared to Col‐0. RNA‐Seq‐based transcriptome analysis ofask1uncovered a large spectrum of SCF functions, which is greater than a 10‐fold increase compared with previous studies. We also identified its hyposensitive responses to auxin and abscisic acid treatments and enhanced far‐red light/phyA‐mediated photomorphogenesis. Such diverse roles are consistent with the 20–30% reduction of ubiquitylation events inask1estimated by immunoblotting analysis in this work. Collectively, we conclude that ASK1 is a predominant Skp1 protein in Arabidopsis and that the fertileask1mutant allowed us to uncover a comprehensive set of SCF functions.

    

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