Abscission is a developmental program that results in the active shedding of infected or nonfunctional organs from a plant body. Here, we establish a signaling pathway that controls abscission in Arabidopsis thaliana from ligand, to receptors, to downstream effectors. Loss of function mutations in Inflorescence Deficient in Abscission (IDA), which encodes a predicted secreted small protein, the receptor-like protein kinases HAESA (HAE) and HAESA-like 2 (HSL2), the Mitogen-Activated Protein Kinase Kinase 4 (MKK4) and MKK5, and a dominant-negative form of Mitogen-Activated Protein Kinase 6 (MPK6) in a mpk3 mutant background all have abscission-defective phenotypes. Conversely, expression of constitutively active MKKs rescues the abscission-defective phenotype of hae hsl2 and ida plants. Additionally, in hae hsl2 and ida plants, MAP kinase activity is reduced in the receptacle, the part of the stem that holds the floral organs. Plants overexpressing IDA in a hae hsl2 background have abscission defects, indicating HAE and HSL2 are epistatic to IDA. Taken together, these results suggest that the sequential action of IDA, HAE and HSL2, and a MAP kinase cascade regulates the programmed separation of cells in the abscission zone.protein phosphorylation ͉ signal transduction A bscission is a physiological process that involves the programmed separation of entire organs, such as leaves, petals, flowers, and fruit. Abscission allows plants to discard nonfunctional or infected organs, and promotes dispersion of progeny. At the cellular level, abscission is the hydrolysis of the middle lamella of an anatomically specialized cell layer, the abscission zone (AZ), by cell wall-modifying and hydrolyzing enzymes. Thus, abscission requires both the formation of the AZ early in the development of a plant organ and the subsequent activation of the cell separation response (1-4).Studies using Arabidopsis thaliana have implicated the involvement of several different genes in the control of abscission including potential signal molecules, receptors and other gene products (4). HAESA (HAE), one of the first Arabidopsis receptor-like protein kinases (RLK) identified, is expressed in floral organ AZs and antisense experiments show a reduction in the level of HAE protein is correlated with the degree of defective floral organ abscission. Expression of HAE is not altered in etr1-1 (an ethylene-insensitive mutation), implying an ethyleneindependent role in abscission (5). Inflorescence Deficient in Abscission (IDA) encodes a small protein with an N-terminal signal peptide. Analysis of ida mutant plants indicates IDA regulates floral organ abscission through an ethylene insensitive pathway (6). Overexpression of IDA results in early abscission and the production of a white substance in the floral AZs. The main components of the white substance are arabinose and galactose (7).Here, we report that components of a MAPK signaling cascade also have roles in the regulation of abscission. A MAPK cascade is a regulatory module with three protein kina...

Sexual reproduction is a salient aspect of plants, and elaborate structures, such as the f lowers of angiosperms, have evolved that aid in this process. Within the f lower the corresponding sex organs, the anther and the ovule, form the male and female sporangia, the pollen sac and the nucellus, respectively. However, despite their central role for sexual reproduction little is known about the mechanisms that control the establishment of these important structures. Here we present the identification and molecular characterization of the NOZZLE (NZZ) gene in the f lowering plant Arabidopsis thaliana. In several nzz mutants the nucellus and the pollen sac fail to form. It indicates that NZZ plays an early and central role in the development of both types of sporangia and that the mechanisms controlling these processes share a crucial factor. In addition, NZZ may have an early function during male and female sporogenesis as well. The evolutionary aspects of these findings are discussed. NZZ encodes a putative protein of unknown function. However, based on sequence analysis we speculate that NZZ is a nuclear protein and possibly a transcription factor.

Proteins containing the forkhead-associated domain (FHA) are known to act in biological processes such as DNA damage repair, protein degradation, and signal transduction. Here we report that DAWDLE (DDL), an FHA domain-containing protein in Arabidopsis, acts in the biogenesis of miRNAs and endogenous siRNAs. Unlike mutants of genes known to participate in the processing of miRNA precursors, such as dcl1, hyponastic leaves1, and serrate, ddl mutants show reduced levels of pri-miRNAs as well as mature miRNAs. Promoter activity of MIR genes, however, is not affected by ddl mutations. DDL is an RNA binding protein and is able to interact with DCL1. In addition, we found that SNIP1, the human homolog of DDL, is involved in miRNA biogenesis and interacts with Drosha. Therefore, we uncovered an evolutionarily conserved factor in miRNA biogenesis. We propose that DDL participates in miRNA biogenesis by facilitating DCL1 to access or recognize pri-miRNAs.class of sequence-specific repressors of gene expression in eukaryotes is 20-to 24-nt small RNAs, which include miRNAs and siRNAs. miRNAs are processed from stem-loop precursor RNAs, called pri-miRNAs. In animals, pri-miRNAs are processed in the nucleus by Drosha to form pre-miRNAs, which are exported to the cytoplasm by exportin 5 and further processed by Dicer to produce mature miRNAs (reviewed in ref. 1). In Arabidopsis, mature miRNAs are produced through two processing steps (primiRNAs to pre-miRNAs and pre-miRNAs to miRNAs) in the nucleus by DCL1 with the assistance of HYL1 and SERRATE (reviewed in ref. 2). After processing, miRNAs are 2Ј-O-methylated by HEN1 (3). siRNAs are produced from long, double-stranded RNAs. Plants contain several classes of endogenous siRNAs, such as transacting siRNAs (ta-siRNAs), natural antisense siRNAs (nat-siRNAs), and siRNAs from endogenous repeat sequences and transposons (reviewed in ref. 4).The forkhead-associated (FHA) domain is an 80-to 100-aa module that is thought to recognize phosphothreonine-containing motifs and mediate protein-protein interactions in prokaryotes and eukaryotes (reviewed in ref. 5). DAWDLE (DDL) is a nuclearlocalized FHA domain-containing protein in Arabidopsis (6). DDL appears to act in multiple developmental processes such as growth, fertility, and root, shoot, and floral morphogenesis (6).Smad nuclear interacting protein 1 (SNIP1) is a human FHA domain-containing protein that functions as an inhibitor of TGF-␤ and NF-B signaling pathways by competing with the TGF-␤ signaling protein Smad4 and the NF-B transcription factor p65/ RelA for binding to the transcriptional coactivator p300 (7,8). Recently, Fujii et al. (9) reported that SNIP1 interacts with the transcription factor/oncoprotein c-Myc and enhances its activity by bridging its interaction with p300.Here we report that DDL is required for the accumulation of miRNAs and endogenous siRNAs in Arabidopsis. Its affinity for RNA, its potential association with DCL1, and the reduction in pri-miRNA levels in ddl loss-of-function mutants suggest that DDL is...

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...