The auxin influx carriers AUX1 and LAX3 are involved in auxin-ethylene interactions during apical hook development in Arabidopsis thaliana seedlings

Dark-grown dicotyledonous seedlings form a hook-like structure at the top of the hypocotyl, which is controlled by the hormones auxin and ethylene. Hook formation is dependent on an auxin signal gradient, whereas hook exaggeration is part of the triple response provoked by ethylene in dark-grown Arabidopsis seedlings. Several other hormones and light are also known to be involved in hook development, but the molecular mechanisms that lead to the initial installation of an auxin gradient are still poorly understood. In this study, we aimed to unravel the cross-talk between auxin and ethylene in the apical hook. Auxin measurements, the expression pattern of the auxin reporter DR5::GUS and the localization of auxin biosynthesis enzymes and influx carriers collectively indicate the necessity for auxin biosynthesis and efficient auxin translocation from the cotyledons and meristem into the hypocotyl in order to support proper hook development. Auxin accumulation in the meristem and cotyledons and in the hypocotyl is increased approximately 2-fold upon treatment with ethylene. In addition, a strong ethylene signal leads to enhanced auxin biosynthesis at the inner side of the hook. Finally, mutant analysis demonstrates that the auxin influx carrier LAX3 is indispensable for proper hook formation, whereas the auxin influx carrier AUX1 is involved in the hook exaggeration phenotype induced by ethylene.

Probing plant membranes with FM dyes: tracking, dragging or blocking?

Remarkable progress in various techniques of in vivo fluorescence microscopy has brought an urgent need for reliable markers for tracking cellular structures and processes. The goal of this manuscript is to describe unexplored effects of the FM (Fei Mao) styryl dyes, which are widely used probes that label processes of endocytosis and vesicle trafficking in eukaryotic cells. Although there are few reports on the effect of styryl dyes on membrane fluidity and the activity of mammalian receptors, FM dyes have been considered as reliable tools for tracking of plant endocytosis. Using plasma membrane-localized transporters for the plant hormone auxin in tobacco BY-2 and Arabidopsis thaliana cell suspensions, we show that routinely used concentrations of FM 4-64 and FM 5-95 trigger transient re-localization of these proteins, and FM 1-43 affects their activity. The active process of re-localization is blocked neither by inhibitors of endocytosis nor by cytoskeletal drugs. It does not occur in A. thaliana roots and depends on the degree of hydrophobicity (lipophilicity) of a particular FM dye. Our results emphasize the need for circumspection during in vivo studies of membrane proteins performed using simultaneous labelling with FM dyes.

Auxin transporters--why so many?

Interacting and coordinated auxin transporter actions in plants underlie a flexible network that mobilizes auxin in response to many developmental and environmental changes encountered by these sessile organisms. The independent but synergistic activity of individual transporters can be differentially regulated at various levels. This invests auxin transport mechanisms with robust functional redundancy and added auxin flow capacity when needed. An evolutionary perspective clarifies the roles of the different transporter groups in plant development. Mathematical and functional analysis of elements of auxin transport makes it possible to rationalize the relative contributions of members of the respective transporter classes to the localized auxin transport streams that then underlie both preprogrammed developmental changes and reactions to environmental stimuli.

The PIN-FORMED (PIN) protein family of auxin transporters

A review of the PIN auxin-efflux transporters, which have important roles in plant development.

Summary

The PIN-FORMED (PIN) proteins are secondary transporters acting in the efflux of the plant signal molecule auxin from cells. They are asymmetrically localized within cells and their polarity determines the directionality of intercellular auxin flow. PIN genes are found exclusively in the genomes of multicellular plants and play an important role in regulating asymmetric auxin distribution in multiple developmental processes, including embryogenesis, organogenesis, tissue differentiation and tropic responses. All PIN proteins have a similar structure with amino- and carboxy-terminal hydrophobic, membrane-spanning domains separated by a central hydrophilic domain. The structure of the hydrophobic domains is well conserved. The hydrophilic domain is more divergent and it determines eight groups within the protein family. The activity of PIN proteins is regulated at multiple levels, including transcription, protein stability, subcellular localization and transport activity. Different endogenous and environmental signals can modulate PIN activity and thus modulate auxin-distribution-dependent development. A large group of PIN proteins, including the most ancient members known from mosses, localize to the endoplasmic reticulum and they regulate the subcellular compartmentalization of auxin and thus auxin metabolism. Further work is needed to establish the physiological importance of this unexpected mode of auxin homeostasis regulation. Furthermore, the evolution of PIN-based transport, PIN protein structure and more detailed biochemical characterization of the transport function are important topics for further studies.

