The predominant protein on the surface of maize pollen is an endoxylanase synthesized by a tapetum mRNA with a long 5' leader

Modifying the Expression of Cysteine Protease Gene PCP Affects Pollen Development, Germination and Plant Drought Tolerance in Maize

Cysteine proteases (CPs) are vital proteolytic enzymes that play critical roles in various plant processes. However, the particular functions of CPs in maize remain largely unknown. We recently identified a pollen-specific CP (named PCP), which highly accumulated on the surface of maize pollen. Here, we reported that PCP played an important role in pollen germination and drought response in maize. Overexpression of PCP inhibited pollen germination, while mutation of PCP promoted pollen germination to some extent. Furthermore, we observed that germinal apertures of pollen grains in the PCP-overexpression transgenic lines were excessively covered, whereas this phenomenon was not observed in the wild type (WT), suggesting that PCP regulated pollen germination by affecting the germinal aperture structure. In addition, overexpression of PCP enhanced drought tolerance in maize plants, along with the increased activities of the antioxidant enzymes and the decreased numbers of the root cortical cells. Conversely, mutation of PCP significantly impaired drought tolerance. These results may aid in clarifying the precise functions of CPs in maize and contribute to the development of drought-tolerant maize materials.

Rapid evolution of pollen and pistil traits as a response to sexual selection in the post-pollination phase of mating

Sexual selection is the basis of some of the most striking phenotypic variation in nature.^1,2 In animals, sexual selection in males can act on traits that improve access to mates prior to copulation,^3-8 but also on sperm traits filtered by sperm competition,^9-14 or female choice expressed simply by the morphology and physiology of genital tracts.^14-16 Although long overlooked as a mode of selection on plant traits, sexual selection should act on land plants too because they are anisogamous: males produce more, and smaller, gametes than females.^17-19 Numerical asymmetry in gamete production is thought to play a central role in selection on traits that affect pollen transfer to mates,^20,21 but very little is known about how pollen competition or cryptic female choice might affect the evolution of traits expressed after pollination.^22,23 Here, we report the divergence of pollen and pistil traits of the dioecious wind-pollinated annual herb Mercurialis annua during evolution over three generations between populations at low versus high plant density, corresponding to low versus higher levels of polyandry;²⁴ we expected selection under higher polyandry to strengthen competition among pollen donors for fertilizing ovules. We found that populations at high density evolved faster-growing pollen tubes (an equivalent of greater sperm velocity), greater expression of pollen proteins involved in pollen growth, and larger stigmas (a trait likely enhancing the number of pollen donors and thus competition for ovules). Our results identify the post-pollination phase of plant mating as an important arena for the action of sexual selection.

GH10 and GH11 endoxylanases in Penicillium subrubescens: comparative characterization and synergy with GH51, GH54, GH62 α-L-arabinofuranosidases from the same fungus

Penicillium subrubescens has an expanded set of genes encoding putative endoxylanases (PsXLNs) compared to most other Penicillia and other fungi. In this study, all GH10 and GH11 PsXLNs were produced heterologously in Pichia pastoris and characterized. They were active towards beech wood xylan (BWX) and wheat flour arabinoxylan (WAX), and showed stability over a wide pH range. Additionally, PsXLNs released distinct oligosaccharides from WAX, and showed significant cooperative action with P. subrubescens α-L-arabinofuranosidases (PsABFs) from GH51 or GH54 for WAX degradation, giving insight into a more diverse XLN and ABF system for the efficient degradation of complex hemicelluloses. Homology modelling analysis pointed out differences in the catalytic center of PsXLNs, which are discussed in view of the different modes of action observed. These findings facilitate understanding of structural requirements for substrate recognition to contribute to recombinant XLN engineering for biotechnological applications.

