| 12/26/03 (VJC) High-affinity salicylic acid-binding protein 2 is required for plant innate immunity and has salicylic acid-stimulated lipase activity Dhirendra Kumar and Daniel F. Klessig PNAS _ December 23, 2003 _ vol. 100 _ no. 26 _ 16101-16106 A high-affinity salicylic acid-binding protein (SABP2) was purified and the corresponding gene cloned from tobacco. The protein possessed a lipase activity stimulated by SA and could potentially act as an SA sensor to mediate defense signaling involving fatty acid signals. The work revealed a novel family of putative plant receptors with associated hydrolase activities for stress hormones. A Gene Expression Map of the Arabidopsis Root The authors used five separate GFP lines (expressing in stele, endodermis, endodermis plus cortex, epidermal atrichoblast cells, and lateral root cap) to dissociate 1 cm root tips into GFP-tagged single cells by protoplasting and a fluorescence-activated cell sorting. The mRNA was analyzed using microarrays and high-resolution spatial and temporal expression profiles (digital in situ) were generated for Arabidopsis roots. A gene expression map was drawn for 15 different zones of the root that correspond to cell types and tissues at progressive developmental stages. The map will serve to guide future studies of regulatory mechanisms underlying cell-type-specific gene expression programs during root development.
The authors showed that the key transcription factor of ethylene signaling, EIN3, was stabilized by ethylene, which required EIN2, EIN5 and EIN6 functions. A search for specific F-box protein genes for EIN3 regulation based on ethylene induction of gene expression pointed to EBF2 and its closest homologue EBF1. Plants over-expressing either F-box gene exhibited ethylene insensitivity and reduced EIN3 levels, while the ebf1 ebf2 double mutant showed constitutive ethylene phenotypes and increased EIN3 accumulation. The work established a specific connection between an unusual MAPK cascade to the SCF-mediated degradation of EIN3 in ethylene signaling. How do EIN2, EIN5 and EIN6 bridge between these biochemical activities will be the next challenging question. EIN3-Dependent Regulation of Plant Ethylene Hormone Signaling by Two Arabidopsis F Box Proteins: EBF1 and EBF2 Cell, Vol. 115, 679-689, December 12, 2003 The yeast two-hybrid screen was used to identify EIN3 and EIL1, key transcription factors of ethylene signaling, as potential interactors of the nuclear-localized F-box protein EBF1. Over-expression of EBF1 conferred ethylene insensitivity, whereas the ebf1 ebf2 double mutant displayed constitutive ethylene signaling. It was shown that ethylene increased EIN3 levels most likely by decreasing its degradation through the SCF complex linked through specific F-box proteins EBF1 and EBF2. The work also revealed a feedback loop through the elevation of EBF2 transcript by ethylene. It will be interesting to determine how EIN3 or/and EBF1/EBF2 are regulated by the upstream components, CTR1 and EIN2, that act negatively and positively in ethylene signal transduction.
The EMBO Journal Vol. 22 No. 21 pp. 5690-5699, 2003 Rui Lu, Isabelle Malcuit, Peter Moffett, Maria T. Ruiz, Jack Peart, Ai-Jiuan Wu, John P. Rathjen, Abdelhafid Bendahmane, Louise Day and David C. Baulcombe The work validated the suitability of using virus-induced gene silencing (VIGS) based on the potato virus X (PVX) vector for a high throughput screen of a normalized tobacco cDNA library (4992 clones) for genes involved in disease resistance. The suppression of most candidate genes (>90%) reduced hypersensitive responses (HR) associated with Pto-mediated resistance against Pseudomonas syringe but not disease resistance, suggesting a complex regulation of HR in defense signaling. The system is powerful and provides extra advantages to identify genes whose deficiency could cause lethality or was more difficult to detect due to redundancy.
The authors used an improved tobacco rattle virus (TRV) vector for a virus-induced gene silencing (VIGS) screen to reveal the importance of nine conserved signaling components in the Pto-mediated disease resistance in tomato. The study revealed the roles of multiple evolutionarily conserved signaling mechanisms in innate immune responses. The system established here will facilitate high-throughput screens for future discoveries of novel and conserved components in diverse signaling pathways in tomato and other plants using the new TRV vector. Ethylene Regulates Arabidopsis Development via the Modulation
of DELLA Protein Growth Repressor Function The study showed that ethylene delayed the GA-induced disappearance of a DELLA nuclear repressor protein RGA via a CTR1-dependent signaling pathway. The finding suggested an integrative role of RGA in ethylene, auxin and GA responses in controlling elongation and differential growth of plant cells. It will be interesting to determine how CTR1 activity modulates the degradation of transcription activator EIN3 in ethylene signaling and transcription repressor RGA in GA signaling.
