A carrot somatic embryo mutant is rescued by chitinase

Wound-induced and developmental activation of a poplar tree chitinase gene promoter in transgenic tobacco

Wounding hybrid poplar (Populus trichocarpa x P. deltoides) trees results in the expression of novel wound-inducible (win) mRNAs thought to encode proteins involved in defense against pests and pathogens. Members of the win6 gene family encode acidic multi-domain chitinases, with combined structure and charge characteristics that differ from previously described chitinases. Win6 expression has been shown to occur in pooled unwounded leaves of a wounded (on multiple leaves) poplar plant. Here we demonstrate that wounding a single leaf induces win6 expression locally, in the wounded leaf, and remotely, in specific unwounded leaves with strong vascular connections to the wounded leaf. We also demonstrate that a win6 promoter-beta-glucuronidase (GUS) gene fusion (win6-GUS) responds to wounding locally and remotely in transgenic tobacco. These data indicate that the poplar win6 promoter has regulatory elements that are responsive to 'wound signals' in the heterologous host. In addition, win6-GUS is developmentally activated in unwounded young leaves and floral tissues of transgenic tobacco. Similar developmental expression patterns are found to occur for win6 in poplar trees, demonstrating that a herbaceous plant can serve as a host for woody tree transgene analysis and can accurately predict expression patterns in tree tissues (e.g. flowers) that would be difficult to study in free-living trees.

Effect of chitinase antisense RNA expression on disease susceptibility of Arabidopsis plants

Chitinases accumulate in higher plants upon pathogen attack are capable of hydrolyzing chitin-containing fungal cell walls and are thus implicated as part of the plant defense response to fungal pathogens. To evaluate the relative role of the predominate chitinase (class I, basic enzyme) of Arabidopsis thaliana in disease resistance, transgenic Arabidopsis plants were generated that expressed antisense RNA to the class I chitinase. Young plants or young leaves of some plants expressing antisense RNA had < 10% of the chitinase levels of control plants. In the oldest leaves of these antisense plants, chitinase levels rose to 37-90% of the chitinase levels relative to vector control plants, most likely because of accumulation and storage of the enzyme in vacuoles. The rate of infection by the fungal pathogen Botrytis cinerea was measured in detached leaves containing 7-15% of the chitinase levels of control plants prior to inoculation. Antisense RNA was not effective in suppressing induced chitinase expression upon infection as chitinase levels increased in antisense leaves to 47% of levels in control leaves within 24 hours after inoculation. Leaves from antisense plants became diseased at a slightly faster rate than leaves from control plants, but differences were not significant due to high variability. Although the tendency to increased susceptibility in antisense plants suggests that chitinases may slow the growth of invading fungal pathogens, the overall contribution of chitinase to the inducible defense responses in Arabidopsis remains unclear.

A hydroxyproline-containing class IV chitinase of sugar beet is glycosylated with xylose

Two acidic chitinase isoforms, SP1 and SP2, have been purified to homogeneity from leaves of sugar beet (Beta vulgaris) infected with Cercospora beticola. SP1 and SP2 are extracellular proteins with an apparent molecular mass of 35 kDa and an approximate pI of 4.2. Since the only major difference was slightly diverging M(r)'s, only the SP2 chitinase was further characterized. Partial amino acid sequence data for SP2 was used to generate a polymerase chain reaction (PCR) clone employed for the isolation of a cDNA clone encoding SP2. SP2 exhibits significant structural identity with the class IV chitinases from sugar beet, rapeseed, bean and maize, but differs from the other members of this class in having a longer hinge region, comprising 22 amino acid residues, with a repeated 'TTP' motif. Western blotting analyses, using antibody raised against SP2, demonstrated an induction of SP protein during infection with C. beticola. The induction was very local, with high protein accumulation found close to the infection site only. Amino acid compositional analysis of SP2 revealed that five out of fourteen prolines are hydroxylated. No glucosamine or galactosamine residues are present. Evidence was obtained that SP2 is glycosylated with a limited number (< or = 7) of xylose residues: (1) SP2 was stained with the periodic acid-Schiff (PAS) reagent, (2) electrospray mass spectrometry on SP2 gave a series of M(r)'s with a consistent increase between two molecular masses of 132 Da, (3) SP2 was recognized by an antibody specific for beta-1,4-D-xylopyranose. The vacuolar class I chitinases A and B in tobacco have recently been shown to comprise a new class of hydroxyproline-containing proteins (Sticher et al., Science 257 (1992) 655-657). The SP2 chitinase differs from these in being glycosylated and, thus, represents a novel type of hydroxyproline-containing glycoproteins in plants.

