Symposia of the Society for Experimental Biology最新文献

Intercellular communication in plants has evolved to occur via elongated cytoplasmic bridges, called plasmodesmata, that traverse the thick cell walls that surround plant cells. Historically, plasmodesmata have been assigned the mostly passive role of creating cytoplasmic continuity between plant cells enabling free transport of the wealth of small plant metabolites and growth hormones under 1 kDa. When it was discovered that plant viruses pirate plasmodesmata for movement of viral genomes during infection, it was proposed that viruses modified plasmodesmata for transport of very large molecules. Now, there is compelling evidence that plasmodesmata are inherently dynamic, rapidly altering their dimensions to increase their transport capabilities, upon contact with viral as well as developmentally important plant proteins. Further, the study of intercellular transport has prompted analyses of intracellular transport pathways, implicating the cytoskeleton as a major tracking system to plasmodesmata. Thus, plasmodesmata form a three-dimensional network of transportation channels and major checkpoints for information transfer. In the following, current knowledge about structure and function of these connective organelles, and about routing of molecules towards plasmodesmata, will be summarized.

The pollen grain coating of Brassica oleracea contains a polymorphic family of highly charged small proteins (PCP-A, pollen coat protein, class A) related to the defensin class of seed proteins. On pollination these proteins are released from the grain and in vitro data suggest that at least one member of the family (PCP-A1) interacts specifically with elements of the stigmatically-expressed S(self-incompatibility) receptor complex. A new in vivo bioassay has demonstrated the male determinant of the self incompatibility system to be contained within the pollen coating, and this determinant to be a low molecular mass protein. A combination of data from interspecific studies and molecular analysis of PCP-A proteins indicates that the primary interaction between PCP-A1 and the receptor complex may be involved in establishing compatibility, while other molecular interactions, perhaps involving other PCP-A class proteins, are responsible for regulating S-specific rejection of self grains. The evolution of the self incompatibility system on the dry sigma of Brassica is discussed in the context of these data.

Hybrid corn seed is traditionally produced using either mechanical/hand detasseling or cytoplasmic male sterility, or a combination of both. In recent years, the development of transgenic systems to produce hybrid seed in several crops has attracted much attention. Here we describe a transgenic mechanism for production of hybrid corn, reversible male sterility (RMS), in which the action of the cytotoxic gene used to introduce male sterility is suppressed by the application of a chemical to the plant. Reversion of the sterility allows the RMS parent to be self-fertilized, a step which overcomes the need to remove fertile sib plants prior to making the hybrid cross. The key enabling technology in RMS is the use of a plant gene promoter which is specifically induced by chemical application. We have exemplified RMS in transgenic corn plants and believe that it provides specific benefits in the production of hybrid corn seed.

Characterization of the DNA sequence of 4 tomato leaf mould disease resistance genes (Cf-2, Cf-4, Cf-5 and Cf-9) leads to the prediction that they encode C-terminally membrane anchored glycopeptides with many extracytoplasmic leucine rich repeats (LRRs). The N terminal LRRs are variable between the Cf-genes, suggesting a role in specificity, and the C terminal LRRs are more conserved, suggesting a role in signal transduction. Genetic analysis has revealed several Rcr genes that are required for Cf-gene function; their isolation will help us understand how Cf-genes work. Cf-9 confers responsiveness to pathogen-encoded Avr9 peptide on introduction to tobacco. Tobacco suspension cultures carrying the Cf-9 gene produce reactive oxygen species in response to Avr9 peptide, whereas untransformed cultures do not. The significance of these observations is discussed.

The plant extracellular matrix has multiple roles in determining pattern during plant development. These include provision of anchorage sites for focal adhesion-like structures which may play a direct signalling role and provide a reference for cytoskeletal elements involved in nuclear rotation and orientation of the cell division plane. The activity of mechanosensitive ion channels in the plasma membrane can also be regulated by the mechanical properties of the cell wall. Moreover, there is increasing evidence from a variety of systems suggesting that the cell wall may be a direct source of factors which specify cell fate in response to position. These may be inserted into the wall by differentiating cells and may act by providing signals to adjacent cells or by providing positive feedback to the protoplast contained therein, maintaining its fate according to its position.

