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Plant Physiology 149:601-605 (2009) © 2009 American Society of Plant Biologists Advancing Cell Biology and Functional Genomics in Maize Using Fluorescent Protein-Tagged Lines1Cold Spring Harbor Laboratory, Cold Spring Harbor, New York 11724 (A.M., S.D., Y.Y., T.Z., D.J.); Department of Molecular Biology, University of Wyoming, Laramie, Wyoming 82071 (A.L., X.L., D.E.T., K.E.W., D.H., A.W.S.); and J. Craig Venter Institute, Rockville, Maryland 20850 (A.C.)
Genomic resources have significantly impacted plant biology research in recent years. Cell biology has been further enabled by an ongoing revolution in visualization technologies. Using fluorescent proteins (FPs), we now have unprecedented views of cellular architecture, and we can study real-time dynamics of cell structure, function, and protein localization. To date, these technologies have been most widely used in Arabidopsis (Arabidopsis thaliana); however, the grasses provide a unique opportunity to study the underlying mechanisms and inter-related controls of cell growth, morphogenesis, and physiology in leading crop models. Here, we present a resource that leverages the emerging maize (Zea mays) genome sequence to develop tools to study protein structure and function in a cellular context. Traditionally, such studies relied on fixed tissue or FP fusions driven by constitutive promoters, which can lead to significant artifacts. The maize genome sequence now provides access to regulatory regions that can be used to drive native expression. We have developed streamlined methods to generate maize FP-tagged lines using these regulatory elements, allowing analysis of tissue-specific expression and localized function. Identification of diverse proteins that function in specific subcellular compartments will provide the tools for understanding basic developmental, biochemical, and physiological processes in maize, with direct application potential for crop improvement.
We developed a protocol to generate fusion proteins with yellow (YFP), cyan (CFP), or red (RFP) color variants of FPs driven by native regulatory elements, based on our previous work in Arabidopsis (Tian et al., 2004 Candidate genes were selected for tagging based on several criteria, including, as first priority, the availability of full genomic sequence plus regulatory regions that included 3 kb upstream and 1 kb downstream of the coding region. A size limit of 8 to 9 kb for the full genomic region with the FP insertion is imposed to ensure good cloning efficiency. Given these size constraints, we next prioritized genes that encoded proteins with robust predicted functions. These decisions were based on homology to other well-studied proteins, known localizations to specific compartments, and/or corroborating antibody or expression data. Genes with available mutations were also given high priority so as to provide a means of functional complementation. Our final criterion was to include candidates that would label the full range of cellular compartments.
Approximately 25% of genes tagged were derived from specific requests that met our designated tagging criteria from researchers in the maize community. The remaining 75% of genes were selected initially by searching approximately 2,500 TIGR (The Institute for Genomic Research) Release 5 AZMs (Assembled Zea mays; Chan et al., 2006
Figure 1 depicts a subset of images representative of the tagged lines generated. The tagged lines mark most compartments in maize cells for use in diverse research programs. For example, proteins localized to the nucleus are ideal as constitutive markers and for studying chromosome dynamics and developmental regulation (Fig. 1, A–D). Here we display two nuclear markers, each serving a unique function. Histone H1 is a linker histone that functions in maintaining higher order chromatin structure and in epigenetic regulation (Misteli et al., 2000
The second nuclear marker displayed is Meiotic Recombination11B (MRE11B) tagged with YFP. MRE11 proteins are tools for studying chromosome repair, due to their interactions with RAD50 and NBS1, which mediate double-stranded break repair and recombination (Waterworth et al., 2007
As a marker for the vacuole, we tagged a maize aquaporin, tonoplast intrinsic protein1 (TIP1; Barrieu et al., 1998
Peroxin11 (PEX11) was tagged as a marker for peroxisomes, small organelles that perform a variety of oxidative cellular functions. PEX11 is targeted to peroxisomes in Arabidopsis and has been implicated in peroxisome proliferation (Lingard et al., 2008
FP tags of tubulin (TUB) are essential for studying in vivo cytoskeletal dynamics and organization. Because
Immediately adjacent to the cortical array of MTs lies the plasma membrane (PM) compartment that serves as a gate between the cytoplasm and the cell wall. We marked this compartment by fusing CFP to a PM intrinsic protein (PIP2-1), an integral membrane aquaporin that transports water, small uncharged solutes, and gases across the PM (Chaumont et al., 2001
Vesicle trafficking is a highly coordinated system of intracompartmental transport in eukaryotes, mediated by small GTPases in the Ras superfamily. ROPs and RABs are two subfamilies that have diverged extensively in plants (Christensen et al., 2003
The actin cytoskeleton is a dynamic structure associated with cell division, expansion, organelle movement, and maintenance of subcellular compartments. Study of actin in plant cells has been largely limited to fixed tissue, possibly due to the failure of actin filament assembly when subunits are tagged. However, the actin-binding protein fimbrin permits in vivo observation of actin cytoskeleton through its direct binding to intact microfilaments (Wang et al., 2008
Mitochondrial markers are important tools to study organelle morphology, development, and function. Heat shock proteins (HSPs) are useful because they can be either conditional or constitutive (Shemetov et al., 2008
Lipoxygenase10 (LOX10) is thought to function in synthesis of oxylipins, including jasmonic acid, which functions in defense responses as well as in plant development (Nemchenko et al., 2006
Currently, approximately 40 proteins have been tagged with FP and T2 generation seeds are available by request through our Web site at http://maize.jcvi.org/tigr-scripts/maize/cellgenomics/seed_request.pl. We aim to continue tagging up to 100 proteins for compartments not yet labeled and for proteins developmentally regulated, physiologically controlled, and conditionally expressed. We will leverage advances in live cell imaging techniques and continue to mine the emerging maize genome sequence for new regulatory regions and genes of interest. In addition to their value for cell and developmental studies, these FP-tagged proteins will be useful for proteomic analysis of protein complexes and for protein-protein interaction studies. Our constructs are designed for easy replacement of the FP tag with any other tag, allowing flexibility in future experiments. We are also developing additional cloning techniques to increase throughput. The techniques described here could easily be applied to other grasses, providing a broad-based cell biological resource to the plant biology community and serving to provide new views into protein and cellular imaging.
We thank Dr. R. Tsien for his generous gift of citrine YFP and mRFP1 plasmids. We thank Tim Mulligan for excellent plant care at Cold Spring Harbor Laboratory, Pete Gallins and Ryan Pendleton for greenhouse assistance at the University of Wyoming, and Kan Wang, Bronwyn Frame, and the staff of the Iowa State University Plant Transformation Facility for outstanding maize transformation services. A.W.S. thanks Dr. Mike Tamkun (Colorado State University) for use of live cell imaging facilities and Dr. Laurie Smith (University of California, San Diego) for use of a spinning disc confocal microscope. Received September 19, 2008; accepted November 15, 2008; published February 6, 2009.
1 This work was supported by the National Science Foundation (DBI no. 0501862).
2 These authors contributed equally to the article.
3 Present address: DuPont Knowledge Centre, ICICI Knowledge Park, Turkapally, Hyderabad–5000078, India.
4 Present address: Technical Center of Animal and Plant Inspection and Quarantine, B–601, Herping Building, Herping Road 2049, Shenzhen City, P.R. China 518001. The author responsible for the distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Anne W. Sylvester (annesyl{at}uwyo.edu). www.plantphysiol.org/cgi/doi/10.1104/pp.108.130146 * Corresponding author; e-mail annesyl{at}uwyo.edu.
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