Forgotten Actors: Glycoside Hydrolases During Elongation Growth of Maize Primary Root

Plant cell enlargement is coupled to dynamic changes in cell wall composition and properties. Such rearrangements are provided, besides the differential synthesis of individual cell wall components, by enzymes that modify polysaccharides in muro. To reveal enzymes that may contribute to these modifications and relate them to stages of elongation growth in grasses, we carried out a transcriptomic study of five zones of the primary maize root. In the initiation of elongation, significant changes occur with xyloglucan: once synthesized in the meristem, it can be linked to other polysaccharides through the action of hetero-specific xyloglucan endotransglycosidases, whose expression boosts at this stage. Later, genes for xyloglucan hydrolases are upregulated. Two different sets of enzymes capable of modifying glucuronoarabinoxylans, mainly bifunctional α-arabinofuranosidases/β-xylosidases and β-xylanases, are expressed in the maize root to treat the xylans of primary and secondary cell walls, respectively. The first set is highly pronounced in the stage of active elongation, while the second is at elongation termination. Genes encoding several glycoside hydrolases that are able to degrade mixed-linkage glucan are downregulated specifically at the active elongation. It indicates the significance of mixed-linkage glucans for the cell elongation process. The possibility that many glycoside hydrolases act as transglycosylases in muro is discussed.

Characterization of feruloyl esterases in maize pollen

Ferulic acid esterases have been identified and partially purified from maize pollen. Results suggest that maize pollen FAEs may play an important role in pollen fertilization. A critical step in maize (Zea mays) seed production involves fertilization of the ovule by pollen, a process that relies on ability of the pollen tube to grow through the highly structured and feruloylated arabinoxylan/cellulose-rich tissue of the silk and stigma. It is known that different cell wall hydrolases are present on the surface of pollen. An important hydrolase reported to date is an endo-xylanase (ZmXYN1). We report presence and characterization of another hydrolase, ferulic acid esterase (FAE), in maize pollen. Using a combination of biochemical approaches, these FAEs were partially purified and characterized with respect to their biochemical properties and putative sequences. Maize pollen FAEs were shown to be expressed early during pollen development, to release significant amounts of both monomeric and dimeric ferulates esterified from maize silks and other grass cell walls, and to synergize with an externally applied fungal endo-1,4-β-xylanase on the release of cell wall ferulates and diferulates. Preliminary analysis of maize silk cell walls following pollination, showed a significant reduction of esterified ferulates up to 96 h following pollination, compared to unpollinated silks. These results suggest that maize pollen FAEs may play an important biological role in pollen fertilization and possibly in seed production.

Genetic variation of HvXYN1 associated with endoxylanase activity and TAX content in barley (Hordeum vulgare L.)

BACKGROUND

Endo-β-1,4-xylanase1 (EA), the key endoxylanase in plants, is involved in the degradation of arabinoxylan during grain germination. In barley (Hordeum vulgare L.), one gene (HvXYN-1) that encode a endo-beta-1,4-xylanase, has been cloned. However, the single nucleotide polymorphisms (SNPs) that affect the endoxylanase activity and total arabinoxylan (TAX) content have yet to be characterized. The investigation of genetic variation in HvXYN1 may facilitate a better understanding of the relationship between TAX content and EA activity in barley.

RESULTS

In the current study, 56 polymorphisms were detected in HvXYN1 among 210 barley accessions collected from 34 countries, with 10 distinct haplotypes identified. The SNPs at positions 110, 305, 1045, 1417, 1504, 1597, 1880 bp in the genomic region of HvXYN1 were significantly associated with EA activity (P < 0.0001), and the sites 110, 305, and 1045 were highly significantly associated with TAX content. The amount of phenotypic variation in a given trait explained by each associated polymorphism ranged from 6.96 to 9.85%. Most notably, we found two variants at positions 1504 bp and 1880 bp in the second exon that significantly (P < 0.0001) affected EA activity; this result could be used in breeding programs to improve beer quality. In addition, African accessions had the highest EA activity and TAX content, and the richest germplasm resources were from Asia, indicating the high potential value of Asian barley.

