Arabinogalactan proteins is an umbrella term applied to a highly diverse class of cell surface glycoproteins, many of which contain glycosylphosphatidylinositol lipid anchors. The structures of protein and glycan moieties of arabinogalactan proteins are overwhelmingly diverse while the "hydroxproline contiguity hypothesis" predicts arabinogalactan modification of members of many families of extracellular proteins. Descriptive studies using monoclonal antibodies reacting with carbohydrate epitopes on arabinogalactan proteins and experimental work using beta-Yariv reagent implicate arabinogalactan proteins in many biological processes of cell proliferation and survival, pattern formation and growth, and in plant microbe interaction. Advanced structural understanding of arabinogalactan proteins and an emerging molecular genetic definition of biological roles of individual arabinogalactan protein species, in conjunction with potentially analogous extracellular matrix components of animals, stimulate hypotheses about their mode of action. Arabinogalactan proteins might be soluble signals, or might act as modulators and coreceptors of apoplastic morphogens; their amphiphilic molecular nature makes them prime candidates of mediators between the cell wall, the plasma membrane, and the cytoplasm.

Monoclonal antibodies recognizing un-esterified (JIM5) and methyl-esterified (JIM7) epitopes of pectin have been used to locate these epitopes by indirect immunofluorescence and immunogold electron microscopy in the root apex of carrot (Daucus carota L.). Both antibodies labelled the walls of cells in all tissues of the developing root apex. Immunogold labelling observed at the level of the electron microscope indicated differential location of the pectin epitopes within the cell walls. The un-esterified epitope was located to the inner surface of the primary cell walls adjacent to the plasma membrane, in the middle lamella and abundantly to the outer surface at intercellular spaces. In contrast, the epitope containing methyl-esterified pectin was located evenly throughout the cell wall. In root apices of certain other species the JIM5 and JIM7 epitopes were found to be restricted to distinct tissues of the developing roots. In the root apex of oat (Avena sativa L.), JIM5 was most abundantly reactive with cell walls at the region of intercellular spaces of the cortical cells. JIM7 was reactive with cells of the cortex and the stele. Neither epitope occurred in walls of the epidermal or root-cap cells. These pattern of expression were observed to derive from the very earliest stages of the development of these tissues in the oat root meristem and were maintained in the mature root. In the coleoptile and leaf tissues of oat seedlings, JIM5 labelled all cells abundantly whereas JIM7 was unreactive. Other members of the Gramineae and also the Chenopodiaceae are shown to express similar restricted spatial patterns of distribution of these pectin epitopes in root apices.

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MYB-related transcription factors are known to regulate different branches of flavonoid metabolism in plants and are believed to play wider roles in the regulation of phenylpropanoid metabolism in general. Here, we demonstrate that overexpression of two MYB genes from Antirrhinum represses phenolic acid metabolism and lignin biosynthesis in transgenic tobacco plants. The inhibition of this branch of phenylpropanoid metabolism appears to be specific to AmMYB308 and AmMYB330, suggesting that they recognize their normal target genes in these transgenic plants. Experiments with yeast indicate that AmMYB308 can act as a very weak transcriptional activator so that overexpression may competitively inhibit the activity of stronger activators recognizing the same target motifs. The effects of the transcription factors on inhibition of phenolic acid metabolism resulted in complex modifications of the growth and development of the transgenic plants. The inhibition of monolignol production resulted in plants with at least 17% less lignin in their vascular tissue. This reduction is of importance when designing strategies for the genetic modification of woody crops.

Evidence is provided that ethylene is a positive regulator of hair cell development in the root epidermis of Arabidopsis thaliana. Treatment of seedlings with increasing concentrations of the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC) results in progressively more root hair cells developing in positions normally occupied by non-hair cells. Consistent with these findings are observations that treatments that block either ethylene synthesis or its perception reduce the number of root hairs. A model is proposed in which either ethylene or ACC is a signal involved in specifying the pattern of cell differentiation in the Arabidopsis root epidermis.

Two polymorphic forms of an extracellular arabinogalactan protein (AGP1 and AGP2), obtained from the conditioned media of two carrot suspension-cultured cell lines, have been identified in terms of binding of the anti-plasma membrane antibodies JIM4 and MAC207. AGP1 and AGP2 have been used as immunogens to generate further anti-AGP monoclonal antibodies. JIM14 identified an epitope carried by AGP2 and also by glycoproteins of low molecular weight localized to the plant cell wall. In addition, further antibodies (JIM13 and JIM15) identified carbohydrate epitopes of the AGPs that also occur on plasma membrane glycoproteins and are expressed by patterns of cells that reflect cell position at the carrot root apex. Indirect immunofluorescence microscopy indicated that JIM13 recognized the surface of cells forming the epidermis and cells marking the region and axis of the future xylem. JIM15 recognized a pattern of cells directly complementary to the JIM13 pattern. The panel of anti-AGP monoclonal antibodies now available indicates groups of cells within the root meristem that may reflect an early pre-pattern of the tissues of the mature root structure and suggests extensive modulation of cell surface AGPs during cell development and the positioning of cells within the apex.

