Benzoxazinoid biosynthesis,a model for evolution of secondary metabolic pathways in plants

Benzoxazinoids are secondary metabolites that are effective in defence and allelopathy. They are synthesised in two subfamilies of the Poaceae and sporadically found in single species of the dicots. The biosynthesis is fully elucidated in maize; here the genes encoding the enzymes of the pathway are in physical proximity. This “biosynthetic cluster” might facilitate coordinated gene regulation. Data from Zea mays, Triticum aestivum and Hordeum lechleri suggest that the pathway is of monophyletic origin in the Poaceae. The branchpoint from the primary metabolism (Bx1 gene) can be traced back to duplication and functionalisation of the alpha-subunit of tryptophan synthase (TSA). Modification of the intermediates by consecutive hydroxylation is catalysed by members of a cytochrome P450 enzyme subfamily (Bx2–Bx5). Glucosylation by an UDP-glucosyltransferase (UGT, Bx8, Bx9) is essential for the reduction of autotoxicity of the benzoxazinoids. In some species 2,4-dihydroxy-1,4-benzoxazin-3-one-glucoside (DIBOA-glc) is further modified by the 2-oxoglutarate-dependent dioxygenase BX6 and the O-methyltransferase BX7. In the dicots Aphelandra squarrosa, Consolida orientalis, and Lamium galeobdolon, benzoxazinoid biosynthesis is analogously organised: The branchpoint is established by a homolog of TSA, P450 enzymes catalyse hydroxylations and at least the first hydroxylation reaction is identical in dicots and Poaceae, the toxic aglucon is glucosylated by an UGT. Functionally, TSA and BX1 are indole-glycerolphosphate lyases (IGLs). Igl genes seem to be generally duplicated in angiosperms. Modelling and biochemical characterisation of IGLs reveal that the catalytic properties of the enzyme can easily be modified by mutation. Independent evolution can be assumed for the BX1 function in dicots and Poaceae.     

Biosynthesis of L-ascorbic acid in plants: new pathways for an old antioxidant

The biosynthetic pathway of L-ascorbic acid (vitamin C) in plants has been established for several years. However, recent reports describe alternative pathways, revealing a more complex picture of L-ascorbic acid biosynthesis than had been expected. GDP-L-gulose and myo-inositol are proposed as new intermediates in L-ascorbic acid biosynthesis, indicating that part of the animal pathway might also be operating in plants. Enzymatic studies on the GDP-mannose- 3',5'-epimerase and L-galactono-1,4-lactone dehydrogenase suggest that they are important regulatory steps for L-ascorbic acid biosynthesis.     

Morphological evolution in land plants: new designs with old genes

The colonization and radiation of multicellular plants on land that started over 470 Ma was one of the defining events in the history of this planet. For the first time, large amounts of primary productivity occurred on the continental surface, paving the way for the evolution of complex terrestrial ecosystems and altering global biogeochemical cycles; increased weathering of continental silicates and organic carbon burial resulted in a 90 per cent reduction in atmospheric carbon dioxide levels. The evolution of plants on land was itself characterized by a series of radical transformations of their body plans that included the formation of three-dimensional tissues, de novo evolution of a multicellular diploid sporophyte generation, evolution of multicellular meristems, and the development of specialized tissues and organ systems such as vasculature, roots, leaves, seeds and flowers. In this review, we discuss the evolution of the genes and developmental mechanisms that drove the explosion of plant morphologies on land. Recent studies indicate that many of the gene families which control development in extant plants were already present in the earliest land plants. This suggests that the evolution of novel morphologies was to a large degree driven by the reassembly and reuse of pre-existing genetic mechanisms.     

Cytocatalytic evolution in plants

Low fertility and vigor, and requirements of ecological niches distinct from diploids are not universal characteristics of autoploids. InClaytonia, Hedyotis, Oldenlandia, and other genera, occurrence and frequency of both polyploids and aneuploids within species populations suggest a greater role of such mutations in the evolution of vascular plants than heretofore presumed.     

