Patterns of soybean proline-rich protein gene expression

Identification and analysis of proline-rich proteins and hybrid proline-rich proteins super family genes from Sorghum bicolor and their expression patterns to abiotic stress and zinc stimuli

Systematic genome-wide analysis of Sorghum bicolor revealed the identification of a total of 48 homologous genes comprising 21 proline-rich proteins (PRPs) and 27 hybrid proline-rich proteins (HyPRPs). Comprehensive scrutiny of these gene homologs was conducted for gene structure, phylogenetic investigations, chromosome mapping, and subcellular localization of proteins. Promoter analysis uncovered the regions rich with phosphorous- (BIHD), ammonium-, sulfur-responsive (SURE), and iron starvation-responsive (IRO2) along with biotic, abiotic, and development-specific cis-elements. Further, PRPs exhibit more methylation and acetylation sites in comparison with HyPRPs. miRNAs have been predicted which might play a role in cleavage and translation inhibition. Several of the SbPRP genes were stimulated in a tissue-specific manner under drought, salt, heat, and cold stresses. Additionally, exposure of plants to abscisic acid (ABA) and zinc (Zn) also triggered PRP genes in a tissue-dependent way. Among them, SbPRP17 has been found upregulated markedly in all tissues irrespective of the stress imposed. The expressions of SbHyPRPs, especially SbHyPRP2, SbHyPRP6, and SbHyPRP17 were activated under all stresses in all three tissues. On the other hand, SbHyPRP8 (root only) and SbHyPRP12 (all three tissues) were highly responsive to cold stress and ABA while SbHyPRP26 was induced by drought and Zn in the stem. Taken together, this study indicates the critical roles that SbPRPs and SbHyPRPs play during diverse abiotic stress conditions and notably the plausible roles that these genes play upon exposure to zinc, the crucial micronutrient in plants.

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.

Cell Wall Proteins Play Critical Roles in Plant Adaptation to Phosphorus Deficiency

Phosphorus is one of the mineral nutrient elements essential for plant growth and development. Low phosphate (Pi) availability in soils adversely affects crop production. To cope with low P stress, remodeling of root morphology and architecture is generally observed in plants, which must be accompanied by root cell wall modifications. It has been documented that cell wall proteins (CWPs) play critical roles in shaping cell walls, transmitting signals, and protecting cells against environmental stresses. However, understanding of the functions of CWPs involved in plant adaptation to P deficiency remains fragmentary. The aim of this review was to summarize advances in identification and functional characterization of CWPs in responses to P deficiency, and to highlight the critical roles of CWPs in mediating root growth, P reutilization, and mobilization in plants.

Proline-rich protein gene PdPRP regulates secondary wall formation in poplar

Proline-rich protein (PRP) is a plant cell wall associated protein. Its distinct patterns of regulation and localization studied in a number of plants indicate that it may play important roles in growth and development. However, the mechanism of how these genes control secondary cell wall development in tree species is largely unknown. Here, we report that a Populus deltoides (Marsh.) proline-rich protein gene PdPRP was preferentially expressed in immature/mature phloem and immature xylem in P. deltoides. PdPRP overexpression increased poplar plant height and diameter as well as the radial width of the phloem and xylem regions, facilitated secondary wall deposition, and induced expression of genes related to microfibril angle (MFA) and secondary wall biosynthesis. Downregulation of PdPRP retarded poplar growth, decreased the radial width of the secondary phloem and secondary xylem regions, reduced secondary wall thickening in fibers and vessels, and decreased the expression of genes related to MFA and secondary wall biosynthesis. These results suggest that PdPRP might positively regulate secondary cell wall formation by promoting secondary wall thickening and expansion in poplar. PdPRP-overexpressing poplar had a lower MFA, indicating that PdPRP may be useful for improving wood stiffness and properties in plants. Together, our results demonstrate that PdPRP is a proline-rich protein associated with cell wall development, playing a critical role in regulating secondary cell wall formation in poplar.

Characterization of NtREL1, a novel root-specific gene from tobacco, and upstream promoter activity analysis in homologous and heterologous hosts

KEY MESSAGE

A novel root-specific gene and its upstream promoter were cloned and characterized for potential application in root-specific expression of transgenes. The root is an important plant organ subjected to many biotic and abiotic stresses, such as infection by Ralstonia solanacearum. To isolate tobacco root-specific promoters for genetic applications, microarray screening was performed to identify genes highly and specifically expressed in the root. One root-specific gene encoding an extensin-like protein (NtREL1) was isolated, and its expression pattern was further characterized by both microarray analysis and reverse transcription-polymerase chain reaction. NtREL1 was highly expressed only in roots but not in any other organ. NtREL1 expression was affected by hormone treatment (salicylic acid, abscisic acid, and ethephon) as well as low temperature, drought, and R. solanacearum infection. A full-length 849 bp cDNA containing a 657-nucleotide open reading frame was cloned by Rapid Amplification of cDNA Ends. Subsequently, a fragment of 1,574 bp upstream of NtREL1 was isolated by flanking PCR and named pNtREL1. This promoter fragment contains TATA, GATA, and CAAT-boxes, the basic elements of a promoter, and six root-specific expression elements, namely OSE1, OSE2, ROOTMOTIFTAPOX1, SURECOREATSULTR11, P1BS, and WUSATAg. A construct containing the bacterial uidA reporter gene (β-glucuronidase, GUS) driven by the pNtREL1 promoter was transformed into tobacco plants. GUS staining was only detected in the root, but not in leaves and stems. Additionally, transgenic tobacco plants containing peanut resveratrol synthase gene (AhRS) driven by the pNtREL1 promoter produced resveratrol only in the root. Thus, the pNtREL1 promoter can be used to direct root-specific expression of target genes to protect the root from stress or for biological studies.

Identification of the Abundant Hydroxyproline-Rich Glycoproteins in the Root Walls of Wild-Type Arabidopsis, an ext3 Mutant Line, and Its Phenotypic Revertant

Extensins are members of the cell wall hydroxyproline-rich glycoprotein (HRGP) superfamily that form covalently cross-linked networks in primary cell walls. A knockout mutation in EXT3 (AT1G21310), the gene coding EXTENSIN 3 (EXT3) in Arabidopsis Landsberg erecta resulted in a lethal phenotype, although about 20% of the knockout plants have an apparently normal phenotype (ANP). In this study the root cell wall HRGP components of wild-type, ANP and the ext3 mutant seedlings were characterized by peptide fractionation of trypsin digested anhydrous hydrogen fluoride deglycosylated wall residues and by sequencing using LC-MS/MS. Several HRGPs, including EXT3, were identified in the wild-type root walls but not in walls of the ANP and lethal mutant. Indeed the ANP walls and walls of mutants displaying the lethal phenotype possessed HRGPs, but the profiles suggest that changes in the amount and perhaps type may account for the corresponding phenotypes.