CONCLUSION

This study provided insight into understanding the relationship, EA activity, TAX content with the SNPs of HvXYN1 in barley. These SNPs can be applied as DNA markers in breeding programs to improve the quality of barley for beer brewing after further validation.

A Genome-Wide Association Study for Partial Resistance to Maize Common Rust

Quantitative resistance to maize common rust (causal agent Puccinia sorghi) was assessed in an association mapping population of 274 diverse inbred lines. Resistance to common rust was found to be moderately correlated with resistance to three other diseases and with the severity of the hypersensitive defense response previously assessed in the same population. Using a mixed linear model accounting for the confounding effects of population structure and flowering time, genome-wide association tests were performed based at 246,497 single-nucleotide polymorphism loci. Three loci associated with maize common rust resistance were identified. Candidate genes at each locus had predicted roles, mainly in cell wall modification. Other candidate genes included a resistance gene and a gene with a predicted role in regulating accumulation of reactive oxygen species.

The Pollen Coat Proteome: At the Cutting Edge of Plant Reproduction

The tapetum is a single layer of secretory cells which encloses the anther locule and sustains pollen development and maturation. Upon apoptosis, the remnants of the tapetal cells, consisting mostly of lipids and proteins, fill the pits of the sculpted exine to form the bulk of the pollen coat. This extracellular matrix forms an impermeable barrier that protects the male gametophyte from water loss and UV light. It also aids pollen adhesion and hydration and retains small signaling compounds involved in pollen-stigma communication. In this study, we have updated the list of the pollen coat's protein components and also discussed their functions in the context of sexual reproduction.

Diversity and function of maize pollen coat proteins: from biochemistry to proteomics

Maize (Zea mays L.) is globally cultivated as one of the most important grain crops. As a wind-pollinated species, maize produces a large quantity of pollen grains that heavier and larger compared to Arabidopsis. Maize is an important model plant in pollen biology of monocots. The pollen coat, the outermost layer of pollen, plays a vital role in pollen-stigma interactions and successful fertilization. Pollen coat proteins (PCPs), which confer species specificity, are required for pollen adhesion, recognition, hydration, and germination on the stigma. Thus, PCPs have attracted intensive research efforts in plant science for decades. However, only a few PCPs in maize have been characterized to date, whereas the functions of most maize PCPs remain unclear. In this review, we summarize the current knowledge of maize PCPs with regard to protein constituents, synthesis and transport, and functions by comparison with the model plant Arabidopsis thaliana and Brassica plants. An understanding of the comprehensive knowledge of maize PCPs will help to illuminate the mechanism by which PCPs are involved in pollen-stigma interactions in maize and other crop plants.

Suppression of xylan endotransglycosylase PtxtXyn10A affects cellulose microfibril angle in secondary wall in aspen wood

Certain xylanases from family GH10 are highly expressed during secondary wall deposition, but their function is unknown. We carried out functional analyses of the secondary-wall specific PtxtXyn10A in hybrid aspen (Populus tremula × tremuloides). PtxtXyn10A function was analysed by expression studies, overexpression in Arabidopsis protoplasts and by downregulation in aspen. PtxtXyn10A overexpression in Arabidopsis protoplasts resulted in increased xylan endotransglycosylation rather than hydrolysis. In aspen, the enzyme was found to be proteolytically processed to a 68 kDa peptide and residing in cell walls. Its downregulation resulted in a corresponding decrease in xylan endotransglycosylase activity and no change in xylanase activity. This did not alter xylan molecular weight or its branching pattern but affected the cellulose-microfibril angle in wood fibres, increased primary growth (stem elongation, leaf formation and enlargement) and reduced the tendency to form tension wood. Transcriptomes of transgenic plants showed downregulation of tension wood related genes and changes in stress-responsive genes. The data indicate that PtxtXyn10A acts as a xylan endotransglycosylase and its main function is to release tensional stresses arising during secondary wall deposition. Furthermore, they suggest that regulation of stresses in secondary walls plays a vital role in plant development.

