A major stress-inducible Mr-42000 wall glycoprotein of French bean (Phaseolus vulgaris L.)

PRPs localized to the middle lamellae are required for cortical tissue integrity in Medicago truncatula roots

A family of repetitive proline-rich proteins interact with acidic pectins and play distinct roles in legume root cell walls affecting cortical and vascular structure. A proline-rich protein (PRP) family, composed of tandemly repeated Pro-Hyp-Val-X-Lys pentapeptide motifs, is found primarily in the Leguminosae. Four distinct size classes within this family are encoded by seven tightly linked genes: MtPRP1, MtPRP2 and MtPRP3, and four nearly identical MtPRP4 genes. Promoter fusions to β-glucuronidase showed strong expression in the stele of hairy roots for all 4 PRP genes tested, with additional expression in the cortex for PRP1, PRP2 and PRP4. All except MtPRP4 are strongly expressed in non-tumorous roots, and secreted and ionically bound to root cell walls. These PRPs are absent from root epidermal cell walls, and PRP accumulation is highly localized within the walls of root cortical and vascular tissues. Within xylem tissue, PRPs are deposited in secondary thickenings where it is spatially exclusive to lignin. In newly differentiating xylem, PRPs are deposited in the regularly spaced paired-pits and pit membranes that hydraulically connect neighboring xylem elements. Hairpin-RNA knock-down constructs reducing PRP expression in Medicago truncatula hairy root tumors disrupted cortical and vascular patterning. Immunoblots showed that the knockdown tumors had potentially compensating increases in the non-targeted PRPs, all of which cross-react with the anti-PRP antibodies. However, PRP3 knockdown differed from knockdown of PRP1 and PRP2 in that it greatly reduced viability of hairy root tumors. We hypothesize that repetitive PRPs interact with acidic pectins to form block-copolymer gels that can play distinct roles in legume root cell walls.

Analysis of the potato calcium-dependent protein kinase family and characterization of StCDPK7, a member induced upon infection with Phytophthora infestans

We describe the potato CDPK family and place StCDPK7 as a player in potato response to Phytophthora infestans infection, identifying phenylalanine ammonia lyase as its specific phosphorylation target in vitro. Calcium-dependent protein kinases (CDPKs) decode calcium (Ca²⁺) signals and activate different signaling pathways involved in hormone signaling, plant growth, development, and both abiotic and biotic stress responses. In this study, we describe the potato CDPK/CRK multigene family; bioinformatic analysis allowed us to identify 20 new CDPK isoforms, three CDPK-related kinases (CRKs), and a CDPK-like kinase. Phylogenetic analysis indicated that 26 StCDPKs can be classified into four groups, whose members are predicted to undergo different acylation patterns and exhibited diverse expression levels in different tissues and in response to various stimuli. With the aim of characterizing those members that are particularly involved in plant-pathogen interaction, we focused on StCDPK7. Tissue expression profile revealed that StCDPK7 transcript levels are high in swollen stolons, roots, and mini tubers. Moreover, its expression is induced upon Phytophthora infestans infection in systemic leaves. Transient expression assays showed that StCDPK7 displays a cytosolic/nuclear localization in spite of having a predicted chloroplast transit peptide. The recombinant protein, StCDPK7:6xHis, is an active Ca²⁺-dependent protein kinase that can phosphorylate phenylalanine ammonia lyase, an enzyme involved in plant defense response. The analysis of the potato CDPK family provides the first step towards the identification of CDPK isoforms involved in biotic stress. StCDPK7 emerges as a relevant player that could be manipulated to deploy disease resistance in potato crops.

Role of hydroxyproline-rich glycoproteins in resistance of pearl millet against downy mildew pathogen Sclerospora graminicola

Hydroxyproline-rich glycoproteins (HRGPs) are important plant cell wall components involved in plant defense response to pathogen attack. In the present study, a resistant pearl millet (Pennisetum glaucum) cultivar, IP18292, was compared with a susceptible cultivar, 7042S, to investigate the contribution of HRGPs in the successful defense against the phytopathogenic oomycete S. graminicola. Northern hybridization using MeHRGP cDNA, a heterologous probe from cassava, indicated steady accumulation of HRGP transcripts, from 2 h.p.i. onwards with a maximum at 6 h.p.i., in the resistant cultivar. This is followed by HRGPs accumulation at about 8 h.p.i. as revealed by Western-blot analysis. Immunocytochemical localization by tissue printing and confocal immunofluorescence microscopy indicated cell walls of parenchymatic cells and the vascular tissue of coleoptile as sites of HRGP deposition. In vitro studies in the presence of horseradish peroxidase and H2O2 showed cross-linking of pearl millet HRGPs, which occurred parallel to isodityrosine accumulation. Inducible high isodityrosine content was also observed in vivo in the resistant cultivar. Here, H2O2 was found to accumulate as twin burst at 1 and 6 h.p.i., whereas in the susceptible cultivar only an early single peak was detectable. Moreover, the amount of hydroxyproline in HRGPs was about twice as high in the resistant as in the susceptible cultivar. These results suggest that cell wall strengthening in S. graminicola-infected resistant pearl millet is brought about by a combination of polypeptide cross-linking of isodityrosine as well as by the high content of hydroxyproline in HRGPs, and H2O2, in contrast to the susceptible plant.

