Special Issue: Phytohormones   

June 2011, Volume 53 Issue 6, Pages 410ĘC506.

Cover Caption: Phytohormones
About the cover: WOX1 is a WUS family transcription factor. Using a gain-offunction mutant wox1-D, Zhang et al (pp 493ĘC506) showed that WOX1 plays an important role in meristem development of Arabidopsis, possibly through regulating the activity of SAdenosylmethionine decarboxylase (SAMDC). Using transgenic plants carrying a GUS reporter gene, these authors showed that WOX1 is actively expressed in cells that are active in divisions.


Plant Hormones: Metabolism, Signaling and Crosstalk  
Author: Li-Jia Qu and Yunde Zhao
Journal of Integrative Plant Biology 2011 53(6): 410-411
Published Online: June 9, 2011
DOI: 10.1111/j.1744-7909.2011.01057.x
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          Invited Expert Reviews
Salicylic Acid and Its Function in Plant Immunity  
Author: Chuanfu An and Zhonglin Mou
Journal of Integrative Plant Biology 2011 53(6): 412-428
Published Online: May 3, 2011
DOI: 10.1111/j.1744-7909.2011.01043.x

The small phenolic compound salicylic acid (SA) plays an important regulatory role in multiple physiological processes including plant immune response. Significant progress has been made during the past two decades in understanding the SA-mediated defense signaling network. Characterization of a number of genes functioning in SA biosynthesis, conjugation, accumulation, signaling, and crosstalk with other hormones such as jasmonic acid, ethylene, abscisic acid, auxin, gibberellic acid, cytokinin, brassinosteroid, and peptide hormones has sketched the finely tuned immune response network. Full understanding of the mechanism of plant immunity will need to take advantage of fast developing genomics tools and bioinformatics techniques. However, elucidating genetic components involved in these pathways by conventional genetics, biochemistry, and molecular biology approaches will continue to be a major task of the community. High-throughput method for SA quantification holds the potential for isolating additional mutants related to SA-mediated defense signaling.

An C, Mou Z (2011) Salicylic acid and its function in plant immunity. J. Integr. Plant Biol. 53(6), 412–428.

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Auxin-Oxylipin Crosstalk: Relationship of Antagonists  
Author: Maik Hoffmann, Mathias Hentrich and Stephan Pollmann
Journal of Integrative Plant Biology 2011 53(6): 429-445
Published Online: June 9, 2011
DOI: 10.1111/j.1744-7909.2011.01053.x

Phytohormones regulate a wide array of developmental processes throughout the life cycle of plants. Herein, the various plant hormones may interact additively, synergistically, or antagonistically. By their cooperation they create a delicate regulatory network whose net output largely depends on the action of specific phytohormone combinations rather than on the independent activities of separate hormones. While most classical studies of plant hormonal control have focused mainly on the action of single hormones or on the synergistic interaction of hormones in regulating various developmental processes, recent work is beginning to shed light on the crosstalk of nominally antagonistic plant hormones, such as gibberellins and auxins with oxylipins or abscisic acid. In this review, we summarize our current understanding of how two of the first sight antagonistic plant hormones, i.e. auxins and oxylipins, interact in controlling plant responses and development.

Hoffmann M, Hentrich M, Pollmann S (2011) Auxin-oxylipin crosstalk: Relationship of antagonists. J. Integr. Plant Biol. 53(6), 429–445.

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Regulation of Meristem Size by Cytokinin Signaling  
Author: Anna Skylar and Xuelin Wu
Journal of Integrative Plant Biology 2011 53(6): 446-454
Published Online: May 9, 2011
DOI: 10.1111/j.1744-7909.2011.01045.x

The plant meristems possess unique features that involve maintaining the stem cell populations while providing cells for continued development. Although both the primary shoot apical meristem (SAM) and the root apical meristem (RAM) are specified during embryogenesis, post-embryonic tissue proliferation is required for their full establishment and maintenance throughout a plants' life. The phytohormone cytokinin (CK) interacts with other systemic signals and is a key regulator of meristem size and functions. The SAM and the RAM respond to CK stimulations in different manners: CK promotes tissue proliferation in the SAM through pathways dominated by homeobox transcription factors, including the class I KNOX genes, STIP, and WUS; and curiously, it favors proliferation at low levels and differentiation at a slightly higher concentration in the RAM instead. Here we review the current understanding of the molecular mechanisms underlying CK actions in regulating meristematic tissue proliferation.