SummaryThe Spo11 protein is a eukaryotic homologue of the archaeal DNA topoisomerase VIA subunit (topo VIA). In archaea it is involved, together with its B subunit (topo VIB), in DNA replication. However, most eukaryotes, including yeasts, insects and vertebrates, instead have a single gene for Spo11/topo VIA and no homologues for topo VIB. In these organisms, Spo11 mediates DNA double-strand breaks that initiate meiotic recombination. Many plant species, in contrast to other eukaryotes, have three homologues for Spo11/topo VIA and one for topo VIB. The homologues in Arabidopsis, AtSPO11-1, AtSPO11-2 and AtSPO11-3, all share 20-30% sequence similarity with other Spo11/topo VIA proteins, but their functional relationship during meiosis or other processes is not well understood. Previous genetic evidence suggests that AtSPO11-1 is a true orthologue of Spo11 in other eukaryotes and is required for meiotic recombination, whereas AtSPO11-3 is involved in DNA endo-reduplication as a part of the topo VI complex. In this study, we show that plants homozygous for atspo11-2 exhibit a severe sterility phenotype. Both male and female meiosis are severely disrupted in the atspo11-2 mutant, and this is associated with severe defects in synapsis during the first meiotic division and reduced meiotic recombination. Further genetic analysis revealed that AtSPO11-1 and AtSPO11-2 genetically interact, i.e. plants heterozygous for both atspo11-1 and atspo11-2 are also sterile, suggesting that AtSPO11-1 and AtSPO11-2 have largely overlapping functions. Thus, the three Arabidopsis Spo11 homologues appear to function in two discrete processes, i.e. AtSPO11-1 and AtSPO11-2 in meiotic recombination and AtSPO11-3 in DNA replication.

Petals of animal-pollinated angiosperms have adapted to attract pollinators. Factors influencing pollinator attention include colour and overall size of flowers. Colour is determined by the nature of the pigments, their environment and by the morphology of the petal epidermal cells. Most angiosperms have conical epidermal cells, which enhance the colour intensity and brightness of petal surfaces. The MYB-related transcription factor MIXTA controls the development of conical epidermal cells in petals of Antirrhinum majus. Another gene encoding an R2R3 MYB factor very closely related to MIXTA, AmMYBML2, is also expressed in flowers of A. majus. We have analysed the roles of AmMYBML2 and two MIXTA-related genes, PhMYB1 from Petunia hybrida and AtMYB16 from Arabidopsis thaliana, in petal development. The structural similarity between these genes, their comparable expression patterns and the similarity of the phenotypes they induce when ectopically expressed in tobacco, suggest they share homologous functions closely related to, but distinct from, that of MIXTA. Detailed phenotypic analysis of a phmyb1 mutant confirmed the role of PhMYB1 in the control of cell morphogenesis in the petal epidermis. The phmyb1 mutant showed that epidermal cell shape affects petal presentation, a phenotypic trait also observed following re-examination of mixta mutants. This suggests that the activity of MIXTA-like genes also contributes to petal form, another important factor influencing pollinator attraction.KEY WORDS: Petal, Cell shape, Petunia, Antirrhinum, MYB transcription factor Development 134, 1691Development 134, -1701Development 134, (2007 DEVELOPMENT 1692Emerging data from the study of transcription factors belonging to large families of structurally related proteins suggest that very similar members of phylogenetically-clustered subgroups usually share closely related functions, even though functions may have diverged over the entire family. Structural similarity has led to claims of orthology and functional equivalence between MIXTA, PhMYB1 and AtMYB16 (van Houwelingen et al., 1998;Romero et al., 1998). However, to achieve a general understanding of the control of morphogenesis of petal epidermal cells, the function of new genes needs to be assayed and compared with that of the prototype, MIXTA. In addition, the relevance of these genes to cell shaping and, specifically, to their roles in adapting petals for pollinator attraction, needs to be established in different angiosperm species.We have examined the function of three of the genes encoding proteins very closely related to MIXTA; PhMYB1 from Petunia hybrida, AmMYBML2 from A. majus and AtMYB16 from Arabidopsis thaliana. Structurally, these proteins are most closely related to each other and their genes are therefore orthologous. All three proteins promote directional cell expansion in a bioassay in tobacco. The similarities between the phenotypes induced by these proteins in this bioassay (their biochemical functions) and the similarities in their expression patter...