Symposia of the Society for Experimental Biology最新文献

We are studying several Arabidopsis mutants that show altered regulation of flowering time in response to daylength. One of the mutations we are studying, constans, delays flowering under long days but has no effect under short days. Analysis of the expression of the CONSTANS gene, and modification of its expression in transgenic plants, suggests that this gene promotes flowering in response to long days and that the delay in flowering that occurs in wild-type plants under short days is at least in part due to regulation of CONSTANS gene transcription. We describe genetic approaches that we are taking to identify genes that act in the same genetic pathway as CONSTANS, and in particular the relationship between CONSTANS and two other genes that we are studying. These are LATE ELONGATED HYPOCOTYL, for which we have a dominant mutant allele that causes late flowering, and EARLY SHORT DAYS 4, whose inactivation causes early flowering. In addition to their effects on flowering time, the over-expression of CONSTANS and the inactivation of EARLY SHORT DAYS 4 cause the Arabidopsis shoot to become determinate and therefore to terminate development prematurely. This phenotype is discussed in light of other genes that have previously been shown to be required to maintain indeterminate development of the shoot.

Control of the transition to flowering is critical for reproductive success of a plant. Studies in Arabidopsis have led us to suggest how this species has harnessed the environmental cue of a period of low temperature to ensure flowering occurs at an appropriate time. We propose that Arabidopsis has both vernalization-independent and vernalization-dependent pathways for the initiation of inflorescence development in the shoot apex. The vernalization-independent pathway may be concerned with the supply of carbohydrate to the shoot apex. In late flowering ecotypes which respond to vernalization the vernalization-independent pathway is blocked by the action of two dominant repressors of flowering, FRI and FLC, which interact to produce very late flowering plants which respond strongly to vernalization. We have isolated a gene which may correspond to FLC. We suggest the vernalization-dependent pathway, which may be concerned with apical GA biosynthesis, is blocked by methylation of a gene critical for flowering. This gene may correspond to that encoding kaurenoic acid hydroxylase (KAH), an enzyme catalysing a step in the GA biosynthetic pathway. Under this scheme vernalization causes unblocking of this pathway by demethylation possibly of the KAH gene and consequent biosynthesis of active GAs in the apex.

As sessile organisms, higher plants are characterized by a high degree of developmental pattern plasticity in response to environmental signals, and in many cases respond to the changing environment by tailoring their developmental patterns in a way that maximizes their chances of survival and reproduction. Given the importance of photosynthesis to plant survival, light signals are arguably among the most important environmental cues to plant development. Genetic analysis of the light control of the Arabidopsis seedling development pattern has revealed that the pleiotropic COP/DET/FUS genes play a key role in integrating light signals and modulating developmental pattern formation. Recent studies support a working model in which COP1 act within the nucleus to sequester and inactivate transcription factors in darkness, while light abrogates this association by modulating COP1 nuclear abundance. This results in activation of the transcription factors and expression of genes responsible for photomorphogenic development. The other pleiotropic COP/DET/FUS proteins act to maintain the proper nuclear localization or retention of COP1 in darkness.

Classical studies in plant development have indicated that the fate of plant cells is fixed late, after cell division has ceased. Earlier commitment events are therefore considered reversible. To gain a mechanisatic understanding of the processes involved in specification and fixation of cell fate in plants, we are using the Arabidopsis root epidermis as a model system. The Arabidopsis root epidermis is composed of two cell types whose pattern of differentiation is directed by positional cues during development. Examination of mutations has identified genes involved in the establishment of cell fate specification in this tissue. TRANSPARENT TESTA GLABRA (TTG) and GLABRA2 (GL2) are positive regulators of non-hair fate and are active during the early differentiation of the epidermis in the meristem. GL2 encodes a homeobox protein which is expressed in non-hair cells in the meristem and is positively regulated by TTG. Mutations in genes involved in the regulation of ethylene biosynthesis and signal transduction indicate that ethylene is a positive regulator of hair cell fate. Treatment of ttg and gl2 plants with modulators of ethylene biosynthesis indicate that ethylene acts down stream of TTG and GL2 during the fate specification process. The relationship between meristem organisation and the mechanism underpinning the establishment of cell fate in other systems is also discussed.

The biosynthesis of complex lipids involves specific acylation reactions catalysed by acyltransferases. These reactions are important for the formation of both storage lipids, triacylglycerols, as well as structural lipids such as phospholipids and galactolipids. The current status of our understanding of the specificity, selectivity, structure, cloning and genetic manipulation of these enzymes is reviewed. These studies clearly indicate the possibilities of selecting appropriate acyltransferases to produce designer lipids with defined acyl groups at different positions of the triglyceride molecule. In separate transgenic studies manipulation of these enzymes has demonstrated a dramatic alteration in the chilling sensitivity of plants.

