Anthocyanin accumulation is one measure of ripening in the strawberry (Fragaria ananassa Duch.), a non-climacteric fruit. Neither aminoethoxyvinylglycine, an inhibitor of 1-aminocyclopropane carboxylic acid synthase, nor inhibitors of ethylene action (silver, norbornadiene) affected anthocyanin accumulation in ripening fruit. When the achenes were removed from one half of an unripe fruit there was an accelerated accumulation of anthocyanin and induction of phenylalanine ammonia lyase on the de-achened portion of the ripening fruit. These effects of achene removal could be prevented by the application of the synthetic auxins 1-naphthaleneacetic acid or 2,4-dichlorophenoxyacetic acid to the de-achened surface. The introduction of 1-naphthalene acetic acid into intact unripe strawberry fruit through the peduncle delayed their subsequent ripening, as measured by the accumulation of anthocyanin, loss of chlorophyll and decrease in firmness. These findings suggest that the decline in the concentration of auxin in the achenes as strawberry fruit mature modulates the rate of fruit ripening.

The structure of auxin-binding protein 1 (ABP1) from maize has been determined at 1.9 A Ê resolution, revealing its auxin-binding site. The structure con®rms that ABP1 belongs to the ancient and functionally diverse germin/seed storage 7S protein superfamily. The binding pocket of ABP1 is predominantly hydrophobic with a metal ion deep inside the pocket coordinated by three histidines and a glutamate. Auxin binds within this pocket, with its carboxylate binding the zinc and its aromatic ring binding hydrophobic residues including Trp151. There is a single disul®de between Cys2 and Cys155. No conformational rearrangement of ABP1 was observed when auxin bound to the protein in the crystal, but examination of the structure reveals a possible mechanism of signal transduction.

The eects of elevated CO 2 concentrations on stomatal movement, anion-and K + -channel activities were examined in guard cells from epidermal strips of Vicia faba. Membrane voltage was measured using intracellular, double-barrelled microelectrodes and ionchannel currents were recorded under voltage clamp during exposure to media equilibrated with ambient (350 ll á l A1 ), 1000 ll á l A1 and 10 000 ll á l A1 CO 2 in 20% O 2 and 80% N 2 . The addition of 1000 ll á l A1 CO 2 to the bathing solution caused stomata to close with a halftime of approx. 40 min, and with 10 000 ll á l A1 CO 2 closure occurred with a similar time course. Under voltage clamp, exposure to 1000 ll á l A1 and 10 000 ll á l A1 CO 2 resulted in a rapid increase (mean, 1.5 0.2-fold, n = 8; range 1.3-to 2.5-fold) in the magnitude of current carried by outward-rectifying K + channels (I K,out ). The eect of CO 2 on I K,out was essentially complete within 30 s and was independent of clamp voltage, but was associated with 25±40% (mean, 30 4%) decrease in the halftime for current activation. Exposure to CO 2 also resulted in a four-fold increase in background current near the free-running membrane voltage, recorded as the instantaneous current at the start of depolarising and hyperpolarising voltage steps, and a decrease in the magnitude of current carried by inward-rectifying K + channels (I K,in ). The eect of CO 2 on I K,in was generally slower than on I K,out ; it was allied with a transient acceleration of its activation kinetics during the ®rst 60±120 s of treatment; and it was associated with a negative shift in the voltage-sensitivity of gating over a period of 3±5 min. Measurements carried out to isolate the background currents attributable to anion channels (I Cl ), using tetraethylammonium chloride and CsCl, showed that CO 2 also stimulated I Cl and dramatically altered its relaxation kinetics. Within the timeframe of CO 2 action at the membrane, no signi®cant eect was observed on cytosolic pH, measured using the¯uorescent dye 2¢,7¢-bis-(2-carboxyethyl)-5,6-carboxy¯ourescein (BCECF) and ratio¯uorescence microphotometry. These results are broadly consistent with the pattern of guard-cell response to abscisic acid, and indicate that guard cells control both anion and K + channels to achieve net solute loss in CO 2 . By contrast with the eects of abscisic acid, however, the data indicate that CO 2 action is not mediated through changes in cytosolic pH and thereby implicate new and, as yet, unidenti®ed pathway(s) for channel regulation in the guard cells.Abbreviations: ABA = abscisic acid; BCECF = 2¢,7¢-bis-(2-carboxyethyl)-5,6-carboxy¯ourescein; I K,out , I K,in = outward-, inward-rectifying K + channel; pH i = cytosolic pH; TEA = tetraethylammonium chlo-ride; t 1/2 = activation halftime Correspondence to:

The electrical response of Zea mays protoplasts to different auxins and to antibodies raised against an ER‐located auxin binding protein from maize (Zm‐ERabp1), was investigated using the patch‐clamp technique (whole‐cell configuration). Following a lag‐phase of 30–40 seconds, indole‐3‐acetic acid and 1‐naphthylacetic acid induced an outwardly directed current of positive charge in a concentration‐dependent manner. This current was further increased by the fungal toxin fusicoccin (FC). The current was observed only in the presence of Mg2+‐ATP in the patch‐pipette and was abolished after addition of erythrosin B, an inhibitor of H+‐ATPase, to the protoplasts indicating that the plasma membrane H+‐ATPase is activated by auxins and fusicoccin. Addition of antibodies directed against Zm‐ERabp1 abolished the current induced by auxins, without affecting the response of protoplasts to fusicoccin. Antibodies directed against a peptide representing part of the putative auxin binding domain of Zm‐ERabp1 showed auxin agonist activity, stimulating an outwardly directed membrane current in the absence of auxin. These results suggest that (i) Zm‐ERabp1 or antigenically related proteins represent a site for auxin perception through which the plasma membrane H+‐ATPase is activated, and (ii) that the activation of the H+‐ATPase by such proteins is initiated from outside the plasma membrane.

The major auxin-binding protein in maize membranes is thought to function as a physiological receptor. From earlier information, including the use of site-directed irreversible inhibitors, several of the amino acids likely to form part of the active auxin-binding site were provisionally assigned. Inspection of the amino acid sequence of the auxinbini protein showed a short region containing all but one of these ano acids. We find that antisera raised against a synthetic peptide encompassing this region recognize all isoforms of the mize auxin-binding protein together with homologous polypeptides in other species. We further find that the antibodies hyperpolarIze protoplast transmembrane potential in an auxin-like manner. We conclude that these antibodies display auxin agonist activity and that we have identified an essential portion of the auxin-binding site. 7208The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.