Role of proline in cell wall synthesis and plant development and its implications in plant ontogeny

Proline is a proteogenic amino acid and accumulates both under stress and non-stress conditions as a beneficial solute in plants. Recent discoveries point out that proline plays an important role in plant growth and differentiation across life cycle. It is a key determinant of many cell wall proteins that plays important roles in plant development. The role of extensins, arabinogalactan proteins and hydroxyproline- and proline-rich proteins as important components of cell wall proteins that play pivotal roles in cell wall signal transduction cascades, plant development and stress tolerance is discussed in this review. Molecular insights are also provided here into the plausible roles of proline transporters modulating key events in plant development. In addition, the roles of proline during seed developmental transitions including storage protein synthesis are discussed.

Proline-rich protein-like PRPL1 controls elongation of root hairs in Arabidopsis thaliana

The synthesis and composition of cell walls is dynamically adapted in response to many developmental and environmental signals. In this respect, cell wall proteins involved in controlling cell elongation are critical for cell development. Transcriptome analysis identified a gene in Arabidopsis thaliana, which was named proline-rich protein-like, AtPRPL1, based on sequence similarities from a phylogenetic analysis. The most resemblance was found to AtPRP1 and AtPRP3 from Arabidopsis, which are known to be involved in root hair growth and development. In A. thaliana four proline-rich cell wall protein genes, playing a role in building up the cross-connections between cell wall components, can be distinguished. AtPRPL1 is a small gene that in promoter::GUS (β-glucuronidase) analysis has high expression in trichoblast cells and in the collet. Chemical or mutational interference with root hair formation inhibited this expression. Altered expression levels in knock-out or overexpression lines interfered with normal root hair growth and etiolated hypocotyl development, but Fourier transform-infrared (FT-IR) analysis did not identify consistent changes in cell wall composition of root hairs and hypocotyl. Co-localization analysis of the AtPRPL1-green fluorescent protein (GFP) fusion protein and different red fluorescent protein (RFP)-labelled markers confirmed the presence of AtPRPL1-GFP in small vesicles moving over the endoplasmic reticulum. Together, these data indicate that the AtPRPL1 protein is involved in the cell's elongation process. How exactly this is achieved remains unclear at present.

GmPRP2 promoter drives root-preferential expression in transgenic Arabidopsis and soybean hairy roots

BACKGROUND

Promoters play important roles in gene expression and function. There are three basic types of promoters: constitutive, specific, and inducible. Constitutive promoters are widely used in genetic engineering, but these promoters have limitations. Inducible promoters are activated by specific inducers. Tissue-specific promoters are a type of specific promoters that drive gene expression in specific tissues or organs. Here, we cloned and characterized the GmPRP2 promoter from soybean. The expression pattern indicated that this promoter is root-preferential in transgenic Arabidopsis and the hairy roots of soybean. It can be used to improve the root resistance or tolerance to pathogens, pests, malnutrition and other abiotic stresses which cause extensive annual losses in soybean production.

RESULTS

The GmPRP2 promoter (GmPRP2p-1062) was isolated from soybean cv. Williams 82. Sequence analysis revealed that this promoter contains many cis-acting elements, including root-specific motifs. The GmPRP2p-1062 and its 5'-deletion fragments were fused with the GUS reporter gene and introduced into Arabidopsis and the hairy roots of soybean to further determine promoter activity. Histochemical analysis in transgenic Arabidopsis showed that GUS activity was mainly detected in roots and hypocotyls in all deletion fragments except GmPRP2p-471 (a 5'-deletion fragment of GmPRP2p-1062 with 471 bp length). GUS activity was higher in transgenic Arabidopsis and hairy roots with GmPRP2p-1062 and GmPRP2p-852 (a 5'-deletion fragment of GmPRP2p-1062 with 852 bp length) constructs than the other two constructs. GUS activity was enhanced by NaCl, PEG, IAA and JM treatments and decreased by SA, ABA and GA treatments in transgenic Arabidopsis.

CONCLUSIONS

GmPRP2p-1062 is a root-preferential promoter, and its core fragment for root-preferential expression might lie between -369 and +1. GmPRP2p-852 may be useful in the genetic engineering of novel soybean cultivars in the future.

Cotton PRP5 gene encoding a proline-rich protein is involved in fiber development

Proline-rich proteins contribute to cell wall structure of specific cell types and are involved in plant growth and development. In this study, a fiber-specific gene, GhPRP5, encoding a proline-rich protein was functionally characterized in cotton. GhPRP5 promoter directed GUS expression only in trichomes of both transgenic Arabidopsis and tobacco plants. The transgenic Arabidopsis plants with overexpressing GhPRP5 displayed reduced cell growth, resulting in smaller cell size and consequently plant dwarfs, in comparison with wild type plants. In contrast, knock-down of GhPRP5 expression by RNA interference in cotton enhanced fiber development. The fiber length of transgenic cotton plants was longer than that of wild type. In addition, some genes involved in fiber elongation and wall biosynthesis of cotton were up-regulated or down-regulated in the transgenic cotton plants owing to suppression of GhPRP5. Collectively, these data suggested that GhPRP5 protein as a negative regulator participates in modulating fiber development of cotton.

OsPRP3, a flower specific proline-rich protein of rice, determines extracellular matrix structure of floral organs and its overexpression confers cold-tolerance

Proline-rich protein (PRP), a cell wall protein of plant, has been studied in many plant species. Yet, none of the PRPs has been functionally elucidated. Here we report a novel flower-specific PRP designated OsPRP3 from rice. Expression analysis showed that the OsPRP3 transcript was mainly present in rice flower and accumulated abundantly during the late stage of the flower development. To study the function of OsPRP3, we constructed and transformed a binary vector containing a full clone of OsPRP3 in sense orientation and also an RNAi vector to achieve overexpression and knockout of the gene, respectively. Our overexpression plants showed a significant increase in cold tolerance than the WT plants which is conferred by the accumulation of OsPRP3 protein during cold treatment. Further the microscopic analysis revealed that OsPRP3 enhances the cell wall integrity in the cold tolerant plant and confers cold-tolerance in rice. Microscopic analysis of the RNAi mutant flower revealed that blocking OsPRP3 function caused significant defects in floral organogenesis. Taken together, the results suggested that OsPRP3 is a cell wall protein, playing a crucial role in determining extracellular matrix structure of floral organs.