Proline-rich cell wall proteins accumulate in growing regions and phloem tissue in response to water deficit in common bean seedlings

Plant cell walls undergo dynamic changes in response to different environmental stress conditions. In response to water deficit, two related proline-rich glycoproteins, called p33 and p36, accumulate in the soluble fraction of the cell walls in Phaseolus vulgaris (Covarrubias et al. in Plant Physiol 107:1119-1128, 1995). In this work, we show that p33 and p36 are able to form a 240 kDa oligomer, which is found in the cell wall soluble fraction. We present evidence indicating that the highest accumulation of these proteins in response to water deficit occurs in the growing regions of common bean seedlings, particularly in the phloem tissues. These proteins were detected in P. vulgaris cell suspension cultures, where the p33/p36 ratio was higher under hyperosmotic conditions than in bean seedlings subjected to the same treatment. The results support a role for these proteins during the plant cell response to changes in its water status, and suggest that cell wall modifications are induced in active growing cells of common bean in response to water limitation.

Purification and characterization of proline/hydroxyproline-rich glycoprotein from pearl millet coleoptiles infected with downy mildew pathogen Sclerospora graminicola

Hydroxyproline-rich glycoproteins (HRGPs) are important plant cell wall structural components, which are also involved in response to pathogen attack. In pearl millet, deposition and cross-linking of HRGPs in plant cell walls was shown to contribute to the formation of resistance barriers against the phytopathogenic oomycete Sclerospora graminicola. In the present study, the purification and characterization of HRGPs that accumulated in coleoptiles of pearl millet seedlings in response to S. graminicola inoculation has been carried out. Periodic acid Schiff's staining revealed that the purified protein was a glycoprotein. The protein to carbohydrate ratio was determined to be 95.5%:4.5% (w/w). Proline amounted for 20 mol% of the total amino acids as indicated by amino acid composition analysis. The isolated protein had a pI of 9.8 and was shown to be composed of subunits of 27, 17, and 14 kDa. Cross reactivity with the monoclonal antibody MAC 265 and the presence of the signature amino acid sequence, PVYK, strongly suggested to classify the purified glycoprotein as a member of the P/HRGPs class. In the presence of horseradish peroxidase and H2O2 the purified glycoprotein served as a substrate for oxidative cross-linking processes.

A two component chitin-binding protein from French bean -- association of a proline-rich protein with a cysteine-rich polypeptide

A 42kDa chitin-binding proline-rich protein (PRP) from French bean has been previously characterised through its involvement in plant-pathogen interactions. It is located at the plasmalemma-wall interface, intercellular spaces and binds to the pathogen Colletotrichum lindemuthianum in vitro and in planta. It is also present in cell wall appositions formed in response to an hrp mutant of Xanthomonas campestris. We now show that the 42kDa protein is composed of two components, a 25kDa polypeptide member of the PRP family of legumes and a 6.8kDa cysteine-rich peptide with high similarity to snakin-2 from potato. Snakins bind to pathogens and are antimicrobial. Molecular cloning of the longest PRP corresponding to the N-terminal sequence of the purified protein and the 6.8kDa component is reported. The cognate mRNAs show coordinate expression. The two-component protein complex has already been shown to be involved in binding and immobilising pathogens through oxidative cross-linking of the PRP components but could also function as a two-component chitin-receptor involved in plant-pathogen interactions through antimicrobial activity and/or signalling.

Comparative subcellular immunolocation of polypeptides associated with xylan and callose synthases in French bean (Phaseolus vulgaris) during secondary wall formation