Microfluidic platforms for plant cells studies

Conventional methods of plant cell analysis rely on growing plant cells in soil pots or agarose plates, followed by screening the plant phenotypes in traditional greenhouses and growth chambers. These methods are usually costly, need a large number of experiments, suffer from low spatial resolution and disorderly growth behavior of plant cells, with lack of ability to locally and accurately manipulate the plant cells. Microfluidic platforms take advantage of miniaturization for handling small volume of liquids and providing a closed environment, with the purpose of in vitro single cell analysis and characterizing cell response to external cues. These platforms have shown their ability for high-throughput cellular analysis with increased accuracy of experiments, reduced cost and experimental times, versatility in design, ability for large-scale and combinatorial screening, and integration with other miniaturized sensors. Despite extensive research on animal cells within microfluidic environments for high-throughput sorting, manipulation and phenotyping studies, the application of microfluidics for plant cells studies has not been accomplished yet. Novel devices such as RootChip, RootArray, TipChip, and PlantChip developed for plant cells analysis, with high spatial resolution on a micrometer scale mimicking the internal microenvironment of plant cells, offering preliminary results on the capability of microfluidics to conquer the constraints of conventional methods. These devices have been used to study different aspects of plant cell biology such as gene expression, cell biomechanics, cellular mechanism of growth, cell division, and cells fusion. This review emphasizes the advantages of current microfluidic systems for plant science studies, and discusses future prospects of microfluidic platforms for characterizing plant cells response to diverse external cues.

The impact of environmental stress on male reproductive development in plants: biological processes and molecular mechanisms

In plants, male reproductive development is extremely sensitive to adverse climatic environments and (a)biotic stress. Upon exposure to stress, male gametophytic organs often show morphological, structural and metabolic alterations that typically lead to meiotic defects or premature spore abortion and male reproductive sterility. Depending on the type of stress involved (e.g. heat, cold, drought) and the duration of stress exposure, the underlying cellular defect is highly variable and either involves cytoskeletal alterations, tapetal irregularities, altered sugar utilization, aberrations in auxin metabolism, accumulation of reactive oxygen species (ROS; oxidative stress) or the ectopic induction of programmed cell death (PCD). In this review, we present the critically stress-sensitive stages of male sporogenesis (meiosis) and male gametogenesis (microspore development), and discuss the corresponding biological processes involved and the resulting alterations in male reproduction. In addition, this review also provides insights into the molecular and/or hormonal regulation of the environmental stress sensitivity of male reproduction and outlines putative interaction(s) between the different processes involved.

The cellular mechanics of an invasive lifestyle

Invasive behaviour is the hallmark of a variety of cell types of animal, plant, and fungal origin. Here we review the purpose and mechanism of invasive growth and migration. The focus is on the physical principles governing the process, the source of invasive force, and the cellular mechanism by which the cell penetrates the substrate. The current experimental methods for measuring invasive force and the modelling approaches for studying invasive behaviour are explained, and future experimental strategies are proposed.

An enzyme activity capable of endotransglycosylation of heteroxylan polysaccharides is present in plant primary cell walls