Isolation and characterization of a new defense gene from soybean

A cDNA clone of a single-copy gene designated SbPRP was isolated and characterized from 2-week-old soybean seedlings. It putatively encodes a bimodular protein similar to developmentally regulated proteins in other plant species. The deduced amino acid sequence consists of 126 amino acids with a distinct proline-rich domain (17 amino acids) and a long hydrophobically cysteine-rich domain (84 amino acids), plus a signal peptide of 25 amino acids in N terminal. SbPRP mRNA transcripts accumulated in an organ specific manner. It can be detected in leaves and epicotyls of soybean seedlings, whereas virtually expression signal of SbPRP was not detected in cotyledons, hypocotyls and roots. Further Northern hybridization suggested that SbPRP steady-state mRNA level accumulated differentially not only in response to salicylic acid, but to the inoculation of soybean mosaic virus Sa strain. Also it was responsive to drought treatment and salt (NaCl) stress. Therefore it is likely that SbPRP functions as a defense gene in soybean.

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.

Yellow lupine cyclophilin transcripts are highly accumulated in the nodule meristem zone

Cyclophilin (CyP) is one of the enzymes that act as peptidylprolyl cis-trans isomerases (EC 5.2.1.8). The cDNA and an intronless gene coding for cytosolic CyP have been isolated from yellow lupine. The deduced amino acid sequence of the characterized open reading frame shows approximately 80% homology with cytosolic CyP from other organisms. Southern blots of genomic DNA indicate that there is a small family of genes for CyP-related genes in the yellow lupine genome. RNA blot analyses demonstrate that CyP genes are expressed in all plant organs. The amount of CyP transcripts is dramatically increased in root nodules. In situ hybridization experiments indicate that CyP transcripts are localized mainly in meristematic tissues, with the highest level observed in the nodule meristem zone. The promoter of the sequenced gene contains 5' AAAGAT 3' and AT-rich motifs that are characteristic for some nodulin promoters.

Differential expression of cell-wall-related genes during the formation of tracheary elements in the Zinnia mesophyll cell system

Plants, animals and some fungi undergo processes of cell specialization such that specific groups of cells are adapted to carry out particular functions. One of the more remarkable examples of cellular development in higher plants is the formation of water-conducting cells that are capable of supporting a column of water from the roots to tens of metres in the air for some trees. The Zinnia mesophyll cell system is a remarkable tool with which to study this entire developmental pathway in vitro. We have recently applied an RNA fingerprinting technology, to allow the detection of DNA fragments derived from RNA using cDNA synthesis and subsequent PCR-amplified fragment length polymorphisms (cDNA-AFLP), to systematically characterize hundreds of the genes involved in the process of tracheary element formation. Building hoops of secondary wall material is the key structural event in forming functional tracheary elements and we have identified over 50 partial sequences related to cell walls out of 600 differentially expressed cDNA fragments. The Zinnia system is an engine of gene discovery which is allowing us to identify and characterize candidate genes involved in cell wall biosynthesis and assembly.

Expression of AtPRP3, a proline-rich structural cell wall protein from Arabidopsis, is regulated by cell-type-specific developmental pathways involved in root hair formation

The tightly regulated expression patterns of structural cell wall proteins in several plant species indicate that they play a crucial role in determining the extracellular matrix structure for specific cell types. We demonstrate that AtPRP3, a proline-rich cell wall protein in Arabidopsis, is expressed in root-hair-bearing epidermal cells at the root/shoot junction and within the root differentiation zone of light-grown seedlings. Several lines of evidence support a direct relationship between AtPRP3 expression and root hair development. AtPRP3/beta-glucuronidase (GUS) expression increased in roots of transgenic seedlings treated with either 1-aminocyclopropane-1-carboxylic acid (ACC) or alpha-naphthaleneacetic acid (alpha-NAA), compounds known to promote root hair formation. In the presence of 1-alpha-(2-aminoethoxyvinyl)glycine (AVG), an inhibitor of ethylene biosynthesis, AtPRP3/GUS expression was strongly reduced, but could be rescued by co-addition of ACC or alpha-NAA to the growth medium. In addition, AtPRP3/GUS activity was enhanced in ttg and gl2 mutant backgrounds that exhibit ectopic root hairs, but was reduced in rhd6 and 35S-R root-hair-less mutant seedlings. These results indicate that AtPRP3 is regulated by developmental pathways involved in root hair formation, and are consistent with AtPRP3's contributing to cell wall structure in Arabidopsis root hairs.

StGCPRP, a potato gene strongly expressed in stomatal guard cells, defines a novel type of repetitive proline-rich proteins

Guard cells represent a highly differentiated cell type within the epidermis of plant leaves and stems. They respond to many endogenous and environmental signals and thereby modify the size of the stomatal pore they surround. We identified a novel gene that is highly expressed in guard cells of potato (Solanum tuberosum). It encodes a repetitive proline (Pro)-rich protein of 54 kD (491 amino acids) and was named StGCPRP (S. tuberosum guard cell Pro-rich protein). StGCPRP has a bipartite structure. The C-terminal part of StGCPRP contains a high percentage (46%) of Pro residues organized in distinct repetitive sequence motifs, whereas its extended N terminus is essentially free of Pros. StGCPRP represents the first member of a novel class of hybrid Pro-rich proteins that we designated NHyPRPs. In young but not in mature leaves, StGCPRP transcripts were also present at high levels in mesophyll cells (in addition to guard cells), indicating developmental regulation of StGCPRP gene expression. In addition, StGCPRP expression is regulated by environmental factors, as shown by a decrease in StGCPRP transcript levels under drought stress. Two proteins similar to StGCPRP were found to be encoded by the Arabidopsis genome, indicating that NHyPRPs are more widely distributed in higher plants.

Characterization and expression of four proline-rich cell wall protein genes in Arabidopsis encoding two distinct subsets of multiple domain proteins

We have characterized the molecular organization and expression of four proline-rich protein genes from Arabidopsis (AtPRPs). These genes predict two classes of cell wall proteins based on DNA sequence identity, repetitive motifs, and domain organization. AtPRP1 and AtPRP3 encode proteins containing an N-terminal PRP-like domain followed by a C-terminal domain that is biased toward P, T, Y, and K. AtPRP2 and AtPRP4 represent a second, novel group of PRP genes that encode two-domain proteins containing a non-repetitive N-terminal domain followed by a PRP-like region rich in P, V, K, and C. Northern hybridization analysis indicated that AtPRP1 and AtPRP3 are exclusively expressed in roots, while transcripts encoding AtPRP2 and AtPRP4 were most abundant in aerial organs of the plant. Histochemical analyses of promoter/beta-glucuronidase fusions localized AtPRP3 expression to regions of the root containing root hairs. AtPRP2 and AtPRP4 expression was detected in expanding leaves, stems, flowers, and siliques. In addition, AtPRP4 expression was detected in stipules and during the early stages of lateral root formation. These studies support a model for involvement of PRPs in specifying cell-type-specific wall structures, and provide the basis for a genetic approach to dissect the function of PRPs during growth and development.

