regeneration were carried out as described.I7. Developing shoots were selected on 100 pgmL-' kanamycin.Fifteen kanamycin resistant plants were tested for cyanamide hydratase a~tivity!~] Enzyme activity was found in all plants, the level of expression ranging from 0.03 to 0.79 units per mg protein in the cell extract. The highest value corresponds to about a fifth of the specific activity measured in induced Myrothecium verrucaria extracts. Although the specific activity was highest in the roots, the total enzyme activity per gram fresh weight was distinctly higher in the leaves than in the other plant organs (Table 1). No activity was found in plants transgenic for cah-.

An open question remains as to what coordinates cell behavior during organogenesis, permitting organs to reach their appropriate size and shape. The Arabidopsis gene STRUBBELIG (SUB) defines a receptor-mediated signaling pathway in plants. SUB encodes a putative leucine-rich repeat transmembrane receptorlike kinase. The mutant sub phenotype suggests that SUB affects the formation and shape of several organs by influencing cell morphogenesis, the orientation of the division plane, and cell proliferation. Mutational analysis suggests that the kinase domain is important for SUB function. Biochemical assays using bacterially expressed fusion proteins indicate that the SUB kinase domain lacks enzymatic phosphotransfer activity. Furthermore, transgenes encoding WT and different mutant variants of SUB were tested for their ability to rescue the mutant sub phenotype. These genetic data also indicate that SUB carries a catalytically inactive kinase domain. The SUB receptor-like kinase may therefore signal in an atypical fashion.atypical kinase ͉ flower ͉ organogenesis ͉ ovule ͉ signal transduction I t remains a salient challenge in biology to understand the coordination of cell behavior that underlies organogenesis and allows organs to develop to their correct size and shape. The task should be easier in plants as cell-division patterns are readily traced and plant cells do not move relative to each other (1). Plant organogenesis is a postembryonic event, and the aboveground organs typically originate at the periphery of the shoot apical meristem, located at the apex of the main shoot (2).Signaling involving receptor-like kinases (RLKs) constitutes an essential aspect of plant cell communication and contributes to plant-pathogen interactions, hormone signaling, and development (3-5). In Arabidopsis 417 genes are predicted to encode such proteins (6). A function is known for only a handful of these loci. A major player in meristem development is the RLK CLAVATA1 (CLV1). CLV1 participates in a feedback loop maintaining the size of the stem cell population (for reviews see refs. 7 and 8). The CLV1 extracellular domain is characterized by 21 tandem copies of a leucine-rich repeat (LRR), a motif involved in protein-protein recognition (9, 10). The LRR-RLK ERECTA (ER) plays a more direct role in oganogenesis (11)(12)(13). ER is expressed in the shoot apical meristem and young lateral organs. Plants lacking WT ER function display a more compact stature, shorter inflorescence internodes, shorter pedicels, and shorter fruits with blunted tips. The main cellular basis of the er phenotype appears to be a reduction of cortex cell numbers (14). Members of the ER family of RLKs collectively promote cell proliferation and organ development (15). In corn, CRINKLY4 (CR4) is involved in cell differentiation in the leaf epidermis and specification of the aleurone layer of the endosperm (16,17). Its extracellular domain is characterized by seven ''crinkly'' repeats and a domain containing three repeats also present in mammalian TNF receptor...

The rapidly increasing production of engineered nanoparticles has raised questions regarding their environmental impact and their mobility to overcome biological important barriers. Nanoparticles were found to cross different mammalian barriers, which is summarized under the term translocation. The present work investigates the uptake and translocation of cerium dioxide nanoparticles into maize plants as one of the major agricultural crops. Nanoparticles were exposed either as aerosol or as suspension. Our study demonstrates that 50 microgram cerium per gram leaves was either adsorbed or incorporated into maize leaves. This amount could not be removed by a washing step and did not depend on closed or open stomata investigated under dark and light exposure conditions. However, no translocation into newly grown leaves was found when cultivating the maize plants after airborne particle exposure. The use of inductively coupled mass spectrometer allowed detection limits of less than 1 nanogram cerium per gram leaf. Exposure of plants to well characterized nanoparticle suspensions in the irrigation water resulted also in no detectable translocation. These findings may indicate that the biological barriers of plants are more resistant against nanoparticle translocation than mammalian barriers.