Perception of Rhizobium nodulation factors by tomato cells and inactivation by root chitinases

The bacterial genera Rhizobium and Bradyrhizobium, nitrogen-fixing symbionts of legumes, secrete specific lipo-chitooligosaccharides that induce the formation of nodules on their host plants. When preparations of such nodulation-inducing factors (Nod factors) were added to suspension-cultured tomato cells, a rapid and transient alkalinization of the culture medium occurred. Lipo-oligosaccharide preparations from Rhizobium or Bradyrhizobium treated with flavonoids, known inducers of Nod factor synthesis, were up to 100 times more potent in inducing alkalinization than the ones from untreated bacteria. The activity was absent from preparations of the mutant strain Rhizobium sp. NGR234 delta nodABC, unable to produce any Nod factors. Preparations of Nod factors from various bacteria as well as individual, highly purified Nod factors from Rhizobium sp. NGR(pA28) induced alkalinization in the tomato cell cultures at nanomolar concentrations. This demonstrates that Nod factors can be perceived by tomato, a nonhost of rhizobia. Using the alkalinization response as a sensitive bioassay, Nod factors were found to be inactivated by plant chitinases. Root chitinases purified from different legumes differed in their potential to inactivate differently substituted Nod factors produced by Rhizobium sp. NGR(pA28). This indicates that the specificity of the bacterium-host plant interaction may be due, at least in part, to differential inactivation of Nod factors by root chitinases.

Evolution and multi-functionality of the chitin system

Chitin, that is, the beta-1, 4 linked polysaccharide of N-acetylglucosamine, is best known as a cell wall component of fungi and as skeletal material of invertebrates. In recent years this simple picture has changed dramatically. Three developments have taken place: the discovery of chitinous tissues in vertebrates, the molecular analysis of the chitin-synthase genes, and the discovery that chitin derivatives play a crucial role in the interaction between higher plants and symbiotic bacteria. In this paper the methods for chitin detection and the current data on the evolution of chitin synthesis are reviewed. In addition, data is summarized which suggest that chitin synthesis may serve roles other than the production of skeletal material. In particular, anecdotal evidence suggests that chitin derivatives may play a role as signals in plant as well as animal development. Two major unresolved questions are identified: 1) Is there historical continuity of all the chitin synthesizing systems in protists, animals and, in particular, the deuterostome animals. 2) Are chitin derivatives actually involved in the development of plants and animals?

[beta]-Aminobutyric Acid Induces the Accumulation of Pathogenesis-Related Proteins in Tomato (Lycopersicon esculentum L.) Plants and Resistance to Late Blight Infection Caused by Phytophthora infestans

