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Besides manipulating nitrate uptake and allocation, nitrate transporters (NRTs) are also known to play crucial roles in pathogen defense and stress response. By blasting with the model NRT genes of poplar and Arabidopsis, a total of 408 gene members were identified from 5 maize inbred lines in which the number of NRTs ranged from 72 to 88. Phylogenetic analysis showed that the NRT genes of maize were classified into NRT1/PTR (NPF), NRT2 and NRT3 subfamilies, respectively. Marked divergence of the duplication patterns of NRT genes were identified, which may be a new basis for classification and identification of maize varieties. In terms of biotic stress, NRT2.5A showed an enhanced expression during the pathogen infection of Colletotrichum graminicola, while NRT1c4C was down-regulated, suggesting that maize NRT transporters may have both positive and negative roles in the disease resistance response. This work will promote the further studies of NRT gene families in maize, as well as be beneficial for further understanding of their potential roles in plant-pathogen interactions., Competing Interests: Declaration of Competing Interest The authors declare no conflict of interest., (Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.)

Background: The growing demand for food, combined with a strong social expectation for a diet produced with fewer conventional agrochemical inputs, has led to the development of new alternatives in plant protection worldwide. Among different possibilities, the stimulation of the plant innate immune system by chemicals represents a novel and promising way. The vectorization strategy of an active ingredient that we previously developed with fungicides can potentially extend to salicylic acid (SA) or its halogenated analogues., Results: Using the click chemistry method, six new conjugates combining SA or two mono- or di-halogenated analogues with L-glutamic acid or β-D-glucose via a 1,2,3-triazole nucleus have been synthesized. Conjugate 8a, which is derived from SA and glutamic acid, showed high phloem mobility in the Ricinus model, similar to that of SA alone despite a much higher steric hindrance. In vivo bioassays of the six conjugates against two maize pathogenic fungi Bipolaris maydis and Fusarium graminearum revealed that, unlike SA, the amino acid conjugate 8a with good phloem mobility exerted a protective effect not only locally at the application site, but also in distant stem tissues after foliar application. Moreover, compounds 8a and 8b induced up-regulation of both defense-related genes ZmNPR1 and ZmPR1 similar to their parent compounds upon challenge inoculation with B. maydis., Conclusion: The vectorization of salicylic acid or its halogenated derivatives by coupling them with an α-amino acid can be a promising strategy to stimulate SA-mediated plant defenses responses against pathogens outside the application site. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry., (© 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.)

Background: A new chemical control strategy for grapevine trunk diseases (GTDs) is to develop site-targeted fungicides to protect grapevine vascular tissues. Due to the complexity of GTDs, the effectiveness of a single method is limited. Investigation of the interactions between chemical and biological agents is an essential requirement for integrated control strategies. The effect of a phloem-mobile derivative of the fungicide fenpiclonil (SM 26) in combined use with the plant growth-promoting rhizobacteria, Paraburkholderia phytofirmans PsJN on the Neofusicoccum parvum strain Bourgogne (NpB) was evaluated., Results: SM 26 was found to be translocated to the shoot apices and roots of grapevines through both xylem and phloem after foliage application. In vitro studies demonstrated that SM 26 exhibited no inhibitory effect on the growth of PsJN and could be largely absorbed into the bacterial cells. In vivo evaluation showed that the combined use of SM 26 and PsJN was the most effective following artificial inoculation of NpB on the stems of rooted Chardonnay and Sauvignon cuttings. Finally, the expression of defence-related genes, including the genes associated with secondary metabolism (ANTS, PAL, STS, Vv17.3), defence proteins (GLUC, PR1, PGIP), redox status (GTS1) and ethylene synthesis (ACC), was found to be strongly upregulated in PsJN + SM 26 cotreated plants compared to non-treated plants (controls), especially for Chardonnay., Conclusion: The systemic profungicide SM 26 interacts with the biocontrol agent PsJN to stimulate some plant defence responses, and their combined use may present a potential integrated control strategy against GTDs. © 2020 Society of Chemical Industry., (© 2020 Society of Chemical Industry.)