New cloning technologies and more efficient DNA sequencing now permit comprehensive structural studies of complex eukaryotic genomes. Previous global investigations of genome organisation in plants had shown that abundant repetitive DNAs were intermixed with genes. However, the nature of the major repeats, their possible biological roles, their origins, and their precise patterns of organisation were not clearly defined. My laboratory has used large clones derived from homologous regions of the maize, sorghum and rice genomes to investigate the nature, functional properties and evolution of grass genome organisation. Unexpectedly simple patterns of genome composition and arrangement have been seen, and these appear to be similar in different grasses. Our detailed studies of the maize genome indicate that short (2-20 kb) blocks of gene-containing DNA alternate with large (2-200 kb) blocks of intermixed middle and highly repetitive DNAs. Most of the highly repetitive sequences, and many of the middle repetitive DNAs, are retrotransposons that have inserted within each other. These repetitive DNAs are usually methylated and mostly inactive, but they are homologous to transcripts found in many different tissues. The unmethylated DNA is composed primarily of genes interspersed with lower-copy-number retroelements and inverted-repeat transposable elements. Gene order and sequence are highly conserved, but the mobile DNAs between genes appear to be different due to their rapid evolution and their variable presence or locations in different grasses.

The genetic and molecular bases of morphological evolution in plants are largely unknown. To address questions surrounding this issue, my laboratory has been investigating the evolution of maize from its wild ancestor, teosinte. Our research suggests that a few gene changes of large effect were involved in the evolution of several different traits including plant and ear architecture and kernel color. In cases where gene function could be identified, the genes involved in maize evolution were regulatory in nature. Additional evidence suggests that changes in cis regulatory elements of the regulatory genes rather than changes in protein function underlie the evolution of the traits analyzed. Future work with other plant species, especially wild plants, will be required to test the generality of our observations with maize.

Lamiales s.l., a prominent clade in the Asteridae, commonly have pentamerous monosymmetric flowers with the upper (odd) stamen reduced or lacking. Of the five largest families of the Lamiales s.l. (Gesneriaceae, Scrophulariaceae, Bignoniaceae, Acanthaceae, and Verbenaceae/Lamiaceae), perhaps the most phylogenetically basal, the Gesneriaceae, is the only one with odd staminodes or stamens occurring in all genera. In addition, Gesneriaceae have relatively large odd staminodes often with a differentiation in filament and reduced anther. They also have the largest proportion of genera (approx. 8%) with more or less polysymmetric flowers with five fertile stamens. In the more advanced families of the Lamiales s.l. the pattern varies. In Bignoniaceae an odd staminode is also commonly present. The traditional Scrophulariaceae is the most diverse family with some tribes constantly having an odd staminode (e.g. Antirrhineae) and others missing it (e.g. Manuleae, Pedicularieae), reflecting its probable polyphyly. In Acanthaceae, and still more in Verbenaceae/Lamiaceae with their extremely monosymmetric flowers an odd staminode is more often missing than present. From the systematic distribution of the different floral forms it is most likely that monosymmetric flowers were already present at the base of the Lamiales s.l. but 'reversal' to polysymmetry was still easy in the basal groups with only weak expression of monosymmetry. The evolution of more pronounced monosymmetry proceeded in different lineages and loss of the odd stamen occurred in various clades. The developmentally most intriguing groups are those with loss of the perianth and reduction of floral organ numbers to two or one, Callitriche being the most extreme genus.

The SPINDLY (SPY) locus of Arabidopsis thaliana is believed to be involved in gibberellin (GA) signal transduction. The six known mutations at this locus cause a phenotype that is consistent with constitutive activation of the GA signal transduction pathway. spy alleles are epistatic to gai, a mutation conferring gibberellin-insensitivity, indicating that SPY acts as a negative regulator of GA signal transduction, downstream of GAI. SPY was cloned using a T-DNA insertion in the spy-4 allele. SPY encodes a 914 amino acid protein with an N-terminal TPR region (a likely protein-protein interaction domain) and a novel C-terminal domain. The spy mutants show that both the N- and C-terminal domains of SPY are functionally important, spy-4 is likely to be a null allele and displays some morphological defects not seen in the other alleles. A 35S:SPY construct rescues the spy mutant phenotype, but does not show any gain-of-function SPY phenotypes. Smaller constructs overexpressing different domains of the SPY protein have no effect on plant development.

Phytochromes are regulatory photoreceptors which primarily absorb red (R) and far-red (FR) light. A great deal is known about the spectroscopic properties, primary structure, gene regulation and gross structure of phytochromes, and about the set of developmental changes which they control, but the early steps in signal transduction from phytochrome which result in these changes are still mysterious. In angiosperms, phytochromes are encoded by a small gene family, and as a result of recent work with mutants and transgenic overexpressors it is possible to assign distinct functions to some of the individual types of phytochrome. For two of these, phytochrome A and phytochrome B, overexpression of chimeras has revealed that the determinants for their photosensory specificity and the light-promoted degradation of phytochrome A reside on the amino-terminal halves of the molecules. The interchangeability of the C-terminal halves suggests that they may share a common signal transduction mechanism. These results also invite a reappraisal of the various models that have been proposed over the years to explain the complexity of phytochrome sensitivity to various light regimes.