Heteroxylans in the plant cell wall have been proposed to have a role analogous to that of xyloglucans or heteromannans, forming growth-restraining networks by interlocking cellulose microfibrils. A xylan endotransglycosylase has been identified that can transglycosylate heteroxylan polysaccharides in the presence of xylan-derived oligosaccharides. High activity was detected in ripe fruit of papaya (Carica papaya), but activity was also found in a range of other fruits, imbibed seeds and rapidly growing seedlings of cereals. Xylan endotransglycosylase from ripe papaya fruit used a range of heteroxylans, such as wheat arabinoxylan, birchwood glucuronoxylan and various heteroxylans from dicotyledonous primary cell walls purified from tomato and papaya fruit, as donor molecules. As acceptor molecules, the enzyme preferentially used xylopentaitol over xylohexaitol or shorter-length acceptors. Xylan endotransglycosylase was active over a broad pH range and could perform transglycosylation reactions up to 55 °C. Xylan endotransglycosylase activity was purified from ripe papaya fruit by ultrafiltration and cation exchange chromatography. Highest endotransglycosylase activity was identified in fractions that also contained high xylan hydrolase activity and correlated with the presence of the endoxylanase CpaEXY1. Recombinant CpaEXY1 protein transiently over-expressed in Nicotiana benthamiana leaves showed both endoxylanase and xylan endotransglycosylase activities in vitro, suggesting that CpaEXY1 is a single enzyme with dual activity in planta. Purified native CpaEXY1 showed two- to fourfold higher endoxylanase than endotransglycosylase activity, suggesting that CpaEXY1 may act primarily as a hydrolase. We propose that xylan endotransglycosylase activity (like xyloglucan and mannan endotransglycosylase activities) could be involved in remodelling or re-arrangement of heteroxylans of the cellulose-non-cellulosic cell wall framework.

The maize tapetum employs diverse mechanisms to synthesize and store proteins and flavonoids and transfer them to the pollen surface

In anthers, the tapetum synthesizes and stores proteins and flavonoids, which will be transferred to the surface of adjacent microspores. The mechanism of synthesis, storage, and transfer of these pollen-coat materials in maize (Zea mays) differs completely from that reported in Arabidopsis (Arabidopsis thaliana), which stores major pollen-coat materials in tapetosomes and elaioplasts. On maize pollen, three proteins, glucanase, xylanase, and a novel protease, Zea mays pollen coat protease (ZmPCP), are predominant. During anther development, glucanase and xylanase transcripts appeared at a mid developmental stage, whereas protease transcript emerged at a late developmental stage. Protease and xylanase transcripts were present only in the anther tapetum of the plant, whereas glucanase transcript was distributed ubiquitously. ZmPCP belongs to the cysteine protease family but has no closely related paralogs. Its nascent polypeptide has a putative amino-terminal endoplasmic reticulum (ER)-targeting peptide and a propeptide. All three proteins were synthesized in the tapetum and were present on mature pollen after tapetum death. Electron microscopy of tapetum cells of mid to late developmental stages revealed small vacuoles distributed throughout the cytoplasm and numerous secretory vesicles concentrated near the locular side. Immunofluorescence microscopy and subcellular fractionation localized glucanase in ER-derived vesicles in the cytoplasm and the wall facing the locule, xylanase in the cytosol, protease in vacuoles, and flavonoids in subdomains of ER rather than in vacuoles. The nonoverlapping subcellular locations of the three proteins and flavonoids indicate distinct modes of their storage in tapetum cells and transfer to the pollen surface, which in turn reflect their respective functions in tapetum cells or the pollen surface.

Matrix solubilization and cell wall weakening by β-expansin (group-1 allergen) from maize pollen

Beta-expansins accumulate to high levels in grass pollen, a feature apparently unique to grasses. These proteins, which are major human allergens, facilitate pollen tube penetration of the maize stigma and style (the silk). Here we report that treatment of maize silk cell walls with purified β-expansin from maize pollen led to solubilization of wall matrix polysaccharides, dominated by feruloyated highly substituted glucuronoarabinoxylan (60%) and homogalacturonan (35%). Such action was selective for cell walls of grasses, and indicated a target preferentially found in grass cell walls, probably the highly substituted glucuronoarabinoxylan. Several tests for lytic activities by β-expansin were negative and polysaccharide solubilization had weak temperature dependence, which indicated a non-enzymatic process. Concomitant with matrix solubilization, β-expansin treatment induced creep, reduced the breaking force and increased the plastic compliance of wall specimens. From comparisons of the pH dependencies of these processes, we conclude that matrix solubilization was linked closely to changes in wall plasticity and breaking force, but not so closely coupled to cell wall creep. Because matrix solubilization and increased wall plasticity have not been found with other expansins, we infer that these novel activities are linked to the specialized role of grass pollen β-expansins in promotion of penetration of the pollen tube through the stigma and style, most likely by weakening the middle lamella.