Two Fabaceae exhibiting rapid osmocontractile pulvinar movements were used in this study because this activity is modified by natural auxin and dramatically by 2,4D. A short chain with a carboxylic group being required for auxinic properties, a critical point to analyze is whether the recently synthesized proherbicide ε-(2,4-dichlorophenoxyacetyl)-L-Lys (2-4D-L-Lys) maintains some biological activity despite the increase in length of the chain and the substitution of the carboxyl group by an α-amino acid function. No trace of 2,4D could be detected in the pulvinar tissues treated for 1 h with 2,4D-L-Lys. Complementary approaches (electrophysiology, pH measurements, use of plasma membrane vesicles) suggest that it was less efficient than 2,4D to activate the plasma membrane H + -ATPase (PM-H + -ATPase). However, it modified the various ion-driven reactions of Mimosa pudica and Cassia fasciculata pulvini in a similar way as 2,4D. Additionally, it was much more effective than fusicoccin to inhibit seismonastic movements of M. pudica leaves and, at low concentrations, to promote leaflet opening in dark, indicating that its mode of action is more complex than the only activation of the PM-H + -ATPase. Various substitutions on 2,4D-L-Lys affected its activity in correlation with the molecular descriptor "halogen ratio" of these derivatives. Conjugation with D-Lys also led to a decrease of pulvinar reaction, suggesting that 2,4D-Lys maintains the main signaling properties of 2,4D involved in pulvinar movements providing that the terminal zwitterion is in a suitable orientation. Our data guide future investigations on the effect of 2,4D and 2,4D-L-Lys on the vacuolar pump activity of motor cells., (Copyright © 2020 Elsevier Masson SAS. All rights reserved.)

Systemicity of agrochemicals is an advantageous property for controlling phloem sucking insects, as well as pathogens and pests not accessible to contact products. After the penetration of the cuticle, the plasma membrane constitutes the main barrier to the entry of an agrochemical into the sap flow. The current strategy for developing systemic agrochemicals is to optimize the physicochemical properties of the molecules so that they can cross the plasma membrane by simple diffusion or ion trapping mechanisms. The main problem with current systemic compounds is that they move everywhere within the plant, and this non-controlled mobility results in the contamination of the plant parts consumed by vertebrates and pollinators. To achieve the site-targeted distribution of agrochemicals, a carrier-mediated propesticide strategy is proposed in this review. After conjugating a non-systemic agrochemical with a nutrient (α-amino acids or sugars), the resulting conjugate may be actively transported across the plasma membrane by nutrient-specific carriers. By applying this strategy, non-systemic active ingredients are expected to be delivered into the target organs of young plants, thus avoiding or minimizing subsequent undesirable redistribution. The development of this innovative strategy presents many challenges, but opens up a wide range of exciting possibilities. © 2018 Society of Chemical Industry., (© 2018 Society of Chemical Industry.)

Producing quality food in sufficient quantity while using less agrochemical inputs will be one of the great challenges of the twenty-first century. One way of achieving this goal is to greatly reduce the doses of plant protection compounds by improving the targeting of pests to eradicate. Therefore, we developed a vectorization strategy to confer phloem mobility to fenpiclonil, a contact fungicide from the phenylpyrrole family used as a model molecule. It consists in coupling the antifungal compound to an amino acid or a sugar, so that the resulting conjugates are handled by active nutrient transport systems. The method of click chemistry was used to synthesize three conjugates combining fenpiclonil to glucose or glutamic acid with a spacer containing a triazole ring. Systemicity tests with the Ricinus model have shown that the amino acid promoiety was clearly more favorable to phloem mobility than that of glucose. In addition, the transport of the amino acid conjugate is carrier mediated since the derivative of the L series was about five times more concentrated in the phloem sap than its counterpart of the D series. The systemicity of the L-derivative is pH dependent and almost completely inhibited by the protonophore carbonyl cyanide 3-chlorophenylhydrazone (CCCP). These data suggest that the phloem transport of the L-derivative is governed by a stereospecific amino acid carrier system energized by the proton motive force.