The evolution of the new permeability pathways in Plasmodium falciparum--infected erythrocytes--a kinetic analysis

Malaria parasites demonstrably increase the permeability of the membrane of the erythrocyte in which they develop and propagate. New permeability pathways (NPPs) generated by parasite activity and identified in the erythrocyte membrane are held responsible for these changes. Here, we present a novel analysis of hemolysis curves of infected cells in iso-osmotic solutions of solutes that penetrate selectively into infected cells, as a function of parasite development. The analysis yields three parameters: the t(1/2) of lysis (reciprocally related to permeability), the maximal lysis, and a parameter that expresses the variation of the cell population. Different developmental stages of the parasite were obtained either by sampling synchronized cultures with time or by the fractionation of asynchronous cultures on a Percoll-sorbitol density gradient. While the results confirm previous reports on the stage-dependent evolution of NPPs, they also reveal that the evolution of NPPs is not synchronous: NPPs evolve differentially throughout the ring stage and only at the mid-trophozoite stage they are fully deployed in the majority of the infected cells, but not in all. This leads to desynchronization in the culture and to less than the maximal possible rate of multiplication.     

Hypotheses for the evolution of dioecy in seed plants

Over the last decade, new hypotheses have been proposed for the evolution of dioecy in plants. Most of the selective mechanisms invoked have been suggested and supported by phylogenetic correlations. Here we review (1) the validity of the correlations (especially in light of recent critiques of the comparative method), and (2) the conformity of the proposed mechanisms to empirical data. None of the hypotheses can be flatly rejected on existing evidence, but the strength of their support varies. Future correlational studies must explicitly consider phylogeny; more importantly, such broad studies should also be supplemented by detailed studies of particular transitions to dioecy (e.g. within genera) - studies of the sort that have clarified analogous issues such as heterostyly.     

Proteomics-based dissection of stress-responsive pathways in plants

Abiotic stress has an ability to alter the levels of a number of proteins, which may be soluble or structural in nature or which may exist before and after folding in the plant cell. The most crucial function of plant cell is to respond to stress by developing defence mechanisms. This defence is brought about by alteration in the pattern of gene expression. This leads to modulation of certain metabolic and defensive pathways. Owing to gene expression altered under stress, qualitative and quantitative changes in proteins are obvious. These proteins might play a role in signal transduction, antioxidative defence, antifreezing, heat shock, metal binding, antipathogenesis or osmolyte synthesis. A significant part of the literature shows the quantitative and qualitative changes in proteins, mainly employing western analysis, enzymatic kinetics, fraction isolation, one-dimensional SDS-PAGE electrophoresis, etc. Fortunately, recent developments in sensitivity and accuracy for proteome analysis have provided new dimensions to assess the changes in protein types and their expression levels under stress. The novel aim of this review is to do a side-by-side comparison of the proteins that are induced or overexpressed under abiotic stress, examining those from biochemical literature and the ones observed, sequenced and identified using the advanced proteomics and bioinformatic techniques.     

Making the grade in human evolution

Getting Here: The Story of Human Evolution By WW Howells (1997). Washington: Compass Press. 266 pp. $19.95 (cloth) ISBN 0-929590-16-3. $36.95 (paper) ISBN 0-929590-17-1. From Lucy to Language By DC Johanson and B Edgar. Principal Photography by David Brill (1996). New York: Simon and Schuster, 272 pp. $50.00 (cloth) ISBN 0-684-81023-9. Reconstructing Human Origins: A Modern Synthesis By GC Conroy (1997). New York: WW. Norton.xii + 557 pp. $43.13 (paper) ISBN 0-393-97042-6. Humans Before Humanity By R Foley (1997) Oxford: Blackwell Publishers.vii + 248 pp. $21.95 paper ISBN 0-631-20528-4. Contemporary Issues in Human Evolution By E Meickle, FC Howell and N Jablonski (1996) Wattis Symposium Series in Anthropology. Nina G. Jablonski, Series Editor. California Academy of Sciences Memoir 21. San Francisco: California Academy of Sciences.viii+193pp $35.00 cloth ISBN 0-940228-45-9. © 1998 Wiley-Liss, Inc.     

