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In plants, transport processes are important for the reallocation of defence compounds to protect tissues of high value, as demonstrated in the plant model Arabidopsis, in which the major defence compounds, glucosinolates, are translocated to seeds on maturation. The molecular basis for long-distance transport of glucosinolates and other defence compounds, however, remains unknown. Here we identify and characterize two members of the nitrate/peptide transporter family, GTR1 and GTR2, as high-affinity, proton-dependent glucosinolate-specific transporters. The gtr1 gtr2 double mutant did not accumulate glucosinolates in seeds and had more than tenfold over-accumulation in source tissues such as leaves and silique walls, indicating that both plasma membrane-localized transporters are essential for long-distance transport of glucosinolates. We propose that GTR1 and GTR2 control the loading of glucosinolates from the apoplasm into the phloem. Identification of the glucosinolate transporters has agricultural potential as a means to control allocation of defence compounds in a tissue-specific manner.

Plants protect themselves against herbivory with a diverse array of repellent or toxic secondary metabolites. However, many herbivorous insects have developed counteradaptations that enable them to feed on chemically defended plants without apparent negative effects. Here, we present evidence that larvae of the specialist insect, Pieris rapae (cabbage white butterfly, Lepidoptera: Pieridae), are biochemically adapted to the glucosinolatemyrosinase system, the major chemical defense of their host plants. The defensive function of the glucosinolate-myrosinase system results from the toxic isothiocyanates that are released when glucosinolates are hydrolyzed by myrosinases on tissue disruption. We show that the hydrolysis reaction is redirected toward the formation of nitriles instead of isothiocyanates if plant material is ingested by P. rapae larvae, and that the nitriles are excreted with the feces. The ability to form nitriles is due to a larval gut protein, designated nitrile-specifier protein, that by itself has no hydrolytic activity on glucosinolates and that is unrelated to any functionally characterized protein. Nitrile-specifier protein appears to be the key biochemical counteradaptation that allows P. rapae to feed with impunity on plants containing glucosinolates and myrosinases. This finding sheds light on the ecology and evolution of plant-insect interactions and suggests novel highly selective pest management strategies. O ne of the best-studied groups of plant defense compounds are the glucosinolates (Fig. 1), amino acid-derived thioglycosides found in several plant families (1), including the agriculturally important crops of the Brassicaceae such as oilseed rape, cabbage, and broccoli and the model plant Arabidopsis thaliana (2). Glucosinolates cooccur with myrosinases (thioglucoside glucohydrolases, EC 3.2.3.1), and together these two components constitute an activated plant defense system known as the ''mustard oil bomb'' (3). On tissue damage, the nontoxic glucosinolates are hydrolyzed by myrosinases into biologically active derivatives (Fig. 1 A). The outcome of the hydrolysis reaction depends on the structure of the glucosinolate side chain and the reaction conditions (4). The most common class of hydrolysis products, isothiocyanates (mustard oils), has frequently been shown to be highly toxic to both generalist and specialist insect herbivores (5, 6).Despite the toxicity of isothiocyanates, several lepidopteran insect species use glucosinolate-and myrosinase-containing plants as hosts. Although the neurophysiological bases of host plant choice in these species have been studied extensively (7), relatively little is known about how they overcome the toxicity of their host plants. Among the well known specialist insect herbivores on glucosinolate-containing plants, Pieris rapae is one of the most abundant butterflies in Northern and Central Europe, and it has recently also become a pest in North America. P. rapae has been a model insect for studying herbivore host selection (7,8), but the bioc...

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Root hairs are single cells that develop by tip growth and are specialized in the absorption of nutrients. Their cell walls are composed of polysaccharides and hydroxyproline-rich glycoproteins (HRGPs) that include extensins (EXTs) and arabinogalactan-proteins (AGPs). Proline hydroxylation, an early posttranslational modification of HRGPs that is catalyzed by prolyl 4-hydroxylases (P4Hs), defines the subsequent O-glycosylation sites in EXTs (which are mainly arabinosylated) and AGPs (which are mainly arabinogalactosylated). We explored the biological function of P4Hs, arabinosyltransferases, and EXTs in root hair cell growth. Biochemical inhibition or genetic disruption resulted in the blockage of polarized growth in root hairs and reduced arabinosylation of EXTs. Our results demonstrate that correct O-glycosylation on EXTs is essential for cell-wall self-assembly and, hence, root hair elongation in Arabidopsis thaliana.