Until now, specific inhibitors of sucrose carriers were not available. This led us to study the properties of the recently synthesized D-glucose-fenpiclonil conjugate (D-GFC). This large amphiphilic glucoside exhibited an extremely low phloem systemicity in contrast to L-amino acid-fenpiclonil conjugates. Using Ricinus seedlings, the effect of D-GFC on 0.5 mM [14C]sucrose (Suc), 3-O-[3H]methylglucose, and [3H]glutamine uptake by cotyledon tissues was compared with that of p-chloromercuribenzenesulfonic acid (PCMBS). D-GFC dramatically inhibited H+-Suc symport at the same concentrations as PCMBS (0.5 and 1 mM), but in contrast to the thiol reagent, it did not affect 3-O-methylglucose and glutamine transport, nor the acidification of the incubation medium by cotyledon tissues. Similarly, 0.5 mM D-GFC inhibited active Suc uptake by Vicia faba leaf tissues and by Saccharomyces cerevisiae cells transformed with AtSUC2, a gene involved in Suc phloem loading in Arabidopsis, by approximately 80%. The data indicated that D-GFC was a potent inhibitor of Suc uptake from the endosperm and of Suc phloem loading. It is the first chemical known to exhibit such specificity, at least in Ricinus, and this property permitted the quantification of the two routes involved in phloem loading of endogenous sugars after endosperm removal., (© The Author 2017. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

Background: Excessive agrochemical use poses significant threats to environmental safety and human health. Reducing pesticide use without reducing yield is necessary for sustainable agriculture. Therefore, we developed a vectorisation strategy to enhance agrochemical delivery through plant amino acid carriers., Results: In addition to a fenpiclonil conjugate recently described, three new amino acid conjugates were synthesised by coupling fenpiclonil to an l-α-amino acid. Phloem mobility of these conjugates, which exhibit different structures of the spacer arm introduced between fenpiclonil and the α-amino acid function, was studied using the Ricinus model. Conjugate L-14, which contains a triazole ring with the shortest amino acid chain, showed the best phloem systemicity among the four conjugates. By contrast, removing the triazole ring in the spacer arm did not improve systemicity. L-14 exhibited phloem systemicity at all reported pH values (pH values from 5.0 to 6.5) of the foliar apoplast, while acidic derivatives of fenpiclonil were translocated only at pH values near 5.0., Conclusion: The conjugates were recognised by a pH-dependent transporter system and translocated at distance in the phloem. They exhibited a broader phloem systemicity than fenpiclonil acidic derivatives within the pH value range of the foliar apoplast. © 2017 Society of Chemical Industry., (© 2017 Society of Chemical Industry.)

Early prediction of compound absorption by cells is of considerable importance in the building of an integrated scheme describing the impact of a compound on intracellular biological processes. In this scope, we study the structure-activity relationships of several benzoic acid-related phenolics which are involved in many plant biological phenomena (growth, flowering, allelopathy, defense processes). Using the partial least squares (PLS) regression method, the impact of molecular descriptors that have been shown to play an important role concerning the uptake of pharmacologically active compounds by animal cells was analyzed in terms of the modification of membrane potential, variations in proton flux, and inhibition of the osmocontractile reaction of pulvinar cells of Mimosa pudica leaves. The hydrogen bond donors (HBD) and hydrogen bond acceptors (HBA), polar surface area (PSA), halogen ratio (Hal ratio), number of rotatable bonds (FRB), molar volume (MV), molecular weight (MW), and molar refractivity (MR) were considered in addition to two physicochemical properties (logD and the amount of non-dissociated form in relation to pKa). HBD + HBA and PSA predominantly impacted the three biological processes compared to the other descriptors. The coefficient of determination in the quantitative structure-activity relationship (QSAR) models indicated that a major part of the observed seismonasty inhibition and proton flux modification can be explained by the impact of these descriptors, whereas this was not the case for membrane potential variations. These results indicate that the transmembrane transport of the compounds is a predominant component. An increasing number of implicated descriptors as the biological processes become more complex may reflect their impacts on an increasing number of sites in the cell. The determination of the most efficient effectors may lead to a practical use to improve drugs in the control of microbial attacks on plants.