A proline-, threonine-, and glycine-rich protein down-regulated by drought is localized in the cell wall of xylem elements

A cDNA clone encoding a proline-, threonine-, and glycine-rich protein (PTGRP) was isolated from a wild tomato species (Lycopersicon chilense) (L.X. Yu, H. Chamberland, J.G. Lafontain, Z. Tabaeizadeh [1996] Genome 39: 1185-1193). Northern-blot analysis and in situ hybridization studies revealed that PTGRP is down-regulated by drought stress. The level of the mRNA in leaves and stems of 8-d drought-stressed plants decreased 5- to 10-fold compared with that in regularly watered plants. The mRNA re-accumulated when drought-stressed plants were rewatered. Antibodies raised against a glutathione S-transferase/PTGRP fusion protein were used to elucidate the subcellular localization of the protein by immunogold labeling. In regularly watered L. chilense plants, PTGRP protein was found to be localized in xylem pit membranes and disintegrated primary walls. Examination of sections from drought-stressed plants revealed a significant decrease in the levels of labeling. In these samples, only a few scattered gold particles were detected in the same areas. In the leaf tissues of plants that had been rewatered for 3 d following an 8-d drought stress, the labeling pattern was similar to that of the regularly watered plants. To our knowledge, PTGRP is the first drought-regulated protein that has been precisely localized in the cell wall.

A defective seed coat pattern (Net) is correlated with the post-transcriptional abundance of soluble proline-rich cell wall proteins

The pigmented seed coats of several soybean (Glycine max (L.) Merr.) plant introductions and isolines have unusual defects that result in cracking of the mature seed coat exposing the endosperm and cotyledons. It has previously been shown that the T (tawny) locus that controls the color of trichomes on stems and leaves also has an effect on both the structure and pigmentation of the seed coat. Distribution of pigmentation on the seed coat is controlled by alleles of the I (inhibitor) locus. It was also found that total seed coat proteins were difficult to extract from pigmented seed coats with i T genotypes because they have procyanidins that exhibit tannin properties. We report that the inclusion of poly-L-proline in the extraction buffer out-competes proteins for binding to procyanidins. Once this problem was solved, we examined expression of the proline-rich cell wall proteins PRP1 and PRP2 in pigmented genotypes with the dominant T allele. We found that both homozygous i T and i t genotypes have reduced soluble PRP1 levels. The epistatic interaction of the double recessive genotype at both loci is necessary to produce the pigmented, defective seed coat phenotype characteristic of seed coats with the double recessive i and t alleles. This implies a novel effect of an enzyme in the flavonoid pathway on seed coat structure in addition to its effect on flavonoids, anthocyanidins, and proanthocyanidins. No soluble PRP1 polypeptides were detectable in pigmented seed coats (i T genotypes) of isolines that also display a net-like pattern of seed coat cracking, known as the Net defect. PRP2 was also absent in one of the these lines. However, both PRP1 and PRP2 cytoplasmic mRNAs were found in the Net-defective seed coats. Together with in vitro translation studies, these results suggest that the absence of soluble PRP polypeptides in the defective Net lines is post-translational and could be due to a more rapid or premature insolubilization of PRP polypeptides within the cell wall matrix.

PLANT CELL WALL PROTEINS

The nature of cell wall proteins is as varied as the many functions of plant cell walls. With the exception of glycine-rich proteins, all are glycosylated and contain hydroxyproline (Hyp). Again excepting glycine-rich proteins, they also contain highly repetitive sequences that can be shared between them. The majority of cell wall proteins are cross-linked into the wall and probably have structural functions, although they may also participate in morphogenesis. On the other hand, arabinogalactan proteins are readily soluble and possibly play a major role in cell-cell interactions during development. The interactions of these proteins between themselves and with other wall components is still unknown, as is how wall components are assembled. The possible functions of cell wall proteins are suggested based on repetitive sequence, localization in the plant body, and the general morphogenetic pattern in plants.

Expression of a Soybean Hydroxyproline-Rich Glycoprotein Gene Is Correlated with Maturation of Roots

A novel extensin gene has been identified in soybean (Glycine max L.) that encodes a hydroxyproline-rich glycoprotein (SbHRGP3) with two different domains. In this study expression of SbHRGP3 was investigated during soybean root development. SbHRGP was expressed in roots of mature plants, as well as seedlings, and showed a distinct pattern of expression during root development. The expression of SbHRGP3 increased gradually during root development of seedlings and reached a maximum while the secondary roots were maturing. The maximum expression level was contributed mainly by the secondary roots rather than by the primary root. Furthermore, expression of SbHRGP3 was preferentially detected in the regions undergoing maturation of the primary and secondary roots. These results imply that the expression of SbHRGP3 is regulated in an organ- and development-specific manner and that in soybean SbHRGP3 expression may be required for root maturation, presumably for the cessation of root elongation. Wounding and sucrose in combination enhanced expression of SbHRGP3 in roots, whereas both wounding and sucrose were required for the expression of SbHRGP3 in leaves.

Expression of a novel-type small proline-rich protein gene of alfalfa is induced by 2,4-dichlorophenoxiacetic acid in dedifferentiated callus cells

Differential screening of a cDNA library of 2,4-dichlorophenoxiacetic acid (2,4-D)-treated alfalfa (Medicago sativa) callus tissues resulted in the isolation of a 571 bp cDNA clone (MsPRP5) encoding for a proline-rich protein (84 amino acids) with a specific repeat unit of TPVLPPRK/RGRPPPVPP. In addition, a characteristic amino acid block (PPVYK) previously found in other proline-rich proteins also occurs in the C-terminal region of MsPRP5. At the N-terminal, a signal peptide similar to leader sequences of extracellular proteins can be predicted. According to the northern analysis, the corresponding gene is not expressed or is weakly expressed in differentiated vegetative organs and somatic embryos. However the accumulation of MsPRP5 mRNA is auxin concentration-dependent in dedifferentiated callus tissue. An increase in the amount of steady-state mRNA was detected already 20 min after auxin shock (100 microM 2,4-D). Maximum expression was observed at 24-48 h in the presence of 2,4-D. Elevated expression was also found in cells recovering after heat shock and wounding stress. In synchronized alfalfa cells, the transcript level of MsPRP5 gene fluctuated during cell cycle progression with peaks in G1/S phase cells. Considering the structural features and expression properties of MsPRP5, this clone may represents a new type of proline-rich protein gene which responds to hormonal shock and some other stresses as well.