The authors used a variety of molecular, chemical, immunological, tissue culture and genetic tools to demonstrate the central role of auxin efflux carriers in determining cell polarity from the first asymmetric division of the zygote to the formation of apical-basal axis during embryogenesis. The work provided the first conceptual framework for axis formation through active accumulation of a signaling molecule in plants. It will be interesting to determine the regulatory mechanisms underlying the dramatic pattern changes of auxin efflux carriers throughout plant development.
A set of custom high-density oligonucleotide arrays representing >94% of the Arabidopsis genome sequence (genome tiling arrays) and hybridizations with four RNA populations (seedling, root, flower and suspension culture cell) were used to generate a transcription map for the Arabidopsis genome. The incorporation of full-length cDNA sequences resulted in the dramatic improvement of the genome annotation. The expression of more than 60% of annotated genes was detected and 30% annotated genes showed significant antisense RNA expression, suggesting that double-strand RNA formation may be a general phenomenon in plant cells. Two unusually highly expressed "hot spots" were found within the genetically defined centromeres of chromosomes 2 and 3. The data provide a valuable resource for future functional genomic and proteomic studies. Many powerful applications of the genome tiling arrays, such as identifying non-coding RNAs, determining transcription factor binding sites, and detecting mutations, polymorphisms and modifications of DNA, will be possible when the technology becomes more affordable.
The authors identified a new microRNA, miR-JAW, using an activation
tagging genetic screen and several TCP genes as its putative targets
based on global transcript analysis. A cell surface receptor mediates extracellular Ca2+ sensing
in guard cells An Arabidopsis plasma membrane low-affinity/high-capacity Ca2+ -sensing receptor (CAS) was isolated using a functional screen in human embryo kidney (HEK293) cells based on [Ca2+]o -induced [Ca2+]i increases (CICI) and ratiometric imaging of the fluorescent Ca2+ sensitive dye Fura-2. CAS is a plant specific protein with a single transmembrane domain and N-terminal Ca2+ binding sites, and plays an essential role in guard cell Ca2+o signaling and in bolting. The work illustrated the power of using a heterologous expression system to identify novel genes with unique structures and functions in plant signaling. A Seven-Transmembrane RGS Protein That Modulates Plant Cell
Proliferation This study is focused on the characterization of a novel Arabidopsis seven transmembrane domain protein AtRGS1 with a C terminal RGS-box (Regulators of G-protein signaling). AtRGS1 protein interacted with AtGPA1 preferentially in the transition-state and acted as a negative regulator of AtGPA1 by exerting GAP activity. Mutant plant analyses suggested a role of AtRGS1 in cell proliferation and perhaps in GA and glucose signaling. It will be interesting to investigate the potential ligand of the putative receptor domain of AtRGS1. Evidence that SNF1-related kinase and hexokinase are involved
in separate sugar-signaling pathways modulating post-translational
redox activation of ADP-glucose pyrophosphorylase in potato tubers Potato tuber discs were used as a model system to identify candidate
components linking the post-translational redox modulation of ADP-glucose
pyrophosphorylase (AGPase) in response to sugar supply for starch
synthesis. It was discovered that glucose and sucrose could lead
to redox activation of AGPase through distinct mechanisms that relied
on hexokinase signaling and SNF1-related kinase activities, respectively.
The work added new evidence for the distinction between the glucose
and sucrose signaling pathways in plants. A MAPKK Kinase Gene Regulates Extra-Embryonic Cell Fate in
Arabidopsis An Arabidopsis MAP kinase kinase kinase (MAPKKK) gene YODA has been discovered to execute the first cell fate decision for basal lineage, which forms the extra-embryonic suspensor after the asymmetric division of the zygote. Expression of the constitutively active YODA could suppress proembryo formation and the loss-of-function mutants also exhibit abnormal embryogenesis and postembryonic development. Since YODA transcript is expressed ubiquitously and the same MAPKKK could often function in distinct MAPK pathways, it is possible that YODA might have other important functions throughout embryogenesis and plant development.
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