Summary Spatial distribution of the plant hormone auxin regulates multiple aspects of plant development. These self-regulating auxin gradients are established by the action of PIN auxin transporters, whose activity is regulated by their constitutive cycling between the plasma membrane and endosomes. Here, we show that auxin signaling by the auxin receptor AUXIN-BINDING PROTEIN 1 (ABP1) inhibits the clathrin-mediated internalization of PIN proteins. ABP1 acts as a positive factor in clathrin recruitment to the plasma membrane, thereby promoting endocytosis. Auxin binding to ABP1 interferes with this action and leads tothe inhibition of clathrin-mediated endocytosis. Our study demonstrates that ABP1 mediates a nontranscriptional auxin signaling that regulates the evolutionarily conserved process of clathrin-mediated endocytosis and suggests that this signaling may be essential for the developmentally important feedback of auxin on its own transport.

The plant hormone auxin plays a crucial role in regulating plant development and plant architecture. The directional auxin distribution within tissues depends on PIN transporters that are polarly localized on the plasma membrane. The PIN polarity and the resulting auxin flow directionality are mediated by the antagonistic actions of PINOID kinase and protein phosphatase 2A. However, the contribution of the PIN phosphorylation to the polar PIN sorting is still unclear. Here, we identified an evolutionarily conserved phosphorylation site within the central hydrophilic loop of PIN proteins that is important for the apical and basal polar PIN localizations. Inactivation of the phosphorylation site in PIN1(Ala) resulted in a predominantly basal targeting and increased the auxin flow to the root tip. In contrast, the outcome of the phosphomimic PIN1(Asp) manipulation was a constitutive, PINOID-independent apical targeting of PIN1 and an increased auxin flow in the opposite direction. Furthermore, the PIN1(Asp) functionally replaced PIN2 in its endogenous expression domain, revealing that the phosphorylation-dependent polarity regulation contributes to functional diversification within the PIN family. Our data suggest that PINOID-independent PIN phosphorylation at one single site is adequate to change the PIN polarity and, consequently, to redirect auxin fluxes between cells and provide the conceptual possibility and means to manipulate auxin-dependent plant development and architecture.cell polarity | auxin distribution | plant architecture T he plant hormone auxin acts, on account of its differential distribution (gradients) within tissues, as a major determinant of plant architecture (1-3). Auxin is distributed throughout the plant by a network of carrier proteins (4-8), and the directionality of the auxin flow is determined by asymmetrically localized plasma membrane PIN transporters (9). The differentially expressed and polarly localized PIN proteins constitute the backbone of a transport network for directional auxin distribution in different parts of the plant (10). The local biosynthesis (11-13) together with the PIN-dependent transport (14) largely account for the formation of local auxin maxima and minima that regulate various developmental processes, including embryonic axis establishment, tropic growth, root meristem patterning, lateral organ and fruit formation, and vascular tissue differentiation and regeneration (15, 16). The polar PIN localization determines direction of the auxin flow; thus, any signal that acts upstream to control the cellular PIN localization and activity can be translated into changes in the auxin distribution that modulate multiple aspects of the plant development. Phosphorylation has been shown to be important for auxin transport and distribution (17)(18)(19)(20). So far, the only known regulators that specifically regulate the PIN polar targeting are the serine/threonine protein kinase PINOID (PID) (18-20) and the protein phosphatase 2A (PP2A) (21, 22) that mediate antagonistical...

The phytohormone auxin plays a major role in embryonic and postembryonic plant development. The temporal and spatial distribution of auxin largely depends on the subcellular polar localization of members of the PIN-FORMED (PIN) auxin efflux carrier family. The Ser/Thr protein kinase PINOID (PID) catalyzes PIN phosphorylation and crucially contributes to the regulation of apical-basal PIN polarity. The GTP exchange factor on ADP-ribosylation factors (ARF-GEF), GNOM preferentially mediates PIN recycling at the basal side of the cell. Interference with GNOM activity leads to dynamic PIN transcytosis between different sides of the cell. Our genetic, pharmacological, and cell biological approaches illustrate that PID and GNOM influence PIN polarity and plant development in an antagonistic manner and that the PID-dependent PIN phosphorylation results in GNOM-independent polar PIN targeting. The data suggest that PID and the protein phosphatase 2A not only regulate the static PIN polarity, but also act antagonistically on the rate of GNOM-dependent polar PIN transcytosis. We propose a model that includes PID-dependent PIN phosphorylation at the plasma membrane and the subsequent sorting of PIN proteins to a GNOM-independent pathway for polarity alterations during developmental processes, such as lateral root formation and leaf vasculature development.