Camalexin (3-thiazol-2-yl-indole) is an indole alkaloid phytoalexin produced by Arabidopsis thaliana that is thought to be important for resistance to necrotrophic fungal pathogens, such as Alternaria brassicicola and Botrytis cinerea. It is produced from Trp, which is converted to indole acetaldoxime (IAOx) by the action of cytochrome P450 monooxygenases CYP79B2 and CYP79B3. The remaining biosynthetic steps are unknown except for the last step, which is conversion of dihydrocamalexic acid to camalexin by CYP71B15 (PAD3). This article reports characterization of CYP71A13. Plants carrying cyp71A13 mutations produce greatly reduced amounts of camalexin after infection by Pseudomonas syringae or A. brassicicola and are susceptible to A. brassicicola, as are pad3 and cyp79B2 cyp79B3 mutants. Expression levels of CYP71A13 and PAD3 are coregulated. CYP71A13 expressed in Escherichia coli converted IAOx to indole-3-acetonitrile (IAN). Expression of CYP79B2 and CYP71A13 in Nicotiana benthamiana resulted in conversion of Trp to IAN. Exogenously supplied IAN restored camalexin production in cyp71A13 mutant plants. Together, these results lead to the conclusion that CYP71A13 catalyzes the conversion of IAOx to IAN in camalexin synthesis and provide further support for the role of camalexin in resistance to A. brassicicola.

Characteristic for cruciferous plants is their production of N-and S-containing indole phytoalexins with disease resistance and cancer-preventive properties, previously proposed to be synthesized from indole independently of tryptophan. We show that camalexin, the indole phytoalexin of Arabidopsis thaliana, is synthesized from tryptophan via indole-3-acetaldoxime (IAOx) in a reaction catalyzed by CYP79B2 and CYP79B3. Cyp79B2͞cyp79B3 double knockout mutant is devoid of camalexin, as it is also devoid of indole glucosinolates [Zhao, Y., Hull, A. K., Gupta, N. R., Goss, K. A., Alonso, J., Ecker, J. R., Normanly, J., Chory, J. & Celenza, J. L. (2002) Genes Dev. 16, 3100 -3112], and isotope-labeled IAOx is incorporated into camalexin. These results demonstrate that only CYP79B2 and CYP79B3 contribute significantly to the IAOx pool from which camalexin and indole glucosinolates are synthesized. Furthermore, production of camalexin in the sur1 mutant devoid of glucosinolates excludes the possibility that camalexin is derived from indole glucosinolates. CYP79B2 plays an important role in camalexin biosynthesis in that the transcript level of CYP79B2, but not CYP79B3, is increased upon induction of camalexin by silver nitrate as evidenced by microarray analysis and promoter-␤-glucuronidase data. The structural similarity between cruciferous indole phytoalexins suggests that these compounds are biogenetically related and synthesized from tryptophan via IAOx by CYP79B homologues. The data show that IAOx is a key branching point between several secondary metabolic pathways as well as primary metabolism, where IAOx has been shown to play a critical role in IAA homeostasis.C haracteristic for cruciferous plants is the synthesis of a wide range of species-specific phytoalexins that structurally are sulfur-containing indole alkaloids (1) and the synthesis of glucosinolates (reviewed in ref.2). Both groups of natural products are involved in plant defense and have cancer-preventive properties (3, 4). Very little is known about the biosynthetic pathway of the S-containing indole phytoalexins. Their similar structure with N-and S-containing side chains at C-3 of the indole ring suggests a biogenetic relationship (5). Camalexin (3-thiazol-2Ј-yl-indole) is produced in the model plant Arabidopsis thaliana (6). It is induced by a variety of microorganisms, e.g., Pseudomonas syringae (6) and Alternaria brassisicola (7), and by abiotic factors, such as AgNO 3 (8). These findings make camalexin a good model compound for studying biosynthesis and regulation of cruciferous indole phytoalexins. In vivo feeding experiments where radiolabeled anthranilate and tryptophan were applied on leaves treated with AgNO 3 led to the suggestion that tryptophan was not a precursor in camalexin biosynthesis because tryptophan was much less efficiently incorporated into camalexin compared with anthranilate (8, 9). The data were further supported by labeling studies performed in three tryptophan mutants (8), where reduced levels of camalexin accumulated in ...