The Golgi apparatus of plant cells is thought to be the main site of synthesis of cell wall matrix polysaccharides and the terminal glycosylation of glycoproteins. Much of this evidence still depends on earlier biochemical studies employing subcellular fractionation. However acquiring pure Golgi membranes is still difficult and the question of spatial organisation of glycosyl transferases can be addressed by immunolocation of the enzymes. An antibody to a xylan synthase-associated polypeptide from French bean, the enzyme which synthesises the core polysaccharide for secondary wall xylan, has been raised and shown to inhibit its activity. Xylan is deposited in secondary thickenings and the xylan synthase was only detected in appreciable amounts in developing xylem cells. The location within the Golgi stack was observed throughout the dictyosomes. Some enzyme subunits were also detected in post-Golgi vesicles. A second antibody to a non-catalytic M(r) 65000 subunit of beta 1,3- glucan (callose) synthase was used for a comparative study. Although the bulk of this enzyme has been detected in previous studies at plasmamembrane-wall interfaces in sieve plates and stressed tissue, a Golgi-location can be observed in root tip meristematic cells during cell plate formation. The enzyme was present throughout the stacks. Callose was also immunolocated in a similar manner to xylan in secondary walls and thickenings and in pits in developing xylem. In these cells, the callose synthase was detected at the surface of the growing thickenings and the plasmamembrane within the pits.

Pathogenic infection and the oxidative defences in plant apoplast

The structural and functional continuum of the plant apoplast is the first site of contact with a pathogen and plays a crucial role in initiation and coordination of many defence responses. In this paper, we present an overview of the involvement of the plant apoplast in plant-pathogen interactions. The process of infection of French bean (Phaseolus vulgaris L.) plants by Colletotrichum lindemuthianum is analysed. The ultrastructural features of plant defence responses to fungal infection are then compared with those observed in plants or cell suspensions treated with various elicitors. Changes in cell walls and in whole plant cells responding to infection seem to be highly similar in all systems used. Model systems of French bean and white lupin (Lupinus albus L.) are then utilised to provide some biochemical characteristics of oxidative reactions in the apoplast evoked by elicitor treatment. The species specificity of various mechanisms generating reactive oxygen species is discussed, and some details of pH-dependent H2O2-generating activity of peroxidases are demonstrated. As its exocellular nature is an important feature of the oxidative burst, the major consequence of this event, i.e., the oxidative cross-linking of wall components during the papilla formation and strengthening of the walls, is analysed. Finally, the possible involvement of other wall-associated and developmentally regulated H2O2-generating mechanisms, like amine and oxalate oxidases, in plant defence is demonstrated. It is concluded that under stress conditions, such apoplastic mechanisms might be employed to increase plants' chances of survival.

Ultrastructural localisation and further biochemical characterisation of prolyl 4-hydroxylase from Phaseolus vulgaris: comparative analysis

Prolyl 4-hydroxylase (EC 1.14.11.2), the enzyme responsible for the post-translational hydroxylation of peptide proline, has been well described in animals but has been little studied in plants. The best characterised example is the enzyme from French bean (Phaseolus vulgaris). In this study, the biochemical properties of this plant enzyme were examined in more detail and, using specific polyclonal antibodies, the localisation of the enzyme was determined. Both alpha- and beta-subunits did not show multiple forms, suggesting a relatively broad specificity of the enzyme complex with respect to the target hydroxylated amino acid sequences. Antibodies to the mammalian and Chlamydomonas enzymes cross-react with the higher plant subunits, indicating that some epitopes are highly conserved. The P. vulgaris enzyme was inhibited by analogues of oxoglutarate, but was not susceptible to doxorubicin. Inhibition of the bean enzyme by an oxaloglycine derivative resulted in the retention of the target (hydroxy)proline-rich protein in the endomembrane system. Immunolocalisation of the enzyme showed close association with the endoplasmic reticulum and Golgi apparatus in root tip cells of P. vulgaris or Tropaeolum majus. This localisation was particularly pronounced in Golgi-associated vesicles of young root tip cells of T. majus, cell types where rapid synthesis and deposition of wall material was observed. These data are consistent with the hypothesis, proposed by Bolwell [G.P. Bolwell, Dynamic aspects of the plant extracellular matrix, Int. Rev. Cytol. 146 (1993) 261-324], that protein hydroxylation must be completed before the glycosylation of the target (hydroxy)proline-rich glycoproteins in the Golgi stack.

A metalloprotease from Xanthomonas campestris that specifically degrades proline/hydroxyproline-rich glycoproteins of the plant extracellular matrix

Culture supernatants of Xanthomonas campestris pv. campestris contain an enzymic activity capable of degrading gp120, a proline-rich glycoprotein associated with the extracellular matrix of the vascular bundles in petioles of turnip (Brassica campestris). This activity did not reside in any of the three previously characterized proteases of X. campestris pv. campestris that were identified by their action against the model substrate beta-casein. The novel enzyme was purified by ion-exchange and size-exclusion high-performance liquid chromatography (HPLC). The enzyme, which has no activity against beta-casein, is active against some plant glycoproteins of the hydroxyproline-rich class such as extensin from potato and tomato and gpS-3, a glycoprotein induced in B. campestris petioles by wounding. Other hydroxyproline-rich glycoproteins, such as the solanaceous lectins, were not substrates however. Studies of the products released upon degradation of tomato extensin suggested that the degradative mechanism was proteolysis. Inhibitor studies suggested that the enzyme was a zinc-requiring metalloprotease. Extracellular matrix glycoproteins of the proline-rich and hydroxyproline-rich classes have been implicated in plant resistance to microbial attack, hence their degradation by X. campestris pv. campestris may have considerable significance for black rot pathogenesis.

