Degradation of the auxin response factor ARF1

Auxin-mediated gene expression is largely controlled through a family of DNA-binding proteins known as auxin response factors (ARF). Previous studies on the role of proteolytic regulation in auxin signaling have focused on degradation of their interacting partner, the Aux/IAA proteins. Aux/IAA family members with domain II sequences are rapidly degraded, show auxin-enhanced degradation rates, and interact with the related F-box proteins TIR1 and AFB1-3, which indicates that they are ubiquitylated by a CUL1-dependent E3 ligase. To date, limited data have been generated regarding degradation of ARFs. Here, we focus on the degradation rate of one ARF family member, Arabidopsis thaliana ARF1, and find that the half-lives of N-terminally HA-tagged ARF1 and C-terminally luciferase-tagged ARF1 are both approximately 3–4 h. This half-life appears to be conferred by a component of the middle region (MR), and degradation of the luciferase fusion with the MR is more rapid when the fusion includes an additional nuclear localization signal. ARF1 degradation is proteasome-dependent and rates are not altered in a CUL1 mutant background, suggesting that this ARF is targeted for proteasomal degradation via an alternative set of machinery to that used for Aux/IAA degradation. Consistent with this, exogenous indole acetic acid does not affect the degradation of ARF1. Given increasing evidence that the relative ratio of Aux/IAAs to ARFs rather than the absolute quantity within the cell appears to be the mode through which auxin signaling is modulated, this half-life is likely to be biologically relevant.     

How Does Auxin Enhance Cell Elongation? Roles of Auxin‐Binding Proteins and Potassium Channels in Growth Control

Elongation growth and a several other phenomena in plant development are controlled by the plant hormone auxin. A number of recent discoveries shed light on one of the classical problems of plant physiology: the perception of the auxin signal. Two types of auxin receptors are currently known: the AFB/TIR family of F box proteins and ABP1. ABP1 appears to control membrane transport processes (H+ secretion, osmotic adjustment) while the TIR/AFBs have a role in auxin-induced gene expression. Models are proposed to explain how membrane transport (e.g., K+ and H+ fluxes) can act as a cross-linker for the control of more complex auxin responses such as the classical stimulation of cell elongation.     

Nitric oxide influences auxin signaling through S-nitrosylation of the Arabidopsis TRANSPORT INHIBITOR RESPONSE 1 auxin receptor

Previous studies have demonstrated that auxin (indole-3-acetic acid) and nitric oxide (NO) are plant growth regulators that coordinate several plant physiological responses determining root architecture. Nonetheless, the way in which these factors interact to affect these growth and developmental processes is not well understood. The Arabidopsis thaliana F-box proteins TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F-BOX (TIR1/AFB) are auxin receptors that mediate degradation of AUXIN/INDOLE-3-ACETIC ACID (Aux/IAA) repressors to induce auxin-regulated responses. A broad spectrum of NO-mediated protein modifications are known in eukaryotic cells. Here, we provide evidence that NO donors increase auxin-dependent gene expression while NO depletion blocks Aux/IAA protein degradation. NO also enhances TIR1-Aux/IAA interaction as evidenced by pull-down and two-hybrid assays. In addition, we provide evidence for NO-mediated modulation of auxin signaling through S-nitrosylation of the TIR1 auxin receptor. S-nitrosylation of cysteine is a redox-based post-translational modification that contributes to the complexity of the cellular proteome. We show that TIR1 C140 is a critical residue for TIR1-Aux/IAA interaction and TIR1 function. These results suggest that TIR1 S-nitrosylation enhances TIR1-Aux/IAA interaction, facilitating Aux/IAA degradation and subsequently promoting activation of gene expression. Our findings underline the importance of NO in phytohormone signaling pathways.     

Isolation of auxin efflux carrier cDNA from cucumber.

Cucumber seedlings display not only gravitropism but also peg formation in response to gravity. Gravimorphogenesis is mediated by auxin distribution. As first step to reveal the mechanism that regulates auxin distribution by auxin efflux, we isolated five partial cDNAs of auxin efflux carriers by RT-PCR method. In addition, we isolated two full-length cDNAs (CsPIN2, CsPIN3) from a cucumber cDNA library. CsPIN2, AtPIN3, AtPIN4 and AtPIN7 fall within the same clade. CsPIN3, AtPIN1 and CsPIN1 fall within the same clade. CsPIN5, CsPIN6 and AtPIN2 fall within the same clade. Our phylogenetic analysis of PIN in cucumber and Arabidopsis indicates that cucumber may diversify CsPIN protein compared with AtPIN protein.     

ROP GTPase-mediated auxin signaling regulates pavement cell interdigitation in Arabidopsis thaliana

In multicellular plant organs, cell shape formation depends on molecular switches to transduce developmental or environmental signals and to coordinate cell‐to‐cell communication. Plants have a specific subfamily of the Rho GT Pase family, usually called Rho of Plants(ROP), which serve as a critical signal transducer involved in many cellular processes. In the last decade, important advances in the ROP‐mediated regulation of plant cell morphogenesis have been made by using Arabidopsis thaliana leaf and cotyledon pavement cells.Especially, the auxin‐ROP signaling networks have been demonstrated to control interdigitated growth of pavement cells to form jigsaw‐puzzle shapes. Here, we review findings related to the discovery of this novel auxin‐signaling mechanism at the cell surface. This signaling pathway is to a large extent independent of the well‐known Transport Inhibitor Response(TIR)–Auxin Signaling F‐Box(AFB) pathway, and instead requires Auxin Binding Protein 1(ABP1) interaction with the plasma membrane‐localized, transmembrane kinase(TMK) receptor‐like kinase to regulate ROP proteins. Once activated, ROP influences cytoskeletal organization and inhibits endocytosis of the auxin transporter PIN1. The present review focuses on ROP signaling and its self‐organizing feature allowing ROP proteins to serve as a bustling signal decoder and integrator for plant cell morphogenesis.