Unsaturated amino acids (UnsAA) have been shown to affect the activity of various biological processes. However, their mode of action has been investigated poorly thus far. We show in this work that 2-amino-3-methyl-4-pentenoic acid (C2) and 2-amino-3-methyl-4-pentynoic acid (C3) structurally derived from isoleucine (Ile) exhibited a multisite action on plant cells. For one, C2 and C3 induced early modifications at the plasma membrane level, as shown by the hyperpolarization monitored by microelectrode implantation in the pulvinar cells of Mimosa pudica, indicating that these compounds are able to modify ionic fluxes. In particular, proton (H(+)) fluxes were modified, as shown by the pH rise monitored in the bathing medium of pulvinar tissues. A component of this effect may be linked to the inhibitory effect observed on the proton pumping and the vanadate-sensitive activity of the plasma membrane H(+)-ATPase monitored in plasma membrane vesicles (PMVs) purified from pulvinar tissues of M. pudica and leaf tissues of Beta vulgaris. This effect may explain, in part, the inhibitory effect of the compounds on the uptake capacity of sucrose and valine by B. vulgaris leaf tissues. In contrast, an unexpected action was observed in cell reactions, implicating ion fluxes and water movement. Indeed, the osmocontractile reactions of pulvini induced either by a mechanical shock in M. pudica or by dark and light signals in Cassia fasciculata were increased, indicating that, compared to Ile, these compounds may modify in a specific way the plasma membrane permeability to water and ions., (Copyright © 2016 Elsevier Masson SAS. All rights reserved.)

A study of the structure-activity relationship carried out on several benzoic acid-related phenolics indicates that this type of compounds hinders the osmocontractile reaction of pulvinar cells in the range of 0-100%. Tentatively, we tried to find a way that could explain this differential action. With this aim, the relationship between the inhibitory effect and important molecular physico-chemical parameters (namely lipophilicity and degree of dissociation) was drawn. In addition, the effect of a variety of these compounds was investigated on their capacity to modify the electrical transmembrane potential and induce modifications in proton fluxes. Finally, using plasma membrane vesicles purified from pulvinar tissues, we examined the effects of some selected compounds on the proton pump activity and catalytic activity of the plasma membrane H(+)-ATPase. Taken together, the results indicate that a modification of the molecular structure of phenolics may induce important variation in the activity of the compound on these early membrane events. Among the tested phenolics, salicylic acid (SA) and acetylsalicylic acid (ASA, aspirin) are of particuler note, as they showed atypical effects on the physiological processes studied., (Copyright © 2014 Elsevier Masson SAS. All rights reserved.)

L-meta-tyrosine is an herbicidal nonprotein amino acid isolated some years ago from fine fescue grasses and characterized by its almost immediate microbial degradation in soil (half-life <24 h). Nine monohalogenated or dihalogenated analogs of this allelochemical have been obtained through a seven-step stereoselective synthesis from commercial halogenated phenols. Bioassays showed a large range of biological responses, from a growth root inhibition of lettuce seedling similar to that noted with m-tyrosine [2-amino-3-(2-chloro-5-hydroxyphenyl)propanoic acid or compound 8b] to an increase of the primary root growth concomitant with a delay of secondary root initiation [2-amino-3-[2-fluoro-5-hydroxy-3-(trifluoromethyl)phenyl]propanoic acid or compound 8h]. Compound 8b was slightly less degraded than m-tyrosine in the nonsterilized nutritive solution used for lettuce development, while the concentration of compound 8h remained unchanged for at least 2 weeks. These data indicate that it is possible to manipulate both biological properties and degradation of m-tyrosine by halogen addition.

