Rhizodermic cells as a model for plasticity in post-embryonic development
Our major research interest is to clarify how environmental signals are perceived and translated into changes of intrinsic post-developmental programs and how these changes relate to function. Using the root epidermal surface as a model system, we are taking an integrative approach comprising both hypothesis-driven and non-biased efforts to elucidate ecological relevant alterations in structural and functional aspects of cell differentiation processes. Root epidermal cells can develop into either a root hair cell or a non-hair cell, following a pattern that is genetically determined, but also responsive to extrinsic signals. Environmental cues can interact with, or being superimposed on, the interplay of genetic and positional information. Our long-term goal is to understand how the amalgam of intrinsic and external information is integrated to determine the cell fate of rhizodermal cells.
Forward Genetics: manic, ironic and perfect
We have screened for mutants that display alterations in root hair development in response to phosphate, iron, or manganese deficiency, but do not have a phenotype under control conditions. One of these mutants, named perfect (per) is unable to elongate root hairs under phosphate deficient conditions. In addition, the mutant shows a constitutively elevated number of lateral roots, resembling that of phosphate-deficient plants. The per2 mutation segregates as a single recessive Mendelian locus and has been mapped to a 38-kb interval on the top of chromosome 3. Ironic (iro) is an enhancer trap line that exaggerates the response to iron deficiency and forms root hairs in positions that are normally occupied by non-hair cells. The gene is expressed in the epidermis and in the vasculature, pointing to a possible involvement in long-distance signaling. Screening for mutants that are unable to respond to manganese deficiency has yielded several manic "putants" that are currently being characterized.
Functional "Omics": Identification of genes involved in phenotypic plasticity
The elucidation of signaling networks in response to phosphate, iron, and manganese deficiency is addressed by both transcriptional profiling and by proteomic approaches. Time-course changes in gene expression profiles in wild-type and selected mutants are currently being used to set up a data base covering the complete transcriptome of genes responsive to deficiencies of nutrients with low mobility. To identify new players in cell reprogramming, we are using FACS-aided sorting of protoplasts from enhancer trap lines which show trichoblast- or atrichoblast-specific GFP expression patterns. In addition, the ChIP-on-Chip method is used for analyzing target genes of transcriptional regulators identified by transcriptional profiling.
Modeling cell fate decisions
In order to integrate the data and to refine hypotheses on how environmental signals affect developmental programs, we have developed a model that is congruent with previously obtained experimental data. Based on computer simulations we have put forward the hypothesis that an inhibitor-activator mechanism, acting downstream of the transcription factor cascade that controls root hair pattern in seedlings, confers plasticity to the system during post-embryonic development and aids in generating a pattern that optimally suits for a given developmental stage or a certain environmental situation. The model is currently tested experimentally.
Form follows function
The mobilization of iron and phosphate is aided by the P-type ATPase-mediated extrusion of protons. The acidification of the rhizosphere, on the other hand, is associated with developmental changes in the epidermal cell layer, such as the formation of root hairs and transfer cells. In order to generate recombinant inbred lines (RILs) with high proton extrusion activity in response to Fe deficiency, crosses from parents that have been selected from an initial screening of natural Arabidopsis accessions for high proton pumping potential, show a more pronounced response when compared to the frequently used lab strains such as Col-0. The generation of RILs sets the stage for an in-depth analysis of the linkage between developmental and physiological responses to the environment. Laser Microdissection Pressure Catapulting of stress-induced root hairs from cucumber roots, a species with an extremely high proton extrusion capacity, is used as an additional tool for the analysis of functional aspects of developmental changes.
Thomas Ju Wei Yang, Paula Jay Perry, Silvano Ciani, Sundaravel Pandian, Wolfgang Schmidt. 2008. Manganese deficiency alters the patterning and development of root hairs in Arabidopsis. J. Exp. Bot 59: 3453-3464
Santi S, Schmidt W. 2008. Laser microdissection-assisted analysis of the functional fate of iron deficiency-induced root hairs in cucumber. Journal of Experimental Botany 59: 697-704
Schmidt W. 2008. Inner voices meet outer signals: the plasticity of rhizodermic cells. Plant Science 174: 239-245
Savage NS, Schmidt W. 2008. From priming to plasticity: the changing fate of rhizodermic cells, Bioessays, 30:75-81
Perry P, Linke B, Schmidt, W. 2007. Reprogramming of root epidermal cells. Biochemical Society Transactions35: 161-163 (Cover picture)
Schmidt, W, Linke B. 2007. Nutrients as regulators of root morphology and architecture. The rhizosphere: biochemistry and organic substances at the soil-plant interface. R. Pinton, Z. Varanini, P. Nannipieri, eds., Marcel Dekker Inc., New York, pp 135-150
Schmidt W, Santi S, Pinton R. 2007. Water-extractable humic substances alter root development and epidermal cell pattern in Arabidopsis. Plant and Soil300: 259-267
Schmidt, W. 2006. Iron stress responses in roots of strategy I plants. In: Iron Nutrition in Plants and Rhizoshere Microorganisms. Barton LL, Abadia J, Eds. Kluwer Academic Publishers, Dordrecht, pp 229-250
Schikora A, Thimm O, Linke B, Buckhout T, Müller M, Schmidt W. 2006. Expression, localization, and regulation of the iron transporter LeIRT1 in tomato roots (Lycopersicon esculentum L.). Plant and Soil284: 101-108
Müller M, Schmidt W. 2004 Environmentally induced plasticity of root hair development in Arabidopsis. Plant Physiology134: 409-419
Schmidt W, MichalkeW, Schikora A. 2003. Proton pumping by tomato roots. Effect of Fe deficiency and hormones on the activity and distribution of plasma membrane H+-ATPase in rhizodermal cells. Plant, Cell & Environment26: 361-370
De Simone O, Haase K, Müller E, Junk W, Hartmann K, Schreiber L, Schmidt W. 2003. Apolasmic barriers and oxygen transport properties of hypodermal cell walls in roots from four Amazonian tree species. Plant Physiology132: 206-217
Schmidt W. 2003. Iron solutions: acquisition strategies and signaling pathways. Trends in Plant Science8: 188-193
Schikora A, Schmidt W. 2002. Formation of transfer cells and H+-ATPase expression in tomato roots under P and Fe deficiency. Planta215: 304-311
Schikora A, Schmidt W. 2001. Iron stress-induced epidermal cell fate is regulated independently from physiological acclimations to low iron availability. Plant Physiology125: 1679-1687
Schmidt W, Schikora A. 2001. Different pathways are involved in phosphate and iron stress-induced alterations of root epidermal cell development. Plant Physiology125: 2078-2084
Pich A, Manteuffel R, Hillmer S, Scholz G, Schmidt W. 2001. Fe homeostasis in plant cells: Does nicotianamine play multiple roles in the regulation of cytoplasmic Fe concentration? Planta213: 967-976
Schikora A, Schmidt W. 2001. Acclimative changes in root epidermal cell fate in response to Fe and P deficiency. A specific role for auxin? Protoplasma218: 67-75
Schmidt W. 2001. From faith to fate. Ethylene signaling in morphogenic responses to P and Fe deficiency. Journal of Soil Science and Plant Nutrition164: 147-154