Secondary metabolites during early development in plants

Early development is a critical stage in a plant’s life, as the plant must establish itself in the ecosystem during this period. The secondary metabolites (SM) during this phase is a strategy that contributes to the survival of plant species. Through a review of the literature, a number of reports were found that investigated the presence of SM during germination and early plant development (phases 0 and 1 according to the Zadoks and BBCH scales). A total of 250 reports were found that investigated 99 species and nearly 200 SM that accumulate during this period of the plant life cycle. A large portion of the SM are biosynthesised de novo, whereas the remainder are derived in part or in total from the mother plant. In many cases, the resources for biosynthesis are supplied only by the reserve material of the endosperm or cotyledons, which allows for independent photosynthesis. The presence of SM at these stages confers characteristics of more advanced stages, such as tissue-specific distribution, spatio-temporal regulation, and the individual regulation of all of the biosynthesised SM. The amount and diversity of SM are not universally related to the progress of plant development, but it is a widespread phenomenon. The early production of SM has ecological implications that involve defence mechanisms, relationships with microorganisms, and the role of these compounds as nitrogen reserves. This review contributes to the systematisation of studies on SM in the early stages of development.     

Directed evolution of metabolic pathways

The modification of cellular metabolism is of biotechnological and commercial significance because naturally occurring metabolic pathways are the source of diverse compounds used in fields ranging from medicine to bioremediation. Directed evolution is the experimental improvement of biocatalysts or cellular properties through iterative genetic diversification and selection procedures. The creation of novel metabolic functions without disrupting the balanced intracellular pool of metabolites is the primary challenge of pathway manipulation. The introduction of coordinated changes across multiple genetic elements, in conjunction with functional selection, presents an integrated approach for the modification of metabolism with benign physiological consequences. Directed evolution formats take advantage of the dynamic structures of genomes and genomic sub-structures and their ability to evolve in multiple directions in response to external stimuli. The elucidation, design and application of genome-restructuring mechanisms are key elements in the directed evolution of cellular metabolic pathways.     

Safe and acceptable strategies for producing foreign molecules in plants

The ability to express foreign genes using transgenic technologies has opened up options for producing large quantities of commercially important industrial or pharmaceutical products in plants. These technologies have made it possible to use well-developed systems of commercial agriculture that were developed principally to produce raw material for large-scale food, feed or processing applications for the production of foreign molecules. The possibility of the novel industrial or pharmaceutical molecules produced in such plants, or components derived from them, contaminating the environment and food chains has become especially controversial. This potential contamination has prompted detailed consideration of how such crops and the molecules that they produce can be effectively isolated and contained. First, the crop can be completely isolated physically from its food or feed counterpart during every aspect of its development and commercialization. Second, genetic isolation systems or genetic barriers that prevent normal reproduction can be used to reduce the likelihood of the industrial or pharmaceutical crop entering the food chain.     

Versatile persistence pathways for pathogens of animals and plants

The glyoxylate cycle and the glycine cleavage system are part of conserved metabolic pathways involved in the chronic persistence of microorganisms in animal hosts. In the chromosome of the plant pathogen Rhodococcus fascians, the vic locus has been identified as a region containing genes essential for persistence inside induced leafy galls. Sequence analysis showed that this 18-kb locus is syntenic with chromosomal regions of Mycobacterium species that encompass the 'persistence' loci of these mammalian pathogens. Hence, the ability to switch diet inside the host appears to be governed by 'persistence' enzymes that are conserved between pathogens of animals and plants.