Background: Owing to the complexity of soil composition, accurate predictions of both apoplastic systemicity of lipophilic xenobiotics and their leaching from the soil are made difficult. Therefore, a non-destructive method to assess directly these two components of the spatial behaviour of soil-applied phytochemicals is needed., Results: The plant selected was a dwarf tomato, which can exude an abundant apoplastic fluid through large stomata for several months. The feasibility and reliability of the method were assayed using three triazoles exhibiting different log D values. HPLC-MS analyses indicated that triadimenol (log D = 2.97) was clearly the most mobile compound within the apoplast, especially its diastereoisomer A. Propiconazole (log D = 3.65) and penconazole (log D = 4.64) exhibited a similar low systemicity. The data remained the same when the three fungicides were applied together on the soil. Long time-course studies (1.5 months) of penconazole behaviour indicated that, in contrast to leaching, which decrease sharply, root-to-shoot translocation remained almost unchanged during the whole experiment, in spite of the high lipophilicity of this fungicide., Conclusion: This method must contribute to a better knowledge of the behaviour of commercial soil-applied phytochemicals. It can also be used to screen new xenobiotics within strategies to satisfy environmental requirements., (Copyright © 2011 Society of Chemical Industry.)

Despite its important functions in plant physiology and defense, the membrane transport mechanism of salicylic acid (SA) is poorly documented due to the general assumption that SA is taken up by plant cells via the ion trap mechanism. Using Ricinus communis seedlings and modeling tools (ACD LogD and Vega ZZ softwares), we show that phloem accumulation of SA and hydroxylated analogs is completely uncorrelated with the physicochemical parameters suitable for diffusion (number of hydrogen bond donors, polar surface area, and, especially, LogD values at apoplastic pHs and Delta LogD between apoplast and phloem sap pH values). These and other data (such as accumulation in phloem sap of the poorly permeant dissociated form of monohalogen derivatives from apoplast and inhibition of SA transport by the thiol reagent p-chloromercuribenzenesulfonic acid [pCMBS]) lead to the following conclusions. As in intestinal cells, SA transport in Ricinus involves a pH-dependent carrier system sensitive to pCMBS; this carrier can translocate monohalogen analogs in the anionic form; the efficiency of phloem transport of hydroxylated benzoic acid derivatives is tightly dependent on the position of the hydroxyl group on the aromatic ring (SA corresponds to the optimal position) but moderately affected by halogen addition in position 5, which is known to increase plant defense. Furthermore, combining time-course experiments and pCMBS used as a tool, we give information about the localization of the SA carrier. SA uptake by epidermal cells (i.e. the step preceding the symplastic transport to veins) insensitive to pCMBS occurs via the ion-trap mechanism, whereas apoplastic vein loading involves a carrier-mediated mechanism (which is targeted by pCMBS) in addition to diffusion.

The ability of exogenous salicylic acid (SA) to accumulate in castor bean (Ricinus communis) phloem was evaluated by HPLC and liquid scintillation spectrometry analyses of phloem sap collected from the severed apical part of seedlings. Time-course experiments indicated that SA was transported to the root system via the phloem and redistributed upward in small amounts via the xylem. This helps to explain the peculiarities of SA distribution within the plant in response to biotic stress and exogenous SA application. Phloem loading of SA at 1, 10, or 100 microm was dependent on the pH of the cotyledon incubating solution, and accumulation in the phloem sap was the highest (about 10-fold) at the most acidic pH values tested (pH 4.6 and 5.0). As in animal cells, SA uptake still occurred at pH values close to neutrality (i.e. when SA is only in its dissociated form according to the calculations made by ACD LogD suite software). The analog 3,5-dichlorosalicylic acid, which is predicted to be nonmobile according to the models of Bromilow and Kleier, also moved in the sieve tubes. These discrepancies and other data may give rise to the hypothesis of a possible involvement of a pH-dependent carrier system translocating aromatic monocarboxylic acids in addition to the ion-trap mechanism.

