Annual Review of Plant Biology - Volume 49, 1998

Ovules are the direct precursors of seeds and thus play central roles in sexual plant reproduction and human nutrition. Extensive classical studies have elucidated the evolutionary trends and developmental processes responsible for the current wide variety of ovule morphologies. Recently, ovules have been perceived as an attractive system for the study of genetic regulation of plant development. More than a dozen regulatory genes have now been identified through isolation of ovule mutants. Characterization of these mutants shows that some aspects of ovule development follow independent pathways, while other processes are interdependent. Some of these mutants have ovules resembling those of putative ancestors of angiosperms and may help in understanding plant evolution. Clones of several of the regulatory genes have been used to determine expression patterns and putative biochemical functions of the gene products. Newly constructed models of genetic regulation of ovule development provide a framework for interpretation of future discoveries.

The assembly of chloroplast metalloproteins requires biochemical catalysis. Assembly factors involved in the biosynthesis of metalloproteins might be required to synthesize, chaperone, or transport the cofactor; modify or chaperone the apoprotein; or catalyze cofactor-protein association. Genetic and biochemical approaches have been applied to the study of the assembly of chloroplast iron-sulfur centers, cytochromes, plastocyanin, and the manganese center of photosystem II. These have led to the discovery of NifS-homologues and cysteine desulfhydrase for iron-sulfur center assembly, six loci (CCS1–CCS5, ccsA) for c-type cytochrome assembly, four loci for cytochrome b6 assembly (CCB1–CCB4), the CtpA protease, which is involved in pre-D1 processing, and the PCY2 locus, which is involved in holoplastocyanin accumulation. New assembly factors are likely to be discovered via the study of assembly-defective mutants of Arabidopsis, cyanobacteria, Chlamydomonas, maize, and via the functional analysis of candidate cofactor metabolizing components identified in the genome databases.

The sulfolipid sulfoquinovosyl diacylglycerol is an abundant sulfur-containing nonphosphorous glycerolipid that is specifically associated with photosynthetic membranes of higher plants, mosses, ferns, algae, and most photosynthetic bacteria. The characteristic structural feature of sulfoquinovosyl diacylglycerol is the unique head group constituent sulfoquinovose, a derivative of glucose in which the 6-hydroxyl is replaced by a sulfonate group. While there is growing evidence for the final assembly of the sulfolipid by the transfer of the sulfoquinovosyl moiety from UDP-sulfoquinovose to the sn-3 position of diacylglycerol, very little is known about the biosynthesis of the precursor UDP-sulfoquinovose. Recently, a number of mutants deficient in sulfolipid biosynthesis and the corresponding sqd genes have become available from different organisms. These provide novel tools to analyze sulfolipid biosynthesis by a combination of molecular and biochemical approaches. Furthermore, the analysis of sulfolipid-deficient mutants has provided novel insights into the function of sulfoquinovosyl diacylglycerol in photosynthetic membranes.

The purpose of this review is to highlight the unique and common features of splice site selection in plants compared with the better understood yeast and vertebrate systems. A key question in plant splicing is the role of AU sequences and how and at what stage they are involved in spliceosome assembly. Clearly, intronic U- or AU-rich and exonic GC- and AG-rich elements can influence splice site selection and splicing efficiency and are likely to bind proteins. It is becoming clear that splicing of a particular intron depends on a fine balance in the “strength” of the multiple intron signals involved in splice site selection. Individual introns contain varying strengths of signals and what is critical to splicing of one intron may be of less importance to the splicing of another. Thus, small changes to signals may severely disrupt splicing or have little or no effect depending on the overall sequence context of a specific intron/exon organization.

The assembly of the photosynthetic apparatus at the thylakoid begins with the targeting of proteins from their site of synthesis in the cytoplasm or stroma to the thylakoid membrane. Plastid-encoded proteins are targeted directly to the thylakoid during or after synthesis on plastid ribosomes. Nuclear-encoded proteins undergo a two-step targeting process requiring posttranslational import into the organelle from the cytoplasm and subsequent targeting to the thylakoid membrane. Recent investigations have revealed a single general import machinery at the envelope that mediates the direct transport of preproteins from the cytoplasm to the stroma. In contrast, at least four distinct pathways exist for the targeting of proteins to the thylakoid membrane. At least two of these systems are homologous to translocation systems that operate in bacteria and at the endoplasmic reticulum, indicating that elements of the targeting mechanisms have been conserved from the original prokaryotic endosymbiont.

