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.

Ethylene signaling in plants is mediated by a family of receptors related to bacterial two-component histidine kinases. Of the five members of the Arabidopsis ethylene receptor family, members of subfamily I (ETR1 and ERS1) contain completely conserved histidine kinase domains, whereas members of subfamily II (ETR2, EIN4, and ERS2) lack conserved residues thought to be necessary for kinase activity. To examine the role of the conserved histidine kinase domain in receptor signaling, ers1;etr1 loss-of-function double mutants were generated. The double mutants exhibited a severe constitutive ethylene response phenotype consistent with the negative regulator model for receptor function. The adult ers1-2;etr1-6 and ers1-2;etr1-7 phenotypes included miniature rosette size, delayed flowering, and both male and female sterility, whereas etiolated-seedling responses were less affected. Chimeric transgene constructs in which the ETR1 promoter was used to drive expression of cDNAs for each of the five receptor isoforms were transferred into the ers1-2;etr1-7 double-mutant plants. Subfamily I constructs restored normal growth, whereas subfamily II constructs failed to rescue the double mutant, providing evidence for a unique role for subfamily I in receptor signaling. However, transformation of either the ers1-2;etr1-6 or ers1-2;etr1-7 mutant with a kinase-inactivated ETR1 genomic clone also resulted in complete restoration of normal growth and ethylene responsiveness in the double-mutant background, leading to the conclusion that canonical histidine kinase activity by receptors is not required for ethylene receptor signaling. E thylene serves as an important signaling molecule in plants, both in regulating developmental processes and mediating responses to environmental signals (1). Genetic analysis of ethylene signaling in Arabidopsis revealed the presence of a small family of ethylene receptors that are related to the bacterial two-component histidine kinase superfamily of signaling molecules (2).Although the five members of the ethylene receptor family from Arabidopsis share a high degree of sequence similarity, each has distinguishing characteristics. All members contain an Nterminal, membrane-associated sensor domain that shows highaffinity ethylene binding when expressed in yeast (3, 4). Additional studies with ETR1 indicated that ethylene binding is mediated through a copper cofactor (5). In all receptor isoforms, the ethylene sensor domain is followed by a domain showing varying degrees of sequence similarity to the histidine kinase catalytic domains characteristic of bacterial two-component regulators. The bacterial systems transduce signal via autophosphorylation of a histidine residue in the kinase transmitter domain, followed by transfer of phosphate to an aspartate residue in the receiver domain of a response regulator protein (6). The residues thought to be essential for histidine kinase activity are conserved in ETR1 and ERS1 (7, 8), but are not completely conserved in ETR2, EIN4, and ERS2 (9, 10). Based on ...

Responses to the plant hormone ethylene are mediated by a family of five receptors in Arabidopsis that act in the absence of ethylene as negative regulators of response pathways. In this study, we examined the rapid kinetics of growth inhibition by ethylene and growth recovery after ethylene withdrawal in hypocotyls of etiolated seedlings of wild-type and ethylene receptor-deficient Arabidopsis lines. This analysis revealed that there are two phases to growth inhibition by ethylene in wild type: a rapid phase followed by a prolonged, slower phase. Full recovery of growth occurs approximately 90 min after ethylene removal. None of the receptor null mutations tested had a measurable effect on the two phases of growth inhibition. However, loss-of-function mutations in ETR1, ETR2, and EIN4 significantly prolonged the time for recovery of growth rate after ethylene was removed. Plants with an etr1-6;etr2-3;ein4-4 triple loss-of-function mutation took longer to recover than any of the single mutants, while the ers1;ers2 double mutant had no effect on recovery rate, suggesting that receiver domains play a role in recovery. Transformation of the ers1-2;etr1-7 double mutant with wild-type genomic ETR1 rescued the slow recovery phenotype, while a His kinase-inactivated ETR1 construct did not. To account for the rapid recovery from growth inhibition, a model in which clustered receptors act cooperatively is proposed.Ethylene regulates a number of developmental processes in higher plants, including growth in etiolated seedlings. Inhibition of growth in etiolated seedlings by ethylene is a convenient and useful bioassay that has been used to quantify the dose-response characteristics of ethylene (Chen and Bleecker, 1995) and, in mutant screens, to identify components in the ethylene signal transduction pathway (Bleecker et al., 1988; Guzman and Ecker, 1989). Mutational analysis of the ethylene signaling pathway has led to an increasingly refined model for signaling (Guo and Ecker, 2004). According to this model, responses to ethylene are mediated by a family of five receptors in Arabidopsis that are related to bacterial two-component receptors (Chang et al., 1993;Sakai et al., 1998). The ethylene receptors are thought to transduce signal via Ser/Thr kinase activity in CTR1 (Kieber et al., 1993;Huang et al., 2003). CTR1 may negatively regulate the ethylene response pathway by inhibiting activity of an Nramp-related protein, EIN2, which is required for responses to ethylene (Alonso et al., 1999). Ethylene binding to the receptors reduces the activity of the receptors, leading to reduced activity of CTR1 protein and an increase in activity of EIN2 protein along with subsequent signaling associated with it. At least some responses to ethylene, including the seedling growth response, are mediated by activation of a transcriptional cascade (Solano et al., 1998), suggesting that ethylene responses are mediated by differential gene activation and inactivation. In support of this, a number of genes have been shown to be ethylene res...