Induction of extracellular matrix glycoproteins in Brassica petioles by wounding and in response to Xanthomonas campestris

A panel of monoclonal antibodies that recognize plant extracellular matrix glycoproteins previously implicated in plant-microbe interactions was used to study the effects of pathogen inoculation and wounding on glycoproteins in petioles of Brassica campestris. The panel of monoclonals comprised two sets: JIM11, JIM12, and JIM20 recognize epitopes carried on hydroxyproline-rich glycoproteins (HRGPs) (M. Smallwood, A. Beven, N. Donovan, S. J. Neitl, J. Peart, K. Roberts, and J. P. Knox, Plant J. 5:237-246, 1994); MAC204 and MAC265 recognize glycoproteins of the Rhizobium infection thread (K. A. VandenBosch, D. J. Bradley, S. Perotto, G. W. Butcher, and N. J. Brewin, EMBO J. 8:335-342, 1989). Wounding or inoculation of petioles with avirulent strains of pathovars of Xanthomonas campestris induced the synthesis of two new groups of antigens: gp160 ran as a smear on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) with apparent molecular mass from 120 to 200 kDa and was recognized by JIM20 and MAC204; gpS remained in the stacking gel on SDS-PAGE and was recognized by JIM11, JIM20, and MAC204. The response to virulent strains of pathovars of X. campestris was either less pronounced or absent. gpS comprised several components that were resolved by cation-exchange chromatography. Some of these components were characterized as extensin-like HRGPs. The level of induction of the gpS group of antigens by virulent strains was not altered by mutation of a number of genes required for basic pathogenicity or by heat-killing the bacteria.

Potato lectin: a three-domain glycoprotein with novel hydroxyproline-containing sequences and sequence similarities to wheat-germ agglutinin

Potato (Solanum tuberosum) tuber lectin is a chitin-binding, hydroxyproline-rich glycoprotein, which may be involved in the defence mechanism of the plant. We had previously obtained evidence that it consists of at least two very dissimilar domains. The aim was to use a combination of accurate determinations of molecular weight and protein sequencing to gain more accurate information on the domains. Accurate determinations of the molecular weight of the lectin by a MALDI mass spectrometer have shown that the subunit molecular weight is 65,500 (+/- 1100) and that of a totally deglycosylated sample is 31,250 (+/- 30). This means that the lectin is 52.3 (+/- 1)% carbohydrate with a considerable number of glycoforms being present. Partial sequences and other analyses are consistent with the existence of three distinct domains. These are: (1) an N-terminal region which is rich in proline but poor in hydroxyproline; (2) a glycosylated region with a glycosylated molecular weight of 45,300 (+/- 1100) and a deglycosylated molecular weight of 11,050 (+/- 50) which is extremely rich in glycosylated hydroxyproline residues with a similar sequence to extensins; and (3) a cystine-rich domain which has the sugar binding site shows partial conservation of a repeated motif common to many chitin-binding proteins of the hevin family including wheat-germ agglutinin. The closest similarity seems to be to the sequence of potato basic chitinase.

Specificity in the immobilisation of cell wall proteins in response to different elicitor molecules in suspension-cultured cells of French bean (Phaseolus vulgaris L.)

A characteristic of the defence response is the immobilisation of wall proteins possibly through the formation of covalent cross-links and the subsequent barrier formation against pathogens. A requirement for this is the generation of active oxygen species, particularly hydrogen peroxide. In the present work, we examine in depth the requirement for H2O2 and the specificity of the immobilisation with respect to particular wall proteins. Salt-extractable wall proteins were analysed for hydroxyproline content and the subset of proteins with this post-translational modification was found to be small. About 50 proteins were found to be easily salt-extractable and in response to elicitor treatment about 5 were found to be specifically immobilised. Immobilisation was very rapid and completed within 15 min after elicitation, and dependent upon the type of elicitor and the intensity of the production of active oxygen species. N-terminal sequencing and amino acid analysis revealed that, apart from one polypeptide, all immobilised proteins were (hydroxy)proline-containing glycoproteins with O-linked oligosaccharide side chains. In contrast, N-linked glycoproteins were not immobilised. N-terminal protein sequencing revealed the immobilised HRGPs to be novel, but both extensin and PRP-like. Implications of these findings for both pathogenic and symbiotic processes are also discussed.