Skylar A, Wu X (2011) Regulation of meristem size by cytokinin signaling. J. Integr. Plant Biol. 53(6), 446–454.

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Recent Advances in the Regulation of Brassinosteroid Signaling and Biosynthesis Pathways  
Author: Huaxun Ye, Lei Li and Yanhai Yin
Journal of Integrative Plant Biology 2011 53(6): 455-468
Published Online: May 9, 2011
DOI: 10.1111/j.1744-7909.2011.01046.x

Brassinosteroids (BRs) play important roles in plant growth, development and responses to environmental cues. BRs signal through plasma membrane receptor BRI1 and co-receptor BAK1, and several positive (BSK1, BSU1, PP2A) and negative (BKI1, BIN2 and 14–3-3) regulators to control the activities of BES1 and BZR1 family transcription factors, which regulate the expression of hundreds to thousands of genes for various BR responses. Recent studies identified novel signaling components in the BR pathways and started to establish the detailed mechanisms on the regulation of BR signaling. In addition, the molecular mechanism and transcriptional network through which BES1 and BZR1 control gene expression and various BR responses are beginning to be revealed. BES1 recruits histone demethylases ELF6 and REF6 as well as a transcription elongation factor IWS1 to regulate target gene expression. Identification of BES1 and BZR1 target genes established a transcriptional network for BR response and crosstalk with other signaling pathways. Recent studies also revealed regulatory mechanisms of BRs in many developmental processes and regulation of BR biosynthesis. Here we provide an overview and discuss some of the most recent progress in the regulation of BR signaling and biosynthesis pathways.

Ye H, Li L, Yin Y (2011) Recent advances in the regulation of brassinosteroid signaling and biosynthesis pathways. J. Integr. Plant Biol. 53(6), 455–468.

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Abscisic Acid Receptors: Past, Present and Future  
Author: Jianjun Guo, Xiaohan Yang, David J. Weston and Jin-Gui Chen
Journal of Integrative Plant Biology 2011 53(6): 469-479
Published Online: May 9, 2011
DOI: 10.1111/j.1744-7909.2011.01044.x

Abscisic acid (ABA) is the key plant stress hormone. Consistent with the earlier studies in support of the presence of both membrane- and cytoplasm-localized ABA receptors, recent studies have identified multiple ABA receptors located in various subcellular locations. These include a chloroplast envelope-localized receptor (the H subunit of Chloroplast Mg2+-chelatase/ABA Receptor), two plasma membrane-localized receptors (G-protein Coupled Receptor 2 and GPCR-type G proteins), and one cytosol/nucleus-localized Pyrabactin Resistant (PYR)/PYR-Like (PYL)/Regulatory Component of ABA Receptor 1 (RCAR). Although the downstream molecular events for most of the identified ABA receptors are currently unknown, one of them, PYR/PYL/RCAR was found to directly bind and regulate the activity of a long-known central regulator of ABA signaling, the A-group protein phosphatase 2C (PP2C). Together with the Sucrose Non-fermentation Kinase Subfamily 2 (SnRK2s) protein kinases, a central signaling complex (ABA-PYR-PP2Cs-SnRK2s) that is responsible for ABA signal perception and transduction is supported by abundant genetic, physiological, biochemical and structural evidence. The identification of multiple ABA receptors has advanced our understanding of ABA signal perception and transduction while adding an extra layer of complexity.

Guo J, Yang X, Weston DJ, Chen JG (2011) Abscisic acid receptors: Past, present and future. J. Integr. Plant Biol. 53(6), 469–479.