Developmental and transgenic analysis of two tomato fruit enhanced genes

Tomato fruit development is characterized by distinct developmental stages: fruit set, periods of rapid cell division and cell expansion, and the period where processes associated with ripening are dominant. During each of these stages, different aspects of cellular metabolism are favored. Accompanying these developmental changes are dramatic differences in gene expression, with a subset of genes being expressed early and a subset being expressed later in development. We have isolated and characterized several sequences from tomato that are expressed primarily in immature green fruit. Two of these genes (Tfm7 and Tfm5) have been characterized more extensively and their sequence indicates that they encode proteins corresponding to a proline-rich protein (PRP) and a glycine-rich protein (GRP). RNA blot analysis indicates that the transcripts from these genes are present at the earliest stages of fruit development, and continue to be expressed throughout the growth period of the fruit. Expression analysis during development indicates that the gene encoding the PRP may be down-regulated by ethylene. As a means to understanding the functional significance and the transcriptional contribution of these tissue-limited proteins during development, we constructed promoter-reporter gene fusions to identify which cell types express each of these sequences. GUS protein produced in transgenic plants by both promoter-reporter gene constructs was detected in most tissues of the fruit including the pericarp, columella, and placental tissues of young immature fruit through the mature green stage. However, only one of the promoter sequences conferred expression in the fruit locular tissue.

A novel extensin gene encoding a hydroxyproline-rich glycoprotein requires sucrose for its wound-inducible expression in transgenic plants

A novel hydroxyproline-rich glycoprotein (SbHRGP3) that consists of two different domains is encoded by an extensin gene from soybean. The first domain (domain 1) located at the N terminus is composed of 11 repeats of Ser-Pro4-Lys-His-Ser-Pro4-Tyr3-His, whereas the second domain (domain 2) at the C terminus contains five repeats of Ser-Pro4-Val-Tyr-Lys-Tyr-Lys-Ser-Pro4-Tyr-Lys-Tyr-Pro-Ser-Pro5-Tyr-Lys-T yr- Pro-Ser-Pro4-Val-Tyr-Lys-Tyr-Lys. These two repeat motifs are organized in an extremely well-ordered pattern in each domain, which suggests that SbHRGP3 belongs to a new group of proteins having the repeat motifs of two distinct groups of dicot extensins. The expression of the SbHRGP3 gene increased with seedling maturation, and its expression was relatively high in the mature regions of the hypocotyl and in the root of soybean seedlings. An SbHRGP3-beta-glucuronidase (SbHRGP3-GUS) chimeric gene was constructed and expressed in transgenic tobacco plants. The expression of the SbHRGP3-GUS gene was not induced by wounding alone in transgenic tobacco plants; sucrose was also required. Expression was specific to phloem tissues and cambium cells of leaves and stems. In transgenic tobacco seedlings, SbHRGP3-GUS gene expression was activated by the maturation of the primary root and then inactivated; however, reactivation was specifically at the epidermis of the zone from which the lateral root was to be initiated. Its reactivation occurred just before the lateral root initiation. These results indicate that the SbHRGP3 gene in different tissues responds to different signals.

A novel phloem-specific gene is expressed preferentially in aerial portions of Vicia faba

We have isolated a gene from bean (Vicia faba L.), called Vein1, that encodes a novel protein. The Vein1 cDNA was isolated as a result of a differential screen for genes that are expressed in leaves but not in the most common cell type, the mesophyll cell. Northern blot analysis revealed that Vein 1 transcripts are differentially expressed in the plant with expression in leaves, stems and sepals but not in petals, mesophyll cells or roots. In situ hybridization studies of stem and leaf sections indicate that the expression of Vein1 is localized to the phloem tissue. Interestingly, Vein1 was differentially expressed in stem tissue with the highest expression in the oldest internodes. The deduced Vein1 protein sequence does not share homology with any known protein sequences. The 17 kDa Vein1 protein is highly hydrophilic and contains a histidine-rich motif, where six out of seven amino acids are histidines. The function of Vein1 is unknown, although the expression patterns suggests that it may play a role in mature phloem tissue in the aerial parts of the plant.

Expression patterns of three genes in the stem of lucerne (Medicago sativa)

We have identified three stem abundantly expressed genes in lucerne (alfalfa, Medicago sativa). A cDNA library, constructed from lucerne stem polyadenylated RNA, was screened by differential hybridization. From this screening, cDNA clones that correspond to genes which are preferentially, or specifically, expressed in the stem were isolated. MsaS1 encodes an unidentified protein, MsaS2 encodes an S-adenosyl-homocysteine hydrolase and MsaS3 encodes an extensin-like protein. Northern blot analysis of RNA isolated from individual stem internodes indicated that the three corresponding genes show differing developmental patterns of expression. The expression of MsaS1 was confined to the youngest stem tissue and may be regulated by sucrose. In stem tissue the level of RNA for the three genes decreased in response to wounding. Tissue print hybridization analysis was used to localize the expression of the genes to the xylem side of vascular bundles in lucerne stems.

Xylem-specific gene expression in loblolly pine

Two genes preferentially expressed in differentiating xylem of loblolly pine (Pinus taeda L.) were cloned from cDNA and genomic libraries and designated PtX3H6 and PtX14A9. Transcripts of PtX3H6 and PtX14A9 are very abundant in differentiating xylem, less abundant in needles, and very low or non-detectable in embryos and megagametophytes. PtX3H6 contains a putative signal peptide, a threonine-rich region, a proline-rich region, and a hydrophobic tail. Repeats of Pro-Pro-Pro-Val-X-X are similar to repeats found in proline-rich cell wall proteins. The amino acid compositions of PtX3H6 and PtX14A9 are similar to those of arabinogalactan proteins (AGPs). PtX14A9 contains an 8 amino acid sequence similar to amino terminal sequences of ryegrass, carrot and rose AGPs. Upstream sequences have been determined from genomic clones encoding PtX3H6 and PtX14A9. A 7 bp sequence found in the 5' flanking regions of both genes has previously been shown to be involved in the vascular-specific expression of GRP 1.8, a glycine-rich protein found in bean. The sequence is also present upstream of another glycine-rich protein from bean, GRP 1.0, and may be partially responsible for the xylem-specific expression of pTx3H6 and PtX14A9.

