Auxin-binding protein 1 (ABP1) was discovered nearly 40 years ago and was shown to be essential for plant development and morphogenesis, but its mode of action remains unclear. Here, we report that the plasma membrane–localized transmembrane kinase (TMK) receptor–like kinases interact with ABP1 and transduce auxin signal to activate plasma membrane–associated ROPs [Rho-like guanosine triphosphatases (GTPase) from plants], leading to changes in the cytoskeleton and the shape of leaf pavement cells in Arabidopsis. The interaction between ABP1 and TMK at the cell surface is induced by auxin and requires ABP1 sensing of auxin. These findings show that TMK proteins and ABP1 form a cell surface auxin perception complex that activates ROP signaling pathways, regulating nontranscriptional cytoplasmic responses and associated fundamental processes.

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Delayed cell separation historically was one of the first agricultural traits selected for by man. Successful collection of fruits and seeds of crops such as wheat (Triticum monococcum), rice (Oryza sativa), and a variety of legumes was only possible due to the selection for delayed fruit abscission or pod shatter in which seeds were retained on the stalk rather than rapidly shed. This selection pressure was further imposed as human beings began to harvest by sickle rather than collection in baskets because only the grains that remained longer on the plant were harvested and propagated for future use. A glance at some of the wild relatives of wheat and barley (Hordeum vulgare) shows the presence of brittle, easily shed, seed-bearing stalks rather than the tougher, seed-retaining stalks of some of today's cultivars. In crops like amaranth (Amaranth caudatus) and rice,

The ethylene binding domain (EBD) of the Arabidopsis thaliana ETR1 receptor is modeled as three membrane-spanning helices. We surveyed ethylene binding activity in different kingdoms and performed a bioinformatic analysis of the EBD. Ethylene binding is confined to land plants, Chara, and a group of cyanobacteria but is largely absent in other organisms, consistent with our finding that EBD-like sequences are overrepresented among plant and cyanobacterial species. We made amino acid substitutions in 37 partially or completely conserved residues of the EBD and assayed their effects on ethylene binding and signaling. Mutations primarily in residues in Helices I and II midregions eliminated ethylene binding and conferred constitutive signaling, consistent with the inverse-agonist model of ethylene receptor signaling and indicating that these residues define the ethylene binding pocket. The largest class of mutations, clustered near the cytoplasmic ends of Helices I and III, gave normal ethylene binding activity yet still conferred constitutive signaling. Therefore, these residues may play a role in turning off the signal transmitter domain of the receptor. By contrast, only two mutations were loss of function with respect to signaling. These findings yield insight into the structure and function of the EBD and suggest a conserved role of the EBD as a negative regulator of the signal transmitter domain.

Abscission is an important developmental process in the life cycle of the plant, regulating the detachment of organs from the main body of the plant. This mechanism can be initiated in response to environmental cues such as disease or pathogen, or it can be a programmed shedding of organs that no longer provide essential functions to the plant. We have identified five novel dab (delayed floral organ abscission) mutants (dab1-1, dab2-1, dab3-1, dab3-2, and dab3-3) in Arabidopsis. These mutants each display unique anatomical and physiological characteristics and are governed by three independent loci. Scanning electron microscopy shows delayed development of the flattened fracture plane in some mutants and irregular elongation in the cells of the fracture plane in other mutants. The anatomical observations are also supported by breakstrength measurements that show high breakstrength associated with broken cells, moderate levels for the flattened fracture plane, and low levels associated with the initial rounding of cells. In addition, observations on the expression patterns in the abscission zone of cell wall hydrolytic enzymes, chitinase and cellulose, show altered patterns in the mutants. Last, we have compared these mutants with the ethylene-insensitive mutants etr1-1 and ein2-1 to determine if ethylene is an essential component of the abscission process and find that although ethylene can accelerate abscission under many conditions, the perception of ethylene is not essential. The role of the dab genes and the ethylene response genes during the abscission process is discussed.Abscission, the developmental process regulating detachment of organs from the main body of the plant, can be regarded as valuable to the plant in response to disease or pathogen challenge and the shedding of organs that no longer provide essential functions to the plant. Historically, ethylene treatment has been shown to result in early abscission and increases in cell wall hydrolytic enzymes. In their studies on Prunus serrulata senriko and Parthenocissus quinquefolia, Jackson and Osborne (1970) concluded that ethylene was not only responsible for accelerating abscission but an essential regulator of abscission. Alternatively, crops like Citrus sinensis appear to have limited responses to ethylene treatment (Lewis et al., 1968;Palmer et al., 1969). Although the role of ethylene in hastening abscission has been documented repeatedly in many plant species over the last several decades, it has never been shown definitively that ethylene perception in these plants is essential for abscission (Addicott, 1982; Abeles et al., 1992). Previously, we have illustrated that floral organ abscission in Arabidopsis may be used as a model system to study abscission (Bleecker and Patterson, 1997). In this work, we will further elucidate the role of ethylene in floral organ abscission by the identification and characterization of five novel dab (delayed abscission) mutants (dab1-1, dab2-1, dab3-1, dab3-2, and dab3-3), representing three independent loc...