Regulation of Arabidopsis early anther development by the mitogen-activated protein kinases, MPK3 and MPK6, and the ERECTA and related receptor-like kinases

Mitogen-activated protein kinase (MAPK) and leucine-rich repeat receptor-like kinase (LRR-RLK) signaling pathways have been shown to regulate diverse aspects of plant growth and development. In Arabidopsis, proper anther development relies on intercellular communication to coordinate cell proliferation and differentiation. Two closely related genes encoding MAPKs, MPK3 and MPK6, function redundantly in regulating stomatal patterning. Although the mpk6 mutant has reduced fertility, the function of MPK3 and MPK6 in anther development has not been characterized. Similarly, the ERECTA (ER), ERECTA-LIKE1 (ERL1) and ERL2 genes encoding LRR-RLKs function together to direct stomatal cell fate specification and the er-105 erl1-2 erl2-1 triple mutant is sterile. Because the mpk3 mpk6 double null mutant is embryo lethal, anther development was characterized in the viable mpk3/+ mpk6/- and er-105 erl1-2 erl2-1 mutants. We found that both mutant anthers usually fail to form one or more of the four anther lobes, with the er-105 erl1-2 erl2-1 triple mutant exhibiting more severe phenotypes than those of the mpk3/+ mpk6/- mutant. The somatic cell layers of the differentiated mutant lobes appeared larger and more disorganized than that of wild-type. In addition, the er-105 erl1-2 erl2-1 triple mutant has a reduced number of stamens, the majority of which possess completely undifferentiated or under-differentiated anthers. Furthermore, sometimes, the mpk3/+ mpk6/- mutant anthers do not dehisce, and the er-105 erl1-2 erl2-1 anthers were not observed to dehisce. Therefore, our results indicate that both ER/ERL1/ERL2 and MPK3/MPK6 play important roles in normal anther lobe formation and anther cell differentiation. The close functional relationship between these genes in other developmental processes and the similarities in anther developmental phenotypes of the two types of mutants reported here further suggest the possibility that these genes might also function in the same pathway to regulate anther cell division and differentiation.

An alternative approach for the synthesis of fluorogenic substrates of endo-beta-(1-->4)-xylanases and some applications

Fluorogenic substrates of endo-beta-(1-->4)-xylanases (EXs), 4-methylumbelliferyl beta-glycosides of xylobiose and xylotriose were synthesized from fully acetylated oligosaccharides using the alpha-trichloroacetimidate procedure. A commercially available syrup containing xylose and xylo-oligosaccharides was used as the starting material. Both fluorogenic glycosides were found to be suitable substrates for EXs, particularly for sensitive detection of the enzymes in electrophoretic gels and their in situ localization on sections of fruiting bodies of some plants, such as tomato, potato and eggplant, all of the family Solanaceae.

Papaya fruit softening, endoxylanase gene expression, protein and activity

Papaya (Carica papaya L.) cell wall matrix polysaccharides are modified as the fruit starts to soften during ripening and an endoxylanase is expressed that may play a role in the softening process. Endoxylanase gene expression, protein amount and activity were determined in papaya cultivars that differ in softening pattern and in one cultivar where softening was modified by the ethylene receptor inhibitor 1-methylcyclopropene (1-MCP). Antibodies to the endoxylanase catalytic domain were used to determine protein accumulation. The three papaya varieties used in the study, 'Line 8', 'Sunset', and 'Line 4-16', differed in softening pattern, respiration rate, ethylene production and showed similar parallel relationships during ripening and softening in endoxylanase expression, protein level and activity. When fruit of the three papaya varieties showed the respiratory climacteric and started to soften, the level of endoxylanase gene expression increased and this increase was related to the amount of endoxylanase protein at 32 kDa and its activity. Fruit when treated at less than 10% skin yellow stage with 1-MCP showed a significant delay in the respiratory climacteric and softening, and reduced ethylene production, and when ripe was firmer and had a 'rubbery' texture. The 1-MCP-treated fruit that had the 'rubbery' texture showed suppressed endoxylanase gene expression, protein and enzymatic activity. Little or no delay occurred between endoxylanase gene expression and the appearance of activity during posttranslational processing from 65 to 32 kDa. The close relationship between endoxylanase gene expression, protein accumulation and activity in different varieties and the failure of the 1-MCP-treated fruit to fully soften, supported de novo synthesis of endoxylanase, rapid posttranslation processing and a role in papaya fruit softening.