A new acidic derivative of the fungicide fenpiclonil was synthesized containing a methyl group on the alpha-position of the carboxyl function of N-carboxymethyl-3-cyano-4-(2,3-dichlorophenyl)pyrrole. The phloem mobility of the resulting N-(1-carboxyethyl)-3-cyano-4-(2,3-dichlorophenyl)pyrrole was comparable with that of the former compound, but was higher at external pH 5.0. Unlike the derivatives previously synthesized, it was comparable with fenpiclonil in its fungicidal activity against the pathogenic fungus Eutypa lata., (Copyright 2004 Society of Chemical Industry.)

A series of derivatives of the phenylpyrrole fungicide fenpiclonil was synthesized in which a carboxyl group was present at various sites of this non-phloem-mobile molecule. Using the Kleier model, all these acidic analogues were predicted to be moderately phloem-mobile, especially the N-substituted derivatives. One of these latter molecules, N-carboxymethyl-3-cyano-4-(2,3-dichlorophenyl)pyrrole, exhibited some fungicidal activity on the pathogenic fungus Eutypa lata, and was then tested as a phloem-mobile pesticide in the Ricinus system. The compound was indeed mobile in the sieve tubes and was not degraded to fenpiclonil in the phloem sap under our experimental conditions. Its concentration in the sap was closely correlated to the percentage of the undissociated form of the molecule in the external medium, and was similar under acidic conditions (external pH 4.6-5.0) to that of the herbicide glyphosate.

Plants are attacked, not only by various microorganisms, but also by other enemies, such as molluscs, nematods, mites, and insects. They have evolved complex and efficient mechanisms to defend themselves against pathogens (hypersensitive response, systemic acquired resistance) and herbivores (release of volatile compounds that attract predators of the herbivores, accumulation of proteinase inhibitors). Yet, the confrontation of the plants with their invaders can also turn to the advantage of the latter. In the past, the attacks of crops regularly brought about dramatic economic losses. From the World War II onwards, the development of organic chemistry associated with a growing awareness of the problems of agriculture has resulted in the production of a constantly growing number of plant protection products. They are currently divided into about ten classes, the herbicides, fungicides, and insecticides-acaricides making up more than 90% of the world market. Most of the agrochemical products put on the market over these last three decades are used in relatively low doses and have a more favourable toxicological and ecotoxicological profile than those of the former pesticides, many of which are now withdrawn from the market. Several more or less recent families are derivatives of metabolites from various organisms. Thus, the improvement achieved in the protection of crops is outstanding. However, one on the main side-effect is an environmental imbalance that has entailed a dependency on agrochemicals. Quite judiciously, alternative strategies (elicitors, genetic engineering, etc.) have been initiated or developed over the last decade.

In a previous paper we have shown that epsilon-(phenoxyalkanecarboxylyl)-L-Lys conjugates are potent inhibitors of amino acid transport systems and that it is possible to modulate the uptake inhibition by hydrophobic or hydrophilic additions in the 4-position of the aromatic ring (J.F. Chollet, C. Delétage, M. Faucher, L. Miginiac, J.L. Bonnemain [1997] Biochem Biophys Acta 1336: 331-341). In this report we demonstrate that epsilon-(2,4-dichlorophenoxyacetyl)-L-Lys (2,4D-Lys), one of the largest molecules of the series and one of the most potent inhibitors, is a highly permeant conjugate. Uptake of 2,4D-Lys by broad bean (Vicia faba) leaf discs is mediated by an active carrier system (Km1 = 0.2 mM; Vmax1 = 2.4 nmol x cm(-2) x h(-1) at pH 5.0) complemented by an important diffusive component. Among the compounds tested (neutral, basic, and acidic amino acids, auxin, glutathione, and sugars), only the aromatic amino acids clearly compete with 2,4D-Lys. The conjugate accumulates in the vein network, is exported toward the growing organs, and exhibits a distribution pattern different from that of the herbicide moiety. However, over time 2,4D-Lys progressively splits into 2,4D and lysine. Analyses by high-performance liquid chromatography and liquid scintillation spectrometry of the phloem sap collected from the castor bean system, used as a systemy test, indicate decreasing capacities of 2,4D, 2,4D-Lys, and glyphosate, respectively, to move from the epidermis cell wall to the sieve element. Our results show that it is possible to design synthesis of large-size xenobiotics (approximately 350 D) with a lipophilic pole, exhibiting high mobility within the vascular system.