Subcellular homeostasis of phytohormone auxin is mediated by the ER-localized PIN5 transporter

The plant signalling molecule auxin provides positional information in a variety of developmental processes by means of its differential distribution (gradients) within plant tissues. Thus, cellular auxin levels often determine the developmental output of auxin signalling. Conceptually, transmembrane transport and metabolic processes regulate the steady-state levels of auxin in any given cell. In particular, PIN auxin-efflux-carrier-mediated, directional transport between cells is crucial for generating auxin gradients. Here we show that Arabidopsis thaliana PIN5, an atypical member of the PIN gene family, encodes a functional auxin transporter that is required for auxin-mediated development. PIN5 does not have a direct role in cell-to-cell transport but regulates intracellular auxin homeostasis and metabolism. PIN5 localizes, unlike other characterized plasma membrane PIN proteins, to endoplasmic reticulum (ER), presumably mediating auxin flow from the cytosol to the lumen of the ER. The ER localization of other PIN5-like transporters (including the moss PIN) indicates that the diversification of PIN protein functions in mediating auxin homeostasis at the ER, and cell-to-cell auxin transport at the plasma membrane, represent an ancient event during the evolution of land plants.

Excretion of polyamines in alfalfa and tobacco suspension-cultured cells and its possible role in maintenance of intracellular polyamine contents

Changes in polyamines (PAs) in cells and cultivation media of alfalfa (Medicago sativa L.) and tobacco bright yellow 2 (BY-2) (Nicotiana tabacum L.) cell suspension cultures were studied over their growth cycles. The total content of PAs (both free and conjugated forms) was nearly 10 times higher in alfalfa, with high level of free putrescine (Put) (in exponential growth phase it represented about 65-73% of the intracellular Put pool). In contrast, the high content of soluble Put conjugates was found in tobacco cells (in exponential phase about 70% of the intracellular Put). Marked differences occurred in the amount of PAs excreted into the cultivation medium: alfalfa cells excreted at the first day after inoculation 2117.0, 230.5, 29.0 and 88.0 nmol g(-1) of cell fresh weight (FW) of Put, spermidine (Spd), spermine (Spm) and cadaverine (Cad), respectively, while at the same time tobacco cells excreted only small amount of Put and Spd (12.7 and 2.4 nmol g(-1) FW, respectively). On day 1 the amounts of Put, Spd, Spm and Cad excreted by alfalfa cells represented 21, 38, 12 and 15% of the total pool (intra- plus extra-cellular contents) of Put, Spd, Spm and Cad, respectively. In the course of lag-phase and the beginning of exponential phase the relative contents of extracellular PAs continually decreased (with the exception of Cad). On day 10, the extracellular Put, Spd, Spm and Cad still represented 11.3, 10.9, 2.1 and 27% of their total pools. The extracellular PAs in tobacco cells represented from day 3 only 0.1% from their total pools. The possible role of PA excretion into the cultivation medium in maintenance of intracellular PA contents in the cells of the two cell culture systems, differing markedly in growth rate and PA metabolism is discussed.

Functional characterization of PaLAX1, a putative auxin permease, in heterologous plant systems

We have isolated the cDNA of the gene PaLAX1 from a wild cherry tree (Prunus avium). The gene and its product are highly similar in sequences to both the cDNAs and the corresponding protein products of AUX/LAX-type genes, coding for putative auxin influx carriers. We have prepared and characterized transformed Nicotiana tabacum and Arabidopsis thaliana plants carrying the gene PaLAX1. We have proved that constitutive overexpression of PaLAX1 is accompanied by changes in the content and distribution of free indole-3-acetic acid, the major endogenous auxin. The increase in free indole-3-acetic acid content in transgenic plants resulted in various phenotype changes, typical for the auxin-overproducing plants. The uptake of synthetic auxin, 2,4-dichlorophenoxyacetic acid, was 3 times higher in transgenic lines compared to the wild-type lines and the treatment with the auxin uptake inhibitor 1-naphthoxyacetic acid reverted the changes caused by the expression of PaLAX1. Moreover, the agravitropic response could be restored by expression of PaLAX1 in the mutant aux1 plants, which are deficient in auxin influx carrier activity. Based on our data, we have concluded that the product of the gene PaLAX1 promotes the uptake of auxin into cells, and, as a putative auxin influx carrier, it affects the content and distribution of free endogenous auxin in transgenic plants.

Polar transport of the plant hormone auxin - the role of PIN-FORMED (PIN) proteins

The PIN-FORMED (PIN) protein family is a group of plant transmembrane proteins with a predicted function as secondary transporters. PINs have been shown to play a rate-limiting role in the catalysis of efflux of the plant growth regulator auxin from cells, and their asymmetrical cellular localization determines the direction of cell-to-cell auxin flow. There is a functional redundancy of PINs and their biochemical activity is regulated at many levels. PINs constitute a flexible network underlying the directional auxin flux (polar auxin transport) which provides cells in any part of the plant body with particular positional and temporal information. Thus, the PIN network, together with downstream auxin signalling system(s), coordinates plant development. This review summarizes recent progress in the elucidation of the role of PIN proteins in polar auxin transport at the cellular level, with emphasis on their structure and evolution and regulation of their function.