Although research has determined that reactive oxygen species (ROS) function as signaling molecules in plant development, the molecular mechanism by which ROS regulate plant growth is not well known. An aba overly sensitive mutant, abo8-1, which is defective in a pentatricopeptide repeat (PPR) protein responsible for the splicing of NAD4 intron 3 in mitochondrial complex I, accumulates more ROS in root tips than the wild type, and the ROS accumulation is further enhanced by ABA treatment. The ABO8 mutation reduces root meristem activity, which can be enhanced by ABA treatment and reversibly recovered by addition of certain concentrations of the reducing agent GSH. As indicated by low ProDR5:GUS expression, auxin accumulation/signaling was reduced in abo8-1. We also found that ABA inhibits the expression of PLETHORA1 (PLT1) and PLT2, and that root growth is more sensitive to ABA in the plt1 and plt2 mutants than in the wild type. The expression of PLT1 and PLT2 is significantly reduced in the abo8-1 mutant. Overexpression of PLT2 in an inducible system can largely rescue root apical meristem (RAM)-defective phenotype of abo8-1 with and without ABA treatment. These results suggest that ABA-promoted ROS in the mitochondria of root tips are important retrograde signals that regulate root meristem activity by controlling auxin accumulation/signaling and PLT expression in Arabidopsis.

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In plants, auxin distribution and tissue patterning are coordinated via a feedback loop involving the auxin-regulated cell polarity factor ICR1 and the secretory machinery.

We propose a systematic approach for a better understanding of how HIV viruses employ various combinations of mutations to resist drug treatments, which is critical to developing new drugs and optimizing the use of existing drugs. By probabilistically modeling mutations in the HIV-1 protease or reverse transcriptase (RT) isolated from drug-treated patients, we present a statistical procedure that first detects mutation combinations associated with drug resistance and then infers detailed interaction structures of these mutations. The molecular basis of our statistical predictions is further studied by using molecular dynamics simulations and free energy calculations. We have demonstrated the usefulness of this systematic procedure on three HIV drugs, (Indinavir, Zidovudine, and Nevirapine), discovered unique interaction features between viral mutations induced by these drugs, and revealed the structural basis of such interactions.Bayesian model selection | free energy calculation | Markov chain Monte Carlo | molecular dynamics | mutation interactions H IV drug-resistance, which is caused by mutations of viral proteins that disrupt the drugs' binding but do not affect the viral survival, is a major hurdle that hinders a successful treatment of AIDS (1, 2). Due to the high rate and low fidelity of HIV replication, resistant strains quickly become dominant in a viral population under the selection pressure of a drug. By sequencing viral strains in the treated-patient isolates, genotypic data have been accumulated for the drugs targeting two viral enzymes, protease and reverse transcriptase, that are essential to the virus's replication. Because each mutation of the viral protein is not equally important for drug resistance, the observed, complicated mutation patterns are difficult to interpret (3, 4) and are limited in helping physicians design the best therapeutic regimen for a patient (5) (Fig. 1A).In past decades, many statistical learning methods (3, 4, 67-8) have been employed to help predict phenotypes from genotypes. There are also rule-based systems that infer drug-resistance levels from sequence information such as the Stanford University HIV Drug Resistance Database (Stanford HIVdb). However, these methods provide little insight on the genetic and molecular basis of drug resistance and often give inconsistent results when analyzing the same input mutation data (4, 6).In the present study, we investigated the problem of mutation interactions of the HIV induced by a certain drug treatment. Using a unique probabilistic model, we first detect resistant mutation combinations (9) and infer the interaction dependence structure of these combinations. Then, we use molecular dynamics (MD) simulations to reveal the molecular basis of how these mutations interact with each other to interfere with the drugs' binding. We have shown that our procedure is applicable to different antiretroviral drugs treating different types of HIV infection by analyzing the sequence mutations induced by three different drug treatments: a...

Water deficit is one of the main limiting factors in apple ( × Borkh.) cultivation. Root architecture plays an important role in the drought tolerance of plants; however, research efforts to improve drought tolerance of apple trees have focused on aboveground targets. Due to the difficulties associated with visualization and data analysis, there is currently a poor understanding of the genetic players and molecular mechanisms involved in the root architecture of apple trees under drought conditions. We previously observed that MdMYB88 and its paralog MdMYB124 regulate apple tree root morphology. In this study, we found that MdMYB88 and MdMYB124 play important roles in maintaining root hydraulic conductivity under long-term drought conditions and therefore contribute toward adaptive drought tolerance. Further investigation revealed that MdMYB88 and MdMYB124 regulate root xylem development by directly binding and promoters and thus influence expression of their target genes under drought conditions. In addition, MdMYB88 and MdMYB124 were shown to regulate the deposition of cellulose and lignin root cell walls in response to drought. Taken together, our results provide novel insights into the importance of MdMYB88 and MdMYB124 in root architecture, root xylem development, and secondary cell wall deposition in response to drought in apple trees.