The synthesis of several derivatives of a xenobiotic (phenoxyalkanecarboxylic acid) and an alpha-amino acid (L-lysine) is described. Various substituents were introduced in the aromatic ring of the phenoxyalkanecarboxylic acid and the side-chain was modified. They were tested for their effect on the transport of a neutral (L-threonine), an acidic (L-glutamic acid) and a basic (L-lysine) amino acid, and a sugar (sucrose). All compounds markedly inhibited threonine uptake by leaf tissues of Vicia faba L.--and more specifically phloem loading--with two exceptions, when the aromatic ring was substituted in the 4-position by a primary amino group or when D-lysine was used instead of L-lysine. By contrast, the addition of a chlorine atom in the 4-position of the aromatic ring enhanced the inhibitory activity. Similar results were obtained for inhibition of glutamate uptake and, to a lesser extent, for lysine uptake. pH dependence of the inhibitory activity as well as electrophysiological data indicate that permeases mediating active transport of amino acids are the target of these conjugates. These, in addition to other data obtained with other xenobiotics, show that the amino acid carrier system is capable of recognizing a wide range of conjugates of various sizes, structures and octanol/water partition coefficients.

The transport of [14C]glycyl-glycine (Gly-Gly) has been characterized in leaf discs from mature exporting leaves of broad bean (Vicia faba L.). In terms of glycine (Gly) equivalents, the rate of transport of Gly-Gly was similar to that of Gly uptake. Uptake of Gly-Gly was localized mainly in the mesophyll cells, with little accumulation in the veins. It was optimal at pH 6.0, sensitive to thiol reagents and metabolic inhibitors, and exhibited a single saturable phase with an apparent Michaelis constant of 16 mM. Gly-Gly did not inhibit the uptake of labeled Gly. Addition of Gly-Gly induced a concentration-dependent pH rise in the medium, showing that peptide uptake is mediated with proton co-transport. Gly-Gly also induced a concentration-dependent transmembrane depolarization of mesophyll cells with an apparent Michaelis constant of 15 mM. This depolarization was followed by a transient hyperpolarization. When present at a 10-fold excess, various peptides and tripeptides were able to inhibit Gly-Gly uptake with the following decreasing order of efficiency: Gly-Gly-Gly = leucine-Gly > Gly-tyrosine > Gly-glutamine = Gly-glutamic acid > Gly-phenylalanine > Gly-threonine > Gly-aspartic acid = Gly-asparagine = aspartic acid-Gly. Gly inhibited the uptake of Gly-Gly only slightly, whereas tetraGly and the tripeptide glutathione were not inhibitory. The dipeptides inhibiting Gly-Gly uptake also induced changes in the transmembrane potential difference of mesophyll cells and were able to affect in a complex way the response normally induced by Gly-Gly. Altogether, the data demonstrate the existence of a low-affinity, broad-specificity H+/peptide co-transporter at the plasma membrane of mesophyll cells. The physiological importance of this transporter for the exchange of nitrogenous compounds in mature leaves remains to be determined, as do the details of the electrophysiological events induced by the dipeptides.