Tomato (Lycopersicon esculentum L.) plants were sprayed with aqueous solutions of isomers of aminobutyric acid and were either analyzed for the accumulation of pathogenesis-related (PR) proteins or challenged with the late blight fungal agent Phytophthora infestans. The [beta] isomer of aminobutyric acid induced the accumulation of high levels of three proteins: P14a, [beta]-1,3 glucanase, and chitinase. These proteins either did not accumulate or accumulated to a much lower level in [alpha]- or [gamma]-aminobutyric acid-treated plants. Plants pretreated with [alpha]-, [beta]-, and [gamma]-aminobutyric acid were protected up to 11 d to an extent of 35, 92, and 6%, respectively, against a challenge infection with P. infestans. Protection by [beta]-aminobutyric acid was afforded against the blight even when the chemical was applied 1 d postinoculation. Examination of ethylene evolution showed that [alpha]-aminobutyric acid induced the production of 3-fold higher levels of ethylene compared with [beta]-aminobutyric acid, whereas [gamma]-aminobutyric acid induced no ethylene production. In addition, silver thiosulfate, a potent inhibitor of ethylene action, did not abolish the resistance induced by [beta]-aminobutyric acid. The results are consistent with the possibility that [beta]-aminobutyric acid protects tomato foliage against the late blight disease by a mechanism that is not mediated by ethylene and that PR proteins can be involved in induced resistance.

Primary structure and expression of mRNAs encoding basic chitinase and 1,3-beta-glucanase in potato

Infection of potato leaves (Solanum tuberosum L. cv. Datura) by the late blight fungus Phytophthora infestans, or treatment with fungal elicitor leads to a strong increase in chitinase and 1,3-beta-glucanase activities. Both enzymes have been implicated in the plant's defence against potential pathogens. In an effort to characterize the corresponding genes, we isolated complementary DNAs encoding the basic forms (class I) of both chitinase and 1,3-beta-glucanase, which are the most abundant isoforms in infected leaves. Sequence analysis revealed that at least four genes each are expressed in elicitor-treated leaves. The structural features of the potato chitinases include a hydrophobic signal peptide at the N-terminus, a hevein domain which is characteristic of class I chitinases, a proline- and glycine-rich linker region which varies among all potato chitinases, a catalytic domain, and a C-terminal extension. The potato 1,3-beta-glucanases also contain a N-terminal hydrophobic signal peptide and a C-terminal extension, the latter comprising a potential glycosylation site. RNA blot hybridization experiments showed that basic chitinase and 1,3-beta-glucanase are strongly and coordinately induced in leaves in response to infection, elicitor treatment, ethylene treatment, or wounding. In addition to their activation by stress, both types of genes are regulated by endogenous factors in a developmental and organ-specific manner. Appreciable amounts of chitinase and 1,3-beta-glucanase mRNAs were found in old leaves, stems, and roots, as well as in sepals of healthy, untreated plants, whereas tubers, root tips, and all other flower organs (petals, stamen, carpels) contained very low levels of both mRNAs. In young leaves and stems, chitinase and 1,3-beta-glucanase were differentially expressed. While chitinase mRNA was abundant in these parts of the plant, 1,3-beta-glucanase mRNA was absent. DNA blot analysis indicated that in potato, chitinase and 1,3-beta-glucanase are encoded by gene families of considerable complexity.

Sequence variation, differential expression and chromosomal location of rice chitinase genes

Rice chitinases are encoded by a small multigene family. To clarify the overall organization of rice chitinase genes, we have isolated and characterized the genes Cht-1, Cht-2 and Cht-3. Although all the three genes encode class I chitinase, the nucleotide sequences of the coding regions of Cht-1 and Cht-3 are very similar (90%), while that of Cht-2 is clearly more divergent (78%). Only Cht-2 has a 130 bp intron and encodes a C-terminal peptide sequence similar to that known to function as a vacuolar targeting signal. In 5' flanking regions of Cht-1 and Cht-3, but not of Cht-2, conserved sequences (GGCCGGCYGCCCYAG) were found. Related sequences were found also in the 5' flanking regions of another chitinase gene and a beta-glucanase gene which has also been reported to be stress-induced in rice. RNA blot hybridization analysis demonstrated that the stress-induced expression patterns of the Cht-1 and Cht-3 genes are similar, but quite different from that of Cht-2. However, all three genes are active in unstressed roots. By restriction fragment length polymorphism (RFLP) linkage analysis, Cht-1 and Cht-3 were mapped onto chromosome 6 and shown to be closely linked (0.8 cM). Cht-2 was mapped onto chromosome 5. All these features suggest that the expression patterns of rice class I chitinase genes may be correlated with their levels of sequence divergence and their chromosomal location.