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          Research Articles
A Gain-of-Function Mutation in IAA7/AXR2 Confers Late Flowering under Short-day Light in Arabidopsis  
Author: Yan-Xia Mai, Long Wang and Hong-Quan Yang
Journal of Integrative Plant Biology 2011 53(6): 480-492
Published Online: May 12, 2011
DOI: 10.1111/j.1744-7909.2011.01050.x

Floral initiation is a major step in the life cycle of plants, which is influenced by photoperiod, temperature, and phytohormones, such as gibberellins (GAs). It is known that GAs promote floral initiation under short-day light conditions (SDs) by regulating the floral meristem-identity gene LEAFY (LFY) and the flowering-time gene SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1). We have defined the role of the auxin signaling component INDOLE-3-ACETIC ACID 7 (IAA7)/AUXIN RESISTANT 2 (AXR2) in the regulation of flowering time in Arabidopsis thaliana. We demonstrate that the gain-of-function mutant of IAA7/AXR2, axr2-1, flowers late under SDs. The exogenous application of GAs rescued the late flowering phenotype of axr2-1 plants. The expression of the GA20 oxidase (GA20ox) genes, GA20ox1 and GA20ox2, was reduced in axr2-1 plants, and the levels of both LFY and SOC1 transcripts were reduced in axr2-1 mutants under SDs. Furthermore, the overexpression of SOC1 or LFY in axr2-1 mutants rescued the late flowering phenotype under SDs. Our results suggest that IAA7/AXR2 might act to inhibit the timing of floral transition under SDs, at least in part, by negatively regulating the expressions of the GA20ox1 and GA20ox2 genes.

Mai YX, Wang L, Yang HQ (2011) A gain-of-function mutation in IAA7/AXR2 confers late flowering under short-day light in Arabidopsis. J. Integr. Plant Biol. 53(6), 480–492.

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Over-expression of WOX1 Leads to Defects in Meristem Development and Polyamine Homeostasis in Arabidopsis  
Author: Yanxia Zhang, Renhong Wu, Genji Qin, Zhangliang Chen, Hongya Gu and Li-Jia Qu
Journal of Integrative Plant Biology 2011 53(6): 493-506
Published Online: June 9, 2011
DOI: 10.1111/j.1744-7909.2011.01054.x

In plants, the meristem has to maintain a separate population of pluripotent cells that serve two main tasks, i.e., self-maintenance and organ initiation, which are separated spatially in meristem. Prior to our study, WUS and WUS-like WOX genes had been reported as essential for the development of the SAM. In this study, the consequences of gain of WOX1 function are described. Here we report the identification of an Arabidopsis gain-of-function mutant wox1-D, in which the expression level of the WOX1 (WUSCHEL HOMEOBOX 1) was elevated and subtle defects in meristem development were observed. The wox1-D mutant phenotype is dwarfed and slightly bushy, with a smaller shoot apex. The wox1-D mutant also produced small and dark green leaves, and exhibited a failure in anther dehiscence and male sterility. Molecular evidences showed that the transcription of the stem cell marker gene CLV3 was down-regulated in the meristem of wox1-D but accumulated in the other regions, i.e., in the root-hypocotyl junction and at the sites for lateral root initiation. The fact that the organ size and cell size in leaves of wox1-D are smaller than those in wild type suggests that cell expansion is possibly affected in order to have partially retarded the development of lateral organs, possibly through alteration of CLV3 expression pattern in the meristem. An S-adenosylmethionine decarboxylase (SAMDC) protein, SAMDC1, was found able to interact with WOX1 by yeast two-hybrid and pull-down assays in vitro. HPLC analysis revealed a significant reduction of polyamine content in wox1-D. Our results suggest that WOX1 plays an important role in meristem development in Arabidopsis, possibly via regulation of SAMDC activity and polyamine homeostasis, and/or by regulating CLV3 expression.

Zhang Y, Wu R, Qin G, Chen Z, Gu H, Qu LJ (2011) Over-expression of WOX1 leads to defects in meristem development and polyamine homeostasis in Arabidopsis. J. Integr. Plant Biol. 53(6), 493–506.

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