An Isorhamnetin Rhamnoglycoside Serves as a Costimulant for Sugars and Amino Acids in Feeding Responses of Adult Western Corn Rootworms (Diabrotica virgifera virgifera) to Corn (Zea mays) Pollen

Adult beetles of Diabrotica virgifera virgifera LeConte (western corn rootworm) feed on pollen of Zea mays L. (corn) and other plant species. To identify D. virgifera feeding stimulants, beetle responses to mixtures of known and novel phagostimulants, presented at their naturally occurring concentrations in maize pollen, were compared to individual component responses applying the amount occurring in 0.2 mg of pollen per cellulose feeding disk. On a molar basis, three major sugars (fructose, glucose, and sucrose) were more prevalent in corn pollen buffer extract (CPE) than free amino acids. Western corn rootworm feeding was stimulated by the three sugars (28% disk consumption) and, to an even greater extent, by a mixture of 21 free amino acids (41% disk consumption). However, the combination of three sugars and 21 amino acids elicited a level of D. virgifera feeding (41% disk consumption) similar to that of the 21 amino acids alone. A novel maize pollen phagostimulant was purified from CPE by using solid-phase extraction followed by RP-HPLC. Based on its mass fragment pattern, two UV maxima (254 and 359 nm), and previous isolation from maize pollen, this phagostimulant is tentatively identified as isorhamnetin 3-O-neohesperidoside. This compound interacted additively with the mixture of three sugars and 21 amino acids, to produce 77% of the phagostimulation level of CPE. Therefore, a possible stimulatory mechanism for D. virgifera feeding on corn pollen has been elucidated.

Proteomics identification of differentially expressed proteins associated with pollen germination and tube growth reveals characteristics of germinated Oryza sativa pollen

Mature pollen from most plant species is metabolically quiescent; however, after pollination, it germinates quickly and gives rise to a pollen tube to transport sperms into the embryo sac. Because methods for collecting a large amount of in vitro germinated pollen grains for transcriptomics and proteomics studies from model plants of Arabidopsis and rice are not available, molecular information about the germination developmental process is lacking. Here we describe a method for obtaining a large quantity of in vitro germinating rice pollen for proteomics study. Two-dimensional electrophoresis of approximately 2300 protein spots revealed 186 that were differentially expressed in mature and germinated pollen. Most showed a changed level of expression, and only 66 appeared to be specific to developmental stages. Furthermore 160 differentially expressed protein spots were identified on mass spectrometry to match 120 diverse protein species. These proteins involve different cellular and metabolic processes with obvious functional skew toward wall metabolism, protein synthesis and degradation, cytoskeleton dynamics, and carbohydrate/energy metabolism. Wall metabolism-related proteins are prominently featured in the differentially expressed proteins and the pollen proteome as compared with rice sporophytic proteomes. Our study also revealed multiple isoforms and differential expression patterns between isoforms of a protein. These results provide novel insights into pollen function specialization.

