On the Inside

Abstract

In this issue,Penmetsa et al. (pp. ) identify a novel hyper-nodulation mutant in Medicago truncatula, designatedsunn (for supernumeric nodules). In marked contrast to the parental genotype that develops approximately 8 nodules per root in response to inoculation with Sinorhizobium meliloti, thesunn mutant develops roughly 70 nodules. In this respect, it is similar to the previously described ethylene-insensitive mutantsickle. Penmetsa et al., however, show that these two mutants are readily distinguishable. First, in contrast tosickle, in which insensitivity to ethylene is thought to be causal to the hyper-nodulation phenotype, nodulation in sunnshows normal sensitivity to ethylene. (Curiously, the root growth ofsunn mutants is insensitive to ethylene, although other aspects of the ethylene triple response are normal). Second, the anatomy the two mutants differ in the nodulation zone: Successful infection and nodule development in sunn occur predominantly opposite xylem poles, similar to wild type. In sickle, however, both infection and nodulation occur randomly throughout the circumference of the developing root. Third, genetic analysis indicates that sunn and sickle correspond to separate and unlinked loci, whereas the sunn/sickle double mutant exhibits a novel and additive super-nodulation phenotype. Similar to super-nodulation mutants described in soybean (Glycine max) and Lotus japonicus, grafting experiments demonstrate that the sunn phenotype is determined by the genotype of the shoot, implicating a mobile signal (auxin?) in conditioning nodule number. The authors propose a model for the genetic control of nodule number in M. truncatula, wherein distinct genetic pathways involving sickle andsunn, respectively, mediate rhizobial infection and nodule organogenesis. Eurycoma longifolia, a medicinal tree of Southeast Asia, has been overharvested. Single nucleotide polymorphisms have proven useful in assessing the genetic diversity of this threatened species in the wild. Biotin is a water-soluble vitamin that is synthesized by plants and required by all living organisms for normal cellular function and growth. As a coenzyme, biotin plays a critical role in the regulation of a number of enzymes involved in primary metabolism. In this issue, Che et al. (pp. ) demonstrate a new non-catalytic function for biotin in plants. Specifically, the authors report that biotin regulates the expression of the subunit genes of methylcrotonyl-coenzyme A (CoA) carboxylase (MCCase) by mechanisms independent of its role as a cofactor. The authors took advantage of the bio1 mutant of Arabidopsis, which is blocked in the de novo biosynthesis of biotin, to elucidate the role of biotin in regulating MCCase expression. In response to thebio1-associated depletion of biotin, the normally biotinylated A-subunit of methylcrotonyl-CoA carboxylase (MCCase) accumulates in its inactive apo-form, and both subunits of MCCase hyperaccumulate. This induction occurs either because the translation of each subunit mRNA is enhanced or because the turnover of each subunit protein is reduced. Evidence is also presented that biotin is required for the two MCCase subunit genes to respond to metabolic signals. Under environmental conditions that reduce the carbon status of seedlings (reduced CO2 or darkness), transcription of the MCCase genes is normally induced. However, this induction in gene transcription fails to occur in seedlings that are depleted of biotin. These studies indicate that biotin not only regulates gene expression by modulating transcription (as occurs in bacteria and animals), but also mediates regulation of gene expression at the translational and/or posttranslational level. In contrast to trans-zeatin, the major and ubiquitous cytokinin of higher plants, cis-zeatin has traditionally been viewed as a rare isomer of low biological activity. Recent studies, however, have indicated that the cis-isomers are the dominant form of cytokinins at particular stages of development in certain plant species and have been shown to be associated with male sterility in another. Such observations raise the question of whether cis-zeatin is of parallel importance to trans-zeatin. Indeed, a recent report suggesting the existence of a maize (Zea mays) gene (cisZOG1) encoding an O-glucosyltransferase specific to cis-zeatin lends strong support to this view. (O-glucosylation changes trans-zeatin to a storage and transport form that is resistant to degradation by cytokinin oxidases.) In this issue, Veach et al. (pp. ) describe the isolation of a second maize cisZOG gene, the differential expression of cisZOG1 and cisZOG2, and the identification of substantial amounts of cis-zeatin isomers in maize tissues. The expression of cisZOG1 was high in maize roots and kernels, whereas cisZOG2 expression was high in roots but low in kernels. The O-glucosides of cis-isomers were found in roots, young cobs, and kernels, which is compatible with the expression of cisZOG1 and cisZOG2 in maize. Comparing the two groups of cytokinins, cis-isomers were more prevalent in roots, stems, and leaves, whereas trans-isomers were more abundant in the kernels. O-glucosylation of cis-zeatin appears to be a natural metabolic process in maize. Whether cis-zeatin serves as a precursor to the active trans-isomer or has unique physiological functions remains to be demonstrated. Many secondary compounds of economic and medicinal significance occur in rare and exotic plant species, often at extremely low levels. Therefore, there has been much interest in using cell cultures from such species to produce secondary metabolites. One way that the efficiency of such in vitro systems might be improved is by engineering the flux of metabolites through the biosynthetic pathways of interest. A potential pitfall of this strategy, however, is that the hyperaccumulation of certain secondary metabolites may be autotoxic to the cell culture itself. In this issue, Goossens et al. (pp. ) examine the possibility of genetically engineering ABC transporters into cultured cells as a way of enhancing the extrusion of potentially toxic secondary metabolites. The ABC protein family, the molecular biology of which is coincidentally reviewed in this issue by Jasinski, Ducos, Martinoia, and Boutry (pp. ) is a large, ubiquitous, and diverse group of proteins that transport an impressively wide variety of substrates across biological membranes via the binding and hydrolysis of ATP. The substrates that can be transported by ABC transporter proteins include peptides, carbohydrates, lipids, heavy metal chelates, inorganic acids, steroids, and xenobiotics. ABC transporters are associated with the acquisition of multiple drug resistance by pathogenic organisms and with detoxification pathways that deal with either endogenously synthesized or environmental toxic compounds. With respect to the secretion of plant secondary metabolites, two subfamilies of ABC proteins, pleiotropic drug resistance (PDR) and multidrug resistance-associated proteins (MRP), are of particular interest. To identify yeast (Saccharomyces cerevisiae) ABC transporters with substrate specificity for tropane alkaloids, Goossens et al. assessed the sensitivity of different yeast strains deficient in various species of ABC transporters to poisoning by the tropane alkaloids hyoscyamine and scopolamine. Based on this assay, an ABC transporter (yeast PDR5) with specificity for tropane alkaloids was identified. When PDR5 was transgenically introduced into tobacco (Nicotiana tabacum) cv Bright Yellow-2 (BY-2) cell lines, the secretion of tropane alkaloids from the plant cells was increased. Because of their diverse substrate specificities ABC transporters might eventually be useful in enhancing the production of a wide variety of secondary metabolites in plant cell cultures.

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