Isolation of a mutant Arabidopsis plant that lacks N-acetyl glucosaminyl transferase I and is unable to synthesize Golgi-modified complex N-linked glycans

The complex asparagine-linked glycans of plant glycoproteins, characterized by the presence of beta 1-->2 xylose and alpha 1-->3 fucose residues, are derived from typical mannose9(N-acetylglucosamine)2 (Man9GlcNAc2) N-linked glycans through the activity of a series of glycosidases and glycosyl transferases in the Golgi apparatus. By screening leaf extracts with an antiserum against complex glycans, we isolated a mutant of Arabidopsis thaliana that is blocked in the conversion of high-manne to complex glycans. In callus tissues derived from the mutant plants, all glycans bind to concanavalin A. These glycans can be released by treatment with endoglycosidase H, and the majority has the same size as Man5GlcNAc1 glycans. In the presence of deoxymannojirimycin, an inhibitor of mannosidase I, the mutant cells synthesize Man9GlcNAc2 and Man8GlcNAc2 glycans, suggesting that the biochemical lesion in the mutant is not in the biosynthesis of high-mannose glycans in the endoplasmic reticulum but in their modification in the Golgi. Direct enzyme assays of cell extracts show that the mutant cells lack N-acetyl glucosaminyl transferase I, the first enzyme in the pathway of complex glycan biosynthesis. The mutant plants are able to complete their development normally under several environmental conditions, suggesting that complex glycans are not essential for normal developmental processes. By crossing the complex-glycan-deficient strain of A. thaliana with a transgenic strain that expresses the glycoprotein phytohemagglutinin, we obtained a unique strain that synthesizes phytohemagglutinin with two high-mannose glycans, instead of one high-mannose and one complex glycan.

Identification of a 32-kDa anther marker protein for androgenic response in maize, Zea mays L

Variations in the whole anther protein pattern have been investigated in a highly androgenic maize hybrid during the inductive pretreatment for androgenesis. It was found that a 32-kDa protein (MAR32) is induced and accumulates in the anthers during cold pretreatment of the tassel. A positive correlation between the rate of embryo formation via anther culture and the level of this protein after 7 days of cold treatment was observed. In addition, the in vivo synthesis of this protein by cold-pretreated anthers was demonstrated. Different responsive and non-responsive genotypes were also evaluated, and the accumulation of MAR32-like protein was only observed in certain responsive genotypes. The results suggest that the protein MAR32 is a marker for a form of androgenic responsiveness in maize.

Differential expression of bean chitinase genes by virus infection, chemical treatment and UV irradiation

Three chitinases have been shown previously to be induced upon various stresses of bean leaves. Time course studies of mRNA accumulation of two of them (P3- and P4-chitinases) have been studied upon virus infection, mercuric chloride treatment and UV irradiation. In alfalfa mosaic virus (AlMV)-infected plants both mRNAs, absent in uninfected bean leaves, become detectable 36 h after inoculation. A maximum level of mRNAs is reached 84 h after inoculation and, whereas the amount of P3-ch mRNA decreases soon after having reached the maximum, the amount of P4-ch mRNA remains at high levels for several days. In mercuric chloride-treated leaves P4-ch mRNA becomes detectable 1-1.5 h after onset of treatment and a maximum level is observed between 6 h and 24 h after treatment; P3-ch mRNA becomes detectable later than P4-ch mRNA in treated leaves and reaches a maximum as late as 18 h after treatment has been applied. UV light also induces the synthesis of both mRNAs but, here again, important differences are observed in the accumulation rate of the two transcripts. The relative amounts of each mRNA induced by the different stresses have been compared. The most effective inducer of P3-ch mRNA is AlMV. In contrast, mercuric chloride induces P4-ch mRNA more efficiently than AlMV or UV light. We have also determined the complete nucleotide sequence of the cDNA encoding P3-chitinase that has been isolated from a cDNA library by using the cucumber lysozyme-chitinase cDNA as a probe. The 1072 bp P3-ch cDNA encodes a mature protein of 268 amino acid residues and the 25 residue NH2-terminal signal peptide of the precursor. Because of its high structural homology to the cucumber and Arabidopsis acidic chitinases as well as to the N-terminal amino acid sequence of the bifunctional lysozyme-chitinase from P. quinquifolia, bean P3-chitinase can be considered to belong to the class III chitinases. Southern blot analysis of bean genomic DNA revealed that P3-chitinase is encoded by a single gene.