Vascular expression of the grp1.8 promoter is controlled by three specific regulatory elements and one unspecific activating sequence

The bean grp1.8 full-length promoter is specifically active in vascular tissue during normal development of tobacco. Deletion of a negative regulatory element resulted in ectopic activity of the promoter in cortical cells of hypocotyls, roots and stems. A 169 bp fragment (-205 to -36) of the grp1.8 promoter conferred vascular-specific expression to CaMV 35S minimal promoters whereas a 141 bp fragment (-205 to -64) strongly activated these minimal promoters both in vascular and cortical cells. These experiments defined a new regulatory element (VSE) that is essential for vascular-specific expression and is located between -64 and -36. The 141 bp grp1.8 promoter sequence had enhancer-like properties as it was active in both orientations. A 24 bp sequence (bp -119 to -96, corresponding to the SE1 regulatory element) enhanced expression from several minimal promoters strongly but unspecifically, whereas a 26 bp sequence (-98 to -73, corresponding to the RSE regulatory element) induced vascular-specific expression. Thus, the grp1.8 promoter is regulated by a combinatorial mechanism that can integrate the action of different, non-additively acting regulatory elements into vascular-specific expression.

A novel extracellular matrix protein from tomato associated with lignified secondary cell walls

A cDNA clone representing a novel cell wall protein was isolated from a tomato cDNA library. The deduced amino acid sequence shows that the encoded protein is very small (88 amino acids), contains an N-terminal hydrophobic signal peptide, and is enriched in lysine and tyrosine. We have designated this protein TLRP for tyrosine- and lysine-rich protein. RNA gel blot hybridization identified TLRP transcripts constitutively present in roots, stems, and leaves from tomato plants. The encoded protein seems to be highly insolubilized in the cell wall, and we present evidence that this protein is specifically localized in the modified secondary cell walls of the xylem and in cells of the sclerenchyma. In addition, the protein is localized in the protective periderm layer of the growing root. The highly localized deposition in cells destined to give support and protection to the plant indicates that this cell wall protein alone and/or in collaboration with other cell wall structural proteins may have a specialized structural function by mechanically strengthening the walls.

Alfalfa Enod12 genes are differentially regulated during nodule development by Nod factors and Rhizobium invasion

MsEnod12A and MsEnod12B are two early nodulin genes from alfalfa (Medicago sativa). Differential expression of these genes was demonstrated using a reverse transcription-polymerase chain reaction approach. MsEnod12A RNA was detected only in nodules and not in other plant tissues. In contrast, MsEnod12B transcripts were found in nodules and also at low levels in roots, flowers, stems, and leaves. MsEnod12B expression was enhanced in the root early after inoculation with the microsymbiont Rhizobium meliloti and after treatment with purified Nod factors, whereas MsEnod12A induction was detected only when developing nodules were visible. In situ hybridization showed that in nodules, MsEnod12 expression occurred in the infection zone. In empty Fix- nodules the MsEnod12A transcript level was much reduced, and in spontaneous nodules it was not detectable. These data indicate that MsEnod12B expression in roots is related to the action of Nod factors, whereas MsEnod12A expression is associated with the invasion process in nodules. Therefore, alfalfa possesses different mechanisms regulating MsEnod12A and MsEnod12B expression.

The LOX1 Gene of Arabidopsis Is Temporally and Spatially Regulated in Germinating Seedlings

We examined the temporal and spatial expression patterns of the LOX1 gene during the development of Arabidopsis thaliana seedlings. Measurements of steady-state LOX1 mRNA levels indicated that this gene is transiently expressed during germination. LOX1 mRNA was not detected in seed that had imbibed (T0) but reached a maximum level by 1 d in both light- and dark-grown seedlings. The induction of the LOX1 gene was not light dependent; however, mRNA levels were 4-fold greater in light-grown seedlings. Immunoblot analysis of lipoxygenase protein levels and measurements of enzyme activity suggested that the induction of the LOX1 gene resulted in the production of functional lipoxygenase enzyme. Lipoxygenase protein was not present in dry seed or seed that had imbibed, but was first detected by immunoblot analysis after 1 and 2 d of growth in the light and dark, respectively. In both cases, lipoxygenase protein levels remained high for 2 d and then declined. Lipoxygenase activity paralleled the changes in protein levels. In situ hybridization studies revealed that the LOX1 gene is transiently expressed in the epidermis and the aleurone layer during germination. LOX1 mRNA levels were particularly high in the epidermis of the radicle and the adaxial side of the cotyledons. These results suggest that the LOX1 gene product is produced specifically during early germination and plays a role in the functioning of the epidermis.

Extensin gene expression is induced by mechanical stimuli leading to local cell wall strengthening in Nicotiana plumbaginifolia

Nicotiana plumbaginifolia Viv. harbors a single extensin gene, although related hydroxyproline-rich sequences are present in the genome. Northern analysis showed that the gene is highly expressed in roots and to a lesser extent in stems. Expression in leaves is low but mRNA levels are increased upon infection with the incompatible bacterium Pseudomonas syringae. Extensin transcript levels in leaves were slightly enhanced after wounding and salicylic acid treatment. In-situ hybridization experiments showed high accumulation of extensin mRNA in cells which, at certain stages of development, require reinforcement of their cell walls. The cortical cells in stem nodes and roots, which are put under severe mechanical stress by adjacent developing tissues, tend to express the gene to high levels. Immunolocalization of the extensin protein in stems and roots demonstrated a close association of the protein with lignin deposition. Mature tissues contained more extensin than younger tissues. The extensin promoter was fused to the beta-glucuronidase gene.

Identification and characterization of a proline-rich mRNA that accumulates during pod development in oilseed rape (Brassica napus L.)

Pod development in oilseed rape (Brassica napus) culminates in a process known as dehiscence (shatter) which can result in the loss of seed before the crop is harvested. In order to investigate the biochemical and the genetic basis controlling this process, a cDNA library was constructed from the dehiscence zone of developing pods. This resulted in the isolation of a cDNA clone (SAC51). The mRNA encoded by SAC51 had a transcript size of ca. 700 nucleotides and was found, by northern analysis, to accumulate preferentially in the dehiscence zone of the pod and in no other part of the plant analysed. The predicted polypeptide is rich in the amino acids proline (14.2%) and leucine (14.2%). The sequence of the polypeptide has more than 40% amino acid sequence identity with polypeptides isolated from carrot embryos, maize roots, soybean seeds and young tomato fruit. The function of these proteins is unknown. Genomic Southern analysis suggests that SAC51 is encoded by a single gene or small gene family. The role of the peptide in the development of pods of oilseed rape is discussed.