Major advances have been made in understanding the role of transcription factors in gene expression in yeast, Drosophila, and man. Transcription factor modification, synergistic events, protein-protein interactions, and chromatin structure have been successfully integrated into transcription factor studies in these organisms. While many putative transcription factors have been isolated from plants, most of them are only poorly characterized. This review summarizes examples where molecular biological techniques have been successfully employed to study plant transcription factors. The functional analysis of transcription factors is described as well as techniques for studying the interactions of transcription factors with other proteins and with DNA.

The nucleotide sequence of the entire genome of the unicellular cyanobacterium, Synechocystis sp. PCC6803, has been determined. The length of the circular genome was 3,573,480 bp, and a total of 3168 protein-coding genes were assigned to the genome by a computer-assisted analysis. The functions of approximately 45% of the genes were deduced based on sequence similarity to known genes. Here are distinctive features of genetic information carried by the cyanobacteria, which have a phylogenetic relationship to both bacteria and plants.

While uninucleate and unicellular, Acetabularia acetabulum establishes and maintains functionally and morphologically distinct body regions and executes phase changes like those in vascular plants. Centimeters tall at maturity, this species has allowed unusual experimental approaches. Amputations revealed fates of nucleate and enucleate portions from both wild type and mutants. Historically, graft chimeras between nucleate and enucleate portions suggested that morphological instructions were supplied by the nucleus but resided in the cytoplasm and could be expressed interspecifically. Recently, graft chimeras enabled rescue of mutants arrested in vegetative phase. Since the 1930s, when Acetabularia provided the first evidence for the existence of mRNAs, a dogma has arisen that it uses long-lived mRNAs to effect morphogenesis. While the evidence favors translational control, the postulated mRNAs have not been identified, and the mechanism of morphogenesis remains unknown. Amenable to biochemistry, physiology, and both classical and molecular genetics, Acetabularia may contribute yet new insights into plant development and morphogenesis.

The plant hormone abscisic acid (ABA) plays a major role in seed maturation and germination, as well as in adaptation to abiotic environmental stresses. ABA promotes stomatal closure by rapidly altering ion fluxes in guard cells. Other ABA actions involve modifications of gene expression, and the analysis of ABA-responsive promoters has revealed a diversity of potential cis-acting regulatory elements. The nature of the ABA receptor(s) remains unknown. In contrast, combined biophysical, genetic, and molecular approaches have led to considerable progress in the characterization of more downstream signaling elements. In particular, substantial evidence points to the importance of reversible protein phosphorylation and modifications of cytosolic calcium levels and pH as intermediates in ABA signal transduction. Exciting advances are being made in reassembling individual components into minimal ABA signaling cascades at the single-cell level.

Methylation of cytosine residues in DNA provides a mechanism of gene control. There are two classes of methyltransferase in Arabidopsis; one has a carboxy-terminal methyltransferase domain fused to an amino-terminal regulatory domain and is similar to mammalian methyltransferases. The second class apparently lacks an amino-terminal domain and is less well conserved. Methylcytosine can occur at any cytosine residue, but it is likely that clonal transmission of methylation patterns only occurs for cytosines in strand-symmetrical sequences CpG and CpNpG. In plants, as in mammals, DNA methylation has dual roles in defense against invading DNA and transposable elements and in gene regulation. Although originally reported as having no phenotypic consequence, reduced DNA methylation disrupts normal plant development.

To cope with environmental fluctuations and to prevent invasion by pathogens, plant metabolism must be flexible and dynamic. Active oxygen species, whose formation is accelerated under stress conditions, must be rapidly processed if oxidative damage is to be averted. The lifetime of active oxygen species within the cellular environment is determined by the antioxidative system, which provides crucial protection against oxidative damage. The antioxidative system comprises numerous enzymes and compounds of low molecular weight. While research into the former has benefited greatly from advances in molecular technology, the pathways by which the latter are synthesized have received comparatively little attention. The present review emphasizes the roles of ascorbate and glutathione in plant metabolism and stress tolerance. We provide a detailed account of current knowledge of the biosynthesis, compartmentation, and transport of these two important antioxidants, with emphasis on the unique insights and advances gained by molecular exploration.

The nature of cell wall proteins is as varied as the many functions of plant cell walls. With the exception of glycine-rich proteins, all are glycosylated and contain hydroxyproline (Hyp). Again excepting glycine-rich proteins, they also contain highly repetitive sequences that can be shared between them. The majority of cell wall proteins are cross-linked into the wall and probably have structural functions, although they may also participate in morphogenesis. On the other hand, arabinogalactan proteins are readily soluble and possibly play a major role in cell-cell interactions during development. The interactions of these proteins between themselves and with other wall components is still unknown, as is how wall components are assembled. The possible functions of cell wall proteins are suggested based on repetitive sequence, localization in the plant body, and the general morphogenetic pattern in plants.