Characterization of the non-specific lipid transfer protein EP2 from carrot (Daucus carota L.)

The extracellular protein EP2 was previously identified as non-specific lipid transfer protein based on its cDNA-derived amino acid sequence. Here, the purification of the EP2 protein from the medium of somatic embryo cultures is described. After two cycles of ion-exchange and gel permeation chromatography, a single silver-stained protein band with an apparent molecular mass of 10 kDa was observed on SDS-PAGE. This protein band was recognized by the antiserum raised against a EP2-beta-galactosidase fusion-protein. Employing a fluorescent phospholipid analog, it was shown that the purified EP2 protein is capable of binding phospholipids and is able to enhance their transfer between artificial membranes. Employing a gel permeation assay, it could be demonstrated that the EP2 protein is also capable of binding palmitic and oleic acid as well as oleyl-CoA. Because in plants these fatty acids are used as precursor molecules for cutin, these results are in support of the proposed role of the EP2 protein to transport cutin monomers from their site of synthesis through the cell wall of epidermal cells to sites of cutin polymerization.

Nucellar callus of 'Femminello' lemon, selected for tolerance toPhoma tracheiphila toxin, shows enhanced release of chitinase and glucanase into the culture medium

Phoma tracheiphila is the causative agent of the disease mal secco. Citrus cultivars differ substantially in respect to their sensitivity to the pathogenP. tracheiphila and its toxin. Some cultivars (e.g., 'Femminello' lemon) are inherently sensitive while others (e.g., 'Tarocco' orange) are tolerant. Cell lines derived from nucellar tissue of 'Femminello', 'Tarocco' and a cell line selected for tolerance to the fungal toxin ('Femminello-S') were used to study host-pathogen interaction. Our results showed that calli or conditioned media of 'Tarocco' and 'Femminello-S' inhibited the size of co-cultivated fungal colonies when compared to 'Femminello'. In addition, conditioned medium of 'Tarocco' as well as 'FemminelloS', but not 'Femminello', promoted bursting of hyphal tips. A ten-fold increase in chitinase and glucanase enzymatic activity, as evaluated by radiometric assay and laminarin hydrolysis respectively, was detected in 'Femminello-S' extracellular extracts as compared to 'Femminello'. An increase in chitinase was also shown by immunoblot analysis. Our findings suggest a positive correlation between the presence of chitinase and glucanase in the conditioned media of the cultured cells and the tolerance of those cells toP. tracheiphila toxin.