The semi-dominant abi1-1 mutation of Arabidopsis interferes with multiple aspects of abscisic acid signal transduction resulting in reduced seed dormancy and sensitivity of root growth in ABA. Furthermore, the mutant transpires excessively as a result of abnormal stomatal regulation leading to a wilty phenotype. The ABI1 gene has been cloned. The carboxyl-terminal domain of the predicted ABI1 protein is related to the 2C class of serine-threonine phosphatases while no overt homology was found in the extended amino terminus. A combination of in vitro assays and yeast mutant complementation studies confirmed that ABI1 is a functional protein phosphatase 2C. The abi1-1 mutation converts the amino acid glycine180 to aspartic acid, and in the above test systems, causes a partial loss of the phosphatase activity. In transgenic Nicotiana benthamiana guard cells, the abi1-1 gene causes a reduction in the background current of the outward-rectifying potassium channels, and also in the abscisic acid-sensitivity of both the outward- and the inward-rectifying potassium channels in the plasma membrane. However, normal sensitivity of both potassium channels to, and stomatal closure in, abscisic acid was recovered in the presence of H7 and staurosporine, both broad-range protein kinase antagonists. These results suggest the aberrant potassium channel behavior as a major consequence of abi1-1 action and implicate ABI1 as part of a phosphatase/kinase pathway that modulates the sensitivity of guard-cell potassium channels to abscisic acid-evoked signal cascades.

A model system for adventitious root formation in woody plants was used to identify transcripts that are up-regulated during this process. 1 mm stem-disks from micropropagated shoots of the apple cultivar Jork 9 can be induced to form roots by treatment with the auxin indole butyric acid (IBA). Stem discs are placed on medium containing IBA for 24 hours and then transferred to IBA-free medium. Root initials become visible after 5-6 days and root elongation occurs within 7-9 days. The first visible cell divisions, which will give rise to the adventitious roots, are detectable 48 hours following IBA treatment. We have used this system to identify transcripts that are induced during adventitious root formation. Two techniques were employed in this analysis: differential messenger RNA display (DDRT) and mRNA representational difference analysis (RDA), a technique that couples PCR and subtractive hybridisation. Using both of these techniques a number of clones have been isolated that exhibit differential expression during auxin induced root formation. Both up-regulated and down-regulated transcripts have been identified. Expression of these genes has initially been verified by reverse northern blot analysis. Northern blot analysis with individual clones has confirmed the expression pattern observed in the reverse northern analysis. Tentative identities of some of the clones have been established by sequencing the partial cDNAs. Among the up-regulated transcripts are clones that share sequence homology with polygalacturonase and MAP kinases. A full-length cDNA for the most abundant up-regulated mRNA, a 2-oxoacid dependent dioxygenase, was characterised. This mRNA is expressed between 24 and 72 hours following IBA treatment of apple stem disks.

Control of the pattern of cell divisions is central to plant development. Several different types of control exist: examples include control of overall cell number in floral meristems, control of relative cell numbers in floral whorls, and control of the relative spacing of the cell divisions that establish floral organs. Mutations that change each of these levels of control are described: mutations in the SUPERMAN gene affect relative amounts of cell division in adjacent floral whorls; mutations in the CLAVATA genes affect overall meristematic cell number; and mutations in PERIANTHIA affect the spacing of floral organs.

The mature maize leaf is characterised by a series of parallel veins that are surrounded by concentric rings of bundle sheath (BS) and mesophyll (M) cells. To identify genes that control cellular differentiation patterns in the leaf, we have isolated a group of mutations that specifically disrupt the differentiation of a single cell type. In maize bundle sheath defective (bsd) mutants, C4 photosynthetic development is perturbed in BS cells while M cells appear to develop normally. Two mutants, bsd1 and bsd2, have been characterised in detail. Analysis of these mutants, and the corresponding Bsd1 and Bsd2 genes is providing an insight into cellular processes regulating photosynthetic cell type differentiation in maize.