SummaryProtoplasts of corn coleoptiles and Arabidopsis hypocotyls respond to the plant hormone auxin with a rapid change in volume. We checked the effect of antibodies directed against epitopes of auxin-binding protein 1 from Arabidopsis thaliana (AtERabp1) and Zea mays (ZmERabp1), respectively. Antibodies raised against the C-terminus of AtERabp1 inhibited the response to auxin, while antibodies raised against a part of box a, the putative auxin-binding domain, induced a swelling response similar to that caused by auxin treatment. Synthetic C-terminal oligopeptides of ZmERabp1 also caused a swelling response. These effects occurred regardless of whether the experiments were carried out with homologous (anti-AtERabp1 antibodies on Arabidopsis protoplasts or anti-ZmERabp1 antibodies in maize protoplasts) or heterologous immunological tools. The results indicate that the auxin signal for protoplast swelling is perceived by extracellular ABP1.

SummaryThe transcript abundance of the K -channel gene ZMK1 (Zea mays K channel 1) in maize coleoptiles is controlled by the phytohormone auxin. Thus, ZMK1 is thought to function in auxin-regulated coleoptile elongation, as well as during gravitropism and phototropism. To investigate related growth phenomena in the dicotyledonous plant Arabidopsis thaliana, we screened etiolated seedlings for auxin-induced K -channel genes. Among the members of the Shaker-like K channels, we thereby identi®ed transcripts of the inward recti®ers, KAT1 (K transporter of Arabidopsis thaliana) and KAT2, to be upregulated by auxin. The phloem-associated KAT2 was localised in cotyledons and the apical part of etiolated seedlings. In contrast, the K -channel gene KAT1 was expressed in the cortex and epidermis of etiolated hypocotyls, as well as in¯ower stalks. Furthermore, KAT1 was induced by active auxins in auxin-sensitive tissues characterised by rapid cell elongation. Applying the patch-clamp technique to protoplasts of etiolated hypocotyls, we correlated the electrical properties of K currents with the expression pro®le of K -channel genes. In KAT1-knockout mutants, K currents after auxin stimulation were characterised by reduced amplitudes. Thus, this change in the electrical properties of the K -uptake channel in hypocotyl protoplasts resulted from an auxin-induced increase of active KAT1 proteins. The loss of KAT1-channel subunits, however, did not affect the auxin-induced growth rate of hypocotyls, pointing to compensation by residual, constitutive K transporters. From gene expression and electrophysiological data, we suggest that auxin regulation of KAT1 is involved in elongation growth of Arabidopsis. Furthermore, a role for KAT2 in the auxin-controlled vascular patterning of leaves is discussed.

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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.

Phospholipase A 2 and a particular isoform of lipoxygenase are synthesized and transferred to lipid bodies during the stage of triacylglycerol mobilization in germinating cucumber seedlings. Lipid body lipoxygenase (LBLOX) is post-translationally transported to lipid bodies without proteolytic modification. Fractionation of homogenates from cucumber cotyledons or transgenic tobacco leaves expressing LBLOX showed that a small but significant amount was detectable in the microsomal fraction. A b-barrel-forming N-terminal domain in the structure of LBLOX, as deduced from sequence data, was shown to be crucial for selective intracellular transport from the cytosol to lipid bodies. Although a specific signal sequence for targeting protein domains to the lipid bodies could not be established, it was evident that the b-barrel represents a membrane-binding domain that is functionally comparable with the C2 domains of mammalian phospholipases. The intact b-barrel of LBLOX was demonstrated to be sufficient to target in vitro a fusion protein of LBLOX b-barrel with glutathione S-transferase (GST) to lipid bodies. In addition, binding experiments on liposomes using lipoxygenase isoforms, LBLOX deletions and the GST-fusion protein confirmed the role of the b-barrel as the membrane-targeting domain. In this respect, the cucumber LBLOX differs from cytosolic isoforms in cucumber and from the soybean LOX-1. When the b-barrel of LBLOX was destroyed by insertion of an additional peptide sequence, its ability to target proteins to membranes was abolished.

Many aspects of plant development are regulated by antagonistic interactions between the plant hormones auxin and cytokinin, but the molecular mechanisms of this interaction are not understood. To test whether cytokinin controls plant development through inhibiting an early step in the auxin response pathway, we compared the effects of cytokinin with those of the dgt (diageotropica) mutation, which is known to block rapid auxin reactions of tomato (Lycopersicon esculentum) hypocotyls. Long-term cytokinin treatment of wild-type seedlings phenocopied morphological traits of dgt plants such as stunting of root and shoot growth, reduced elongation of internodes, reduced apical dominance, and reduced leaf size and complexity. Cytokinin treatment also inhibited rapid auxin responses in hypocotyl segments: auxin-stimulated elongation, H ϩ secretion, and ethylene synthesis were all inhibited by cytokinin in wild-type hypocotyl segments, and thus mimicked the impaired auxin responsiveness found in dgt hypocotyls. However, cytokinin failed to inhibit auxin-induced LeSAUR gene expression, an auxin response that is affected by the dgt mutation. In addition, cytokinin treatment inhibited the auxin induction of only one of two 1-aminocyclopropane-1-carboxylic acid synthase genes that exhibited impaired auxin inducibility in dgt hypocotyls. Thus, cytokinin inhibited a subset of the auxin responses impaired in dgt hypocotyls, suggesting that cytokinin blocks at least one branch of the DGT-dependent auxin response pathway.The balance between auxin and cytokinin controls a wide range of processes in plant development, including the formation of roots, shoots, and callus tissue in vitro (Skoog and Miller, 1957), the outgrowth of shoot axillary buds (Sachs and Thimann, 1967), and the formation of lateral roots (Wightman et al., 1980; Hinchee and Rost, 1986). Mutual control of active auxin and cytokinin pools, interactive control of gene expression, and posttranslational effects have been described as possible mechanisms underlying such physiological interactions (Coenen and Lomax, 1997). However, the relationship between classical hormone interactions at the physiological level and molecular auxin-cytokinin interactions is presently not well defined. Auxin-Cytokinin Interactions during Hypocotyl ElongationAuxin-cytokinin interactions can be observed in the elongation response of dicot hypocotyl segments. Auxin-induced elongation of sunflower (Helianthus annuus; DeRopp, 1956) and soybean (Glycine max; Vanderhoef et al., 1973; Vanderhoef and Stahl, 1975) hypocotyl segments is inhibited in the presence of cytokinins. This inhibition is detectable within 10 min of cytokinin addition and is not mediated by changes in ethylene synthesis (Vanderhoef and Stahl, 1975). Based on the time course of auxin-induced elongation growth in the presence and absence of cytokinin, Vanderhoef and Stahl (1975) proposed that cytokinin selectively inhibits the later phase of auxininduced elongation and may not influence rapid growth processes mediate...

Thirteen auxenic compounds were discovered in a screen of 10 000 compounds for auxin-like activity in Arabidopsis roots. One of the most potent substances was 2-(4-chloro-2-methylphenoxy)-N-(4-H-1,2,4-triazol-3-yl)acetamide (WH7) which shares similar structure to the known auxenic herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). A selected set of 20 analogues of WH7 was used to provide detailed information about the structure–activity relationship based on their efficacy at inhibiting and stimulating root and shoot growth, respectively, and at induction of gene expression. It was shown that WH7 acts in a genetically defined auxin pathway. These small molecules will extend the arsenal of substances that can be used to define auxin perception site(s) and to dissect subsequent signalling events.