Purification and characterization of extracellular, acidic chitinase isoenzymes from elicitor-stimulated parsley cells

Treatment of cultured parsley cells (Petroselinum crispum) with fungal elicitor caused large increases in the activities of chitinase and 1,3-beta-glucanase. Chitinase activity accumulated predominantly in the culture medium, whereas 1,3-beta-glucanase activity was located almost exclusively intracellularly. Extracellular chitinase activity was resolved into six different isoenzymes, all of which were purified and characterized. All six isoforms were acidic proteins (pI 3.8-5.3), with molecular mass 30-38 kDa. Four were exochitinases and two were endochitinases. The most abundant isoform also showed lysozyme activity. Three of the exochitinases were glycoproteins and two of these were reactive with an antiserum specific for xylose in complex glycosidic structures. The exochitinases constituted relatively small proportions of the total chitinase activity and may serve a different function in cellular metabolism compared to the more abundant endochitinases.

cDNA cloning of ECP40, an embryogenic-cell protein in carrot, and its expression during somatic and zygotic embryogenesis

A cDNA of 1.2 kb was isolated from a cDNA library of embryogenic cells of carrot (Daucus carota L.) by use of nucleotide sequences that encode two internal amino-acid sequences of ECP40 (an embryogenic-cell protein with a relative molecular mass of 40,000). A genomic Southern blot using the cDNA as probe suggested that there are at least two genes for ECP40 in the carrot genome. The cDNA encoded an open reading frame of 306 amino acids, and the deduced amino-acid sequence was found to share two motifs, namely SSSSSSEDDGXGGRRKKGXXXKIKEKLXGG and EKKXXXDKIKXKLPG, with rab16 protein from rice and dehydrins from barley and maize. The level of expression of these proteins has been reported to be high during late embryogenesis and to be induced by a plant hormone, ABA. Accumulation of ECP40-specific transcripts started 18 days after flowering and continued until maturation of seeds, but the levels decreased within 24 h after imbibition. ECP40 and its mRNAs were detected in the endosperm and zygotic embryos of mature seeds by immunohistochemistry and in situ hybridization. Exogenous application of 0.1 mM ABA to carrot seedlings did not induce expression of the gene for ECP40, while drought treatment induced the accumulation of low levels of the mRNAs. During somatic embryogenesis, the mRNAs were found at high levels in embryogenic cells and at low levels in somatic embryos at the torpedo stage. Immunohistochemical analysis and in situ hybridization showed that both ECP40 and its transcripts were preferentially localized in the peripheral cells of the clusters of embryogenic cells. In somatic embryos, application of ABA resulted in increases in levels of mRNAs for ECP40 up to the levels in embryogenic cells, but no such increases were observed in ABA-treated embryogenic cells. The pattern of expression of the gene for ECP40 during somatic embryogenesis was basically the same as that of ECP31, another ABA-regulable embryogenic-cell protein of carrot, the presence of which has been correlated with the embryogenic competence of cultured cells (T. Kiyosue, S. Satoh, H. Kamada and H. Harada, Plant Physiol 95 (1991) 1077-1083). The various results together imply that a group of ABA-inducible genes is expressed in these embryogenic cells.

Cloning and characterization of avocado fruit mRNAs and their expression during ripening and low-temperature storage

Differential screening of a cDNA library made from RNA extracted from avocado (Persea americana Mill cv. Hass) fruit stored at low temperature (7 degrees C) gave 23 cDNA clones grouped into 10 families, 6 of which showed increased expression during cold storage and normal ripening. Partial DNA sequencing was carried out for representative clones. Database searches found homologies with a polygalacturonase (PG), endochitinase, cysteine proteinase inhibitor and several stress-related proteins. No homologies were detected for clones from six families and their biological role remains to be elucidated. A full-length cDNA sequence for avocado PG was obtained and the predicted amino acid sequence compared with those from other PGs. mRNA encoding PG increased markedly during normal ripening, slightly later than mRNAs for cellulase and ethylene-forming enzyme (EFE). Low-temperature storage delayed ripening and retarded the appearance of mRNAs for enzymes known to be involved in cell wall metabolism and ethylene synthesis, such as cellulase, PG and EFE, and also other mRNAs of unknown function. The removal of ethylene from the atmosphere surrounding stored fruit delayed the appearance of the mRNAs encoding cellulase and PG more than the cold storage itself, although it hardly affected the expression of the EFE mRNA or the accumulation of mRNAs homologous to some other unidentified clones.