A tobacco gene family for flower cell wall proteins with a proline-rich domain and a cysteine-rich domain

Flowering is known to be associated with the induction of many cell wall proteins. We report here five members of a tobacco gene family (CELP, Cys-rich extensin-like protein) whose mRNAs are found predominantly in flowers and encode extensin-like Pro-rich proteins. CELP mRNAs accumulate most abundantly in vascular and epidermal tissues of floral organs. In the pistil, CELP mRNAs also accumulate in a thin layer of cells between the transmitting tissue and the cortex of the style and in a surface layer of cells of the placenta in the ovary. This unique accumulation pattern of CELP mRNAs in the pistil suggests a possible role in pollination and fertilization processes. CELP genes encode a class of plant extracellular matrix proteins that have several distinct structural features: a Pro-rich extensin-like domain with Xaa-Pro3-7 motifs and Xaa-Pro doublets, a Cys-rich region, and a highly charged C terminus. The extensin-like domains in these proteins differ significantly in their length and these differences appear to be results of both long and short deletions within the coding regions of their genes. Furthermore, the number of charged amino acid residues in the C-terminal region varies among the CELPs. These structural differences may contribute to functional versatility in the CELPs. On the other hand, the Cys-rich domain is highly conserved among CELPs and the positions of the Cys residues are conserved, suggesting that this region may have a common functional role. The presence of a Pro-rich domain and a Cys-rich domain in these CELPs is reminiscent of a class of hydroxyproline-rich glycoproteins, solanaceous lectins, that are believed to be important in cell-cell recognition. The structure of these CELPs indicates that they may be multifunctional and that their genes may have arisen from recombinational events.

Differential Expression of Two Soybean (Glycine max L.) Proline-Rich Protein Genes after Wounding

We have investigated the wound-induced expression of two members of the soybean (Glycine max L.) proline-rich cell wall protein gene family and show that SbPRP1 and SbPRP2 exhibit unique patterns of expression after physical damage. SbPRP1 mRNA can be detected in the hook of soybean seedlings within 2 h after wounding and is present at high levels in the hook and elongating hypocotyl 20 h after wounding. In contrast, SbPRP2 mRNA increases transiently and rapidly throughout the soybean seedling after wounding. SbPRP2 is also induced by wounding in soybean leaves, but the pattern of mRNA accumulation in leaves is distinct from that seen in seedlings and reaches high levels of expression 20 h after physical damage. SbPRP2 mRNA levels were also found to increase in the mature hypocotyl and roots of seedlings in response to treatment with 10 [mu]M indoleacetic acid and naphthalene-1-acetic acid. These data indicate that the wound-induced expression of PRPs in soybean is tissue specific and that the regulation of these genes after physical damage may operate through different signal transduction pathways.

Developmental expression and localization of petunia glycine-rich protein 1

An anti-petunia glycine-rich protein 1 (ptGRP1) antibody was used for biotin-streptavidin-alkaline phosphatase localization of this protein. In petunia stem and leaves grown under different light conditions, these studies revealed a complex pattern of cell localization for this protein. Levels of ptGRP1 were shown to decrease with developmental age of the tissue, appearing to correlate directly with expansive growth and inversely with lignification. Significantly, plants grown under low light (approximately 32 mumol m-2 sec-1 at noon) showed at least an eightfold increased level of ptGRP1 protein throughout ptGRP1's expression period when compared to plants grown under higher light (approximately 80 mumol m-2 sec-1 at noon). Evidence also indicated that for one cell type in which ptGRP1 is localized, this protein is imported rather than synthesized. In addition, confocal microscopy studies suggested that ptGRP1 is deposited at the cell wall/membrane interface rather than within the cell wall.

Developmental regulation and phytochrome-mediated induction of mRNAs encoding a proline-rich protein, glycine-rich proteins, and hydroxyproline-rich glycoproteins in Phaseolus vulgaris L

We have studied developmental and light regulation of mRNAs encoding a putative cell wall proline-rich protein (PvPRP1), cell wall glycine-rich proteins (GRPs), and cell wall hydroxyproline-rich glycoproteins (HRGPs) in bean (Phaseolus vulgaris). Light increases the levels of these mRNAs 2- to 150-fold in highly spatially regulated patterns during seedling development. These mRNA changes include differential regulation of transcripts derived from the GRP and HRGP multigene families. In 6-day-old light-grown seedlings, the PvPRP1 and GRP1.0 mRNAs were most abundant in the apical region of hypocotyls, epicotyls, and roots. In contrast, several HRGP transcripts were most abundant in the mature region of hypocotyls and roots in light-grown seedlings. When etiolated 6-day-old seedlings were illuminated with white light for 8 hr, maximal accumulation of PvPRP1 and GRP1.0 mRNAs occurred in the apical hook, whereas HRGP and GRP1.8 mRNAs accumulated in the mature region of hypocotyls. Etiolated seedlings subjected to a pulse of red light accumulated PvPRP1, GRP, and HRGP mRNAs in the hypocotyls. Far-red light inhibited red light induction of these mRNAs, indicating a phytochrome-mediated process. The possible roles of PRPs, GRPs, and HRGPs in cell differentiation and photomorphogenesis are discussed.

Characterization of cDNAs for stylar transmitting tissue-specific proline-rich proteins in tobacco

The pistil of flowers is a specialized organ which contains the female gametophytes and provides the structures necessary for pollination and fertilization. Pollen deposited on the stigmatic surface of a compatible plant germinates a pollen tube which penetrates the stigmatic papillae and grows intercellularly through the style towards the ovules in the ovary. Pollen tube growth is largely restricted to the transmitting tissue in the style. Therefore the stylar transmitting tissue is extremely important for the migration of the pollen cell towards the ovary. We have isolated two related cDNAs, transmitting tissue-specific (TTS)-1 and TTS-2, derived from two proline-rich protein (PRP)-encoding mRNAs that accumulate specifically in the transmitting tissue of tobacco. The deduced PRP sequences share similarities with proline-rich cell wall glycoproteins found in a variety of plants. TTS-1 and TTS-2 mRNAs are induced in very young floral buds, accumulate most abundantly during the later stages of flower development when style elongation is the most rapid, and remain at relatively high levels at anthesis. These mRNAs become undetectable in maturing green fruits. In situ hybridization shows that TTS-1 and TTS-2 mRNA accumulation is restricted to the transmitting tissue of the style. The possible roles that these transmitting tissue-specific PRPs may play in maintaining the structural integrity of the style or in the function of this organ is discussed.