PIN proteins perform a rate-limiting function in cellular auxin efflux

Intercellular flow of the phytohormone auxin underpins multiple developmental processes in plants. Plant-specific pin-formed (PIN) proteins and several phosphoglycoprotein (PGP) transporters are crucial factors in auxin transport-related development, yet the molecular function of PINs remains unknown. Here, we show that PINs mediate auxin efflux from mammalian and yeast cells without needing additional plant-specific factors. Conditional gain-of-function alleles and quantitative measurements of auxin accumulation in Arabidopsis and tobacco cultured cells revealed that the action of PINs in auxin efflux is distinct from PGP, rate-limiting, specific to auxins, and sensitive to auxin transport inhibitors. This suggests a direct involvement of PINs in catalyzing cellular auxin efflux.

Auxin inhibits endocytosis and promotes its own efflux from cells

One of the mechanisms by which signalling molecules regulate cellular behaviour is modulating subcellular protein translocation. This mode of regulation is often based on specialized vesicle trafficking, termed constitutive cycling, which consists of repeated internalization and recycling of proteins to and from the plasma membrane. No such mechanism of hormone action has been shown in plants although several proteins, including the PIN auxin efflux facilitators, exhibit constitutive cycling. Here we show that a major regulator of plant development, auxin, inhibits endocytosis. This effect is specific to biologically active auxins and requires activity of the Calossin-like protein BIG. By inhibiting the internalization step of PIN constitutive cycling, auxin increases levels of PINs at the plasma membrane. Concomitantly, auxin promotes its own efflux from cells by a vesicle-trafficking-dependent mechanism. Furthermore, asymmetric auxin translocation during gravitropism is correlated with decreased PIN internalization. Our data imply a previously undescribed mode of plant hormone action: by modulating PIN protein trafficking, auxin regulates PIN abundance and activity at the cell surface, providing a mechanism for the feedback regulation of auxin transport.

Do phytotropins inhibit auxin efflux by impairing vesicle traffic?

Phytotropins such as 1-N-naphthylphthalamic acid (NPA) strongly inhibit auxin efflux, but the mechanism of this inhibition remains unknown. Auxin efflux is also strongly decreased by the vesicle trafficking inhibitor brefeldin A (BFA). Using suspension-cultured interphase cells of the BY-2 tobacco (Nicotiana tabacum L. cv Bright-Yellow 2) cell line, we compared the effects of NPA and BFA on auxin accumulation and on the arrangement of the cytoskeleton and endoplasmic reticulum (ER). The inhibition of auxin efflux (stimulation of net accumulation) by both NPA and BFA occurred rapidly with no measurable lag. NPA had no observable effect on the arrangement of microtubules, actin filaments, or ER. Thus, its inhibitory effect on auxin efflux was not mediated by perturbation of the cytoskeletal system and ER. BFA, however, caused substantial alterations to the arrangement of actin filaments and ER, including a characteristic accumulation of actin in the perinuclear cytoplasm. Even at saturating concentrations, NPA inhibited net auxin efflux far more effectively than did BFA. Therefore, a proportion of the NPA-sensitive auxin efflux carriers may be protected from the action of BFA. Maximum inhibition of auxin efflux occurred at concentrations of NPA substantially below those previously reported to be necessary to perturb vesicle trafficking. We found no evidence to support recent suggestions that the action of auxin transport inhibitors is mediated by a general inhibition of vesicle-mediated protein traffic to the plasma membrane.

Auxin efflux carrier activity and auxin accumulation regulate cell division and polarity in tobacco cells

Division and growth of most types of in vitro-cultured plant cells require an external source of auxin. In such cultures, the ratio of external to internal auxin concentration is crucial for the regulation of the phases of the standard growth cycle. In this report the internal concentration of auxin in suspension-cultured cells of Nicotiana tabacum L., strain VBI-0, was manipulated either (i) by increasing 10-fold the normal concentration of 1-naphthaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid in the external medium; or (ii) by addition 1-N-naphthylphthalamic acid (NPA; an inhibitor of auxin efflux and of auxin efflux carrier traffic). Both treatments delayed the onset of cell division for 6-7 days without loss of cell viability. In both cases, cell division activity subsequently resumed coincident with a reduction in the ability of cells to accumulate [(3)H]NAA from an external medium. Following renewed cell division, a significant proportion of the NPA-treated cells but not those grown at high auxin concentration, exhibited changes in the orientation of new cell divisions and loss of polarity. We conclude that cell division, but not cell elongation, is prevented when the internal auxin concentration rises above a critical threshold value and that the directed traffic of auxin efflux carriers to the plasma membrane may regulate the orientation of cell divisions.