Floral organ-specific and constitutive expression of an Arabidopsis thaliana heat-shock HSP18.2::GUS fusion gene is retained even after homeotic conversion of flowers by mutation

Organ-specific and constitutive expression of the Arabidopsis HSP18.2 gene under normal growth conditions (22 degrees C) was observed in transgenic A. thaliana, which carried a fusion gene composed of the promoter region of HSP18.2, one of the genes for low molecular weight heat-shock proteins in Arabidopsis, and the gene for beta-glucuronidase (GUS) from Escherichia coli. In order to clarify the organ-specific nature of promoter expression, various mutations that affect flower morphology were introduced into the transgenic Arabidopsis line, AHS9. The results show that the pattern of expression observed in sepals, filaments, and styles is regulated in a structure-dependent manner, and suggest that the HSP18.2 gene might have an important role in the process of differentiation of flower buds, as do several other stress-related genes.

Emerging strategies for enhancing crop resistance to microbial pathogens

There are marked differences in the pattern of host gene expression in incompatible plant:microbial pathogen interactions compared with compatible interactions, associated with the elaboration of inducible defenses. Constitutive expression of genes encoding a chitinase or a ribosome-inactivating protein in transgenic plants confers partial protection against fungal attack, and a large repertoire of such antimicrobial genes has been identified for further manipulation. In addition, strategies are emerging for the manipulation of multigenic defenses such as lignin deposition and synthesis of phytoalexin antibiotics by overexpression of genes encoding rate determining steps, modification of transcription factors or other regulatory genes, and engineering production of novel phytoalexins by interspecies transfer of biosynthetic genes. The imminent cloning of disease resistance genes, further molecular dissection of stress signal perception and transduction mechanisms, and identification of genes that affect symptom development will provide attractive new opportunities for enhancing crop protection. Combinatorial integration of these novel strategies into ongoing breeding programs should make an important contribution to effective, durable field resistance.

Identification of a transitional cell state in the developmental pathway to carrot somatic embryogenesis

We have located a novel carbohydrate epitope in the cell walls of certain single cells in embryogenic, but not in non-embryogenic, suspension cultures of carrot. Expression of this epitope, recognized by the mAb JIM8, is regulated during initiation, proliferation, and prolonged growth of suspension cultures such that changes in the abundance of JIM8-reactive cells always precede equivalent changes in embryogenic potential. Therefore, a direct correlation exists between the presence of the JIM8-reactive cell wall epitope and somatic embryo formation. The JIM8-reactive cell wall epitope is expressed in the cell walls of three types of single cells and one type of cell cluster. One of the single cell types seems able to follow one of two phytohormone-controlled developmental pathways, either a cell elongation pathway that eventually leads to cell death, or a cell division pathway that gives rise to proembryogenic masses. We demonstrate that all JIM8-reactive cell types in embryogenic carrot suspension cultures are developmentally related, and that the switch by one of them to somatic embryogenesis is accompanied by the immediate dissipation of the JIM8-reactive cell wall epitope. The cell wall carbohydrate epitope recognized by JIM8 therefore represents a cell wall marker for a very early transitional cell state in the developmental pathway to carrot somatic embryogenesis.

Rhizobial lipo-oligosaccharides: answers and questions

Rhizobium bacteria produce certain lipo-oligosaccharides (modified chitin oligomers) after induction of nodulation (nod) gene transcription by the host plant. The function of the rhizobial nod genes in the biosynthesis of these lipo-oligosaccharides, focusing on their host specific aspects, is discussed. The lipo-oligosaccharides can elicit various responses in the host plants, like the formation of pre-infection threads and nodule meristems. Speculating on their function in plant morphogenesis the question is raised: do the rhizobial lipo-oligosaccharides resemble unknown plant signal molecules?