Deletion analysis and localization of SbPRP1, a soybean cell wall protein gene, in roots of transgenic tobacco and cowpea

SbPRP1 is a member of the soybean (Glycine max L. Merr) proline-rich cell wall protein family and is expressed at high levels in root tissue. To characterize the sequences required for this expression, we have fused 1.1 kb of upstream flanking DNA sequence from an SbPRP1 genomic clone to a gene encoding beta-glucuronidase (GUS). This construct was introduced into tobacco using Agrobacterium tumefaciens-mediated transformation. Histochemical staining of GUS activity in transgenic tobacco indicated that SbPRP1 is expressed in the apical and elongating region of both primary and lateral roots, most strongly in the epidermis. A similar localization pattern was found in transformed hairy roots when this construct was introduced into cowpea (Vigna aconitifolia) using Agrobacterium rhizogenes-mediated transformation. Nested 5'-deletion analysis of the SbPRP1 promoter indicated that a minimal promoter for SbPRP1 expression in roots is located within the first 262 bases of upstream flanking DNA and that the region between -1080 and -262 is required for maximal expression of this gene. Gel retardation assays showed that nuclear factors can be detected in soybean roots which specifically bind to sequences located between -1080 and -623, a region which is needed for maximal expression of the SbPRP1 promoter. Northern hybridization analysis was also used to show that little SbPRP1 mRNA was present in roots during the first 24 h after imbibition. These studies indicate that SbPRP1 expression is localized to the actively growing region of the root and that this expression is temporally regulated during very early stages of seedling growth.

Expression of DcPRP1 is linked to carrot storage root formation and is induced by wounding and auxin treatment

A carrot (Daucus carota, L.) genomic clone (DcPRP1) was isolated on the basis of its homology to previously described cDNAs encoding a wound-inducible, proline-rich cell wall protein. DNA sequence analysis showed that DcPRP1 contains a single open-reading frame encoding a 235-amino acid protein that is colinear with that predicted from the cDNA sequence with the exception of four amino acids at the N terminus and a 60-nucleotide insertion present within the genomic clone. Genomic Southern hybridization analysis showed that the cloned sequence hybridized with a single restriction enzyme fragment using several restriction enzymes. Primer extension and northern hybridization analysis indicated that the expression of DcPRP1 is developmentally regulated and linked to the formation of storage roots, where this gene is expressed at high levels after wounding. The level of DcPRP1 mRNA was greatest in tissue immediately adjacent to the wound site. Treatment of unwounded carrot storage roots with 10 microM 2,4-dichlorophenoxy-acetic acid, indoleacetic acid, or naphthalene-1-acetic acid also resulted in the accumulation of DcPRP1 transcripts to a level equal to that seen in wounded tissue.

Variation of proline rich cell wall proteins in soybean lines with anthocyanin mutations

The I locus controls inhibition of anthocyanin accumulation in the epidermal cells of the soybean seed coat and affects abundance of PRP1, a proline-rich cell wall protein in the seed coat. Saline-soluble PRP1 is abundant in the developing seed coats of cultivar Richland (homozygous I, yellow), while it is significantly decreased in the pigmented isogenic mutant T157 (homozygous i, imperfect black). In this report, we examined soluble PRP1 in several cultivars containing alleles of the I locus which affect spatial distribution of pigmentation in the seed coat. We also characterized PRP1 in isolines with allelic variants of several other loci involved in seed coat pigmentation, including T and Im. The T gene is pleiotropic and affects both pubesence color and seed coat pigmentation and structure. Soluble PRP1 was abundant in the developing seed coats of lines with yellow seed (I or ii alleles) regardless of pubescence color, just as in Richland. Likewise, soluble PRP1 was decreased in pigmented seed coats (ik or i alleles) with grey (t) pubescence, as in T157. However, the total seed coat proteins were not extractable from pigmented seed coats with tawny pubescence (i, T genotypes) because they have proanthocyanidins that exhibit tannin properties. The dominant Im allele inhibits seed coat mottling (irregular patches of pigmentation) that occurs if plants are infected with soybean mosaic virus. PRP1 was 35 kDa in mottled (im) isolines and 34 kDa in non-mottled (Im) isolines. PRP2, which is expressed later in seed coat development and in the hypocotyl hooks of soybean seedlings, was also smaller in Im isolines. In summary, some of the anthocyanin mutations affect the quantity of soluble PRP1 polypeptides. while others correlate with structural changes in developmentally regulated proline-rich proteins.

Rhizobium meliloti elicits transient expression of the early nodulin gene ENOD12 in the differentiating root epidermis of transgenic alfalfa

To study the molecular responses of the host legume during early stages of the symbiotic interaction with Rhizobium, we have cloned and characterized the infection-related early nodulin gene MtENOD12 from Medicago truncatula. In situ hybridization experiments have shown that, within the indeterminate Medicago nodule, transcription of the MtENOD12 gene begins in cell layers of meristematic origin that lie ahead of the infection zone, suggesting that these cells are undergoing preparation for bacterial infection. Histochemical analysis of transgenic alfalfa plants that express an MtENOD12 promoter-beta-glucuronidase gene fusion has confirmed this result and further revealed that MtENOD12 gene transcription occurs as early as 3 to 6 hr following inoculation with R. meliloti in a zone of differentiating root epidermal cells which lies close to the growing root tip. It is likely that this transient, nodulation (nod) gene-dependent activation of the ENOD12 gene also corresponds to the preparation of the plant for bacterial infection. We anticipate that this extremely precocious response to Rhizobium will provide a valuable molecular marker for studying early signal exchange between the two symbiotic organisms.

Epidermis-specific gene expression in Pachyphytum

Transcripts of exclusively epidermis-specific expression were obtained by differential screening of a cDNA library prepared from isolated epidermis tissue of a succulent plant, Pachyphytum. Six transcripts were selected and characterized by RNA gel blot hybridization. Five cDNAs represented abundant mRNAs found exclusively in the epidermis. In situ hybridizations with three of these transcripts further substantiated their epidermal location. While two transcripts were present in all cells of the epidermis, one transcript was enriched in subsidiary cells of the stomatal complexes. All of the transcripts were highly expressed in the protoderm of the shoot apical meristem. DNA sequence determination indicated that the mRNAs identified represent novel sequences, encoding yet unknown plant functions. These transcripts, their control elements, and their encoded functions should help to advance our understanding of epidermal cell determination and differentiation.

Developmental expression of tobacco pistil-specific genes encoding novel extensin-like proteins

We have sought to identify pistil-specific genes that can be used as molecular markers to study pistil development. For this purpose, a cDNA library was constructed from poly(A)+ RNA extracted from tobacco stigmas and styles at different developmental stages. Differential screening of this library led to the isolation of cDNA clones that correspond to genes preferentially or specifically expressed in the pistil. Seven of these cDNA clones encode proteins containing repetitions of the pentapeptide Ser-Pro4, which is a typical motif found in extensins. Unlike extensin genes, the extensin-like genes described here are not induced under stress conditions. RNA gel blot hybridizations demonstrated the organ-specific expression of the extensin-like genes and their temporal regulation during pistil development. After pollination, the transcript levels of the pistil-specific extensin-like genes change relative to levels in unpollinated pistils. In situ hybridization experiments showed that at least one of these pistil-specific genes is specifically expressed in cells of the transmitting tissue. The possible roles of the extensin-like proteins in pistils are discussed.