Cytochrome P450-dependent monooxygenases are a large group of heme-containing enzymes, most of which catalyze NADPH- and O2-dependent hydroxylation reactions. The cloning of plant P450s has been hampered because these membrane-localized proteins are typically present in low abundance and are often unstable to purification. Since the cloning of the first plant P450 gene in 1990, there has been an explosion in the rate at which genes encoding plant P450s have been identified. These successes have largely been the result of advances in purification techniques, as well as the application of alternative methods such as mutant- and PCR-based cloning strategies. The availability of these cloned genes has made possible the analysis of P450 gene regulation and may soon reveal aspects of the evolution of P450s in plants. This new knowledge will significantly improve our understanding of many metabolic pathways and may permit their manipulation in the near future.

The timing of the transition from vegetative to reproductive development is of great fundamental and applied interest but is still poorly understood. Recently, molecular-genetic approaches have been used to dissect this process in Arabidopsis. The genetic variation present among a large number of mutants with an early- or late-flowering phenotype, affecting the control of both environmental and endogenous factors that influence the transition to flowering, is described. The genetic, molecular, and physiological analyses have led to identification of different components involved, such as elements of photoperception and the circadian rhythm. Furthermore, elements involved in the signal transduction pathways to flowering have been identified by the cloning of some floral induction genes and their target genes.

During meiosis, homologous chromosomes are brought together to be recombined and segregated into separate haploid gametes. This requires two cell divisions, an elaborate prophase with five substages, and specialized mechanisms that regulate the association of sister chromatids. This review focuses on plant chromosomes and chromosome-associated structures, such as recombination nodules and kinetochores, that ensure accurate meiotic chromosome segregation.

The cytochromes that function in photosynthesis in cyanobacteria, algae, and higher plants have, like the other photosynthetic catalysts, been largely conserved in their structure and function during evolution. Cyanobacteria and algae contain cytochrome c6, which is not found in higher plants and which may enhance survival in their planktonic mode of life. Cyanobacteria and algae contain another cytochrome, low-potential c549, which is not found in higher plants. This cytochrome has a structural role in PSII and may contribute to anaerobic survival. There is a third unique cytochrome, cytochrome M, in the planktonic photosynthesizers, and its function is unknown. New evidence is appearing to indicate evolution of cytochrome interaction mechanisms during the evolution of photosynthesis. The ease of cytochrome gene manipulation in cyanobacteria and in Chlamydomonas reinhardtii now provides great advantages in understanding of photosynthesis. The solution of tertiary and quaternary structures of cytochromes and cytochrome complexes will provide structural and functional detail at atomic resolution.

Brassinosteroids (BRs) are growth-promoting natural products found at low levels in pollen, seeds, and young vegetative tissues throughout the plant kingdom. Detailed studies of BR biosynthesis and metabolism, coupled with the recent identification of BR-insensitive and BR-deficient mutants, has greatly expanded our view of steroids as signals controlling plant growth and development. This review examines the microchemical and molecular genetic analyses that have provided convincing evidence for an essential role of BRs in diverse developmental programs, including cell expansion, vascular differentiation, etiolation, and reproductive development. Recent advances relevant to the molecular mechanisms of BR-regulated gene expression and BR signal transduction are also discussed.

The nucleus must coordinate organelle biogenesis and function on a cell and tissue-specific basis throughout plant development. The vast majority of plastid and mitochondrial proteins and components involved in organelle biogenesis are encoded by nuclear genes. Molecular characterization of nuclear mutants has illuminated chloroplast development and function. Fewer mutants exist that affect mitochondria, but molecular and biochemical approaches have contributed to a greater understanding of this organelle. Similarities between organelles and prokaryotic regulatory molecules have been found, supporting the prokaryotic origin of chloroplasts and mitochondria. A striking characteristic for both mitochondria and chloroplast is that most regulation is posttranscriptional.

New and exciting developments in boron research in the past few years greatly contributed to better understanding of the role of boron in plants. Purification and identification of the first boron-polyol transport molecules resolved much of the controversy about boron phloem mobility. Isolation and characterization of the boron-polysaccharide complex from cell walls provided the first direct evidence for boron crosslinking of pectin polymers. Inhibition and recovery of proton release upon boron withdrawal and restitution in plant culture medium demonstrated boron involvement in membrane processes. Rapid boron-induced changes in membrane function could be attributed to boron-complexing membrane constituents. Boron may affect metabolic pathways by binding apoplastic proteins to cis-hydroxyl groups of cell walls and membranes, and by interfering with manganese-dependent enzymatic reactions. In addition, boron has been implicated in counteracting toxic effects of aluminum on root growth of dicotyledonous plants. Molecular investigations of boron nutrition have been initiated by the discovery of a novel mutant of Arabidopsis thaliana with an altered requirement for boron.