A Rice Family 9 Glycoside Hydrolase Isozyme with Broad Substrate Specificity for Hemicelluloses in Type II Cell Walls

An auxin analog, 2,4-D, stimulates the activity of endo-1,4-beta-glucanase (EGase) in rice (Oryza sativa L.). The auxin-induced activity from three protein fractions was purified to homogeneity from primary root tissues (based on SDS-PAGE and isoelectric focusing after Coomassie brilliant blue staining). Amino acid sequencing indicated that the 20 N-terminal amino acid sequence of the three proteins was identical, suggesting that these proteins may be cognates of one EGase gene. An internal amino acid sequence of the the rice EGase (LVGGYYDAGDNVK) revealed that this enzyme belongs to glycosyl hydrolase family 9 (GHF9). The major isoform of this rice GHF9 [molecular weight based on matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS): 51,216, isoelectric point (pI): 5.5] specifically hydrolyzed 1,4-beta-glycosyl linkages of carboxymethyl (CM)-cellulose, phosphoric acid-swollen cellulose, 1,3-1,4-beta-glucan, arabinoxylan, xylan, glucomannan, cellooligosaccharides [with a degree of polymerization (DP) >3] and 1,4-beta-xylohexaose, indicating a broader substrate range compared with those of other characterized GHF9 enzymes or EGases from higher plants. Hydrolytic products of two major hemicellulosic polysaccharides in type II cell walls treated with the purified enzyme were profiled using high-performance anion exchange chromatography (HPAEC). The results suggested that endolytic attack by rice EGase is not restricted to either the cellulose-like domain of 1,3-1,4-beta-glucan or the unsubstituted 1,4-beta-xylosyl backbone of arabinoxylan, but results in the release of smaller oligosaccharides (DP <6) from graminaceous hemicelluloses. The comparatively broader substrate range of this EGase with respect to beta-1,4-glycan backbones (glucose and xylose) may partly reflect different roles of gramineous and non-gramineous GHF9 enzymes.

Maize pollen coat xylanase facilitates pollen tube penetration into silk during sexual reproduction

Cell wall hydrolases are well documented to be present on pollen, but their roles on the stigma during sexual reproduction have not been previously demonstrated. We explored the function of the tapetum-synthesized xylanase, ZmXYN1, on maize (Zea mays L.) pollen. Transgenic lines (xyl-less) containing little or no xylanase in the pollen coat were generated with use of an antisense construct of the xylanase gene-coding region driven by the XYN1 gene promoter. Xyl-less and wild-type plants had similar vegetative growth. Electron microscopy revealed no appreciable morphological difference in anther cells and pollen between xyl-less lines and the wild type, whereas immunofluorescence microscopy and biochemical analyses indicated an absence of xylanase on xyl-less pollen. Xyl-less pollen germinated as efficiently as wild-type pollen in vitro in a liquid medium but less so on gel media of increasing solidity or on silk, which is indicative of partial impaired water uptake. Once germinated in vitro or on silk, the xyl-less and wild-type pollen tubes elongated at comparable rates. Tubes of germinated xyl-less pollen on silk did not penetrate into the silk as efficiently as tubes of wild-type pollen, and this lower efficiency could be overcome by the addition of xylanase to the silk. For wild-type pollen, coat xylanase activity on oat spelled xylan in vitro and tube penetration into silk were inhibited by xylose but not glucose. The overall findings indicate that maize pollen coat xylanase facilitates pollen tube penetration into silk via enzymatic xylan hydrolysis.

Localization of group-1 allergen Zea m 1 in the coat and wall of maize pollen

The pollen surface consists of an outermost coat and an underlying wall. It makes the initial contact with the stigma surface during sexual reproduction. To date, only two proteins have been identified from the maize pollen coat. Zea m 1 (beta-expansin 1) is the major group-1 allergen in maize pollen, but its presence and localization in the pollen coat and wall has not yet been explored. In the present study, immunoblot analysis using an antibody directed against group-1 allergen revealed that a small amount of Zea m 1 exists in the pollen coat fraction prepared using a diethyl ether wash. Immunogold labeling also showed that the extracellular localization of Zea m 1 was mainly restricted to the tectum and the foot layer of the exine (the outer pollen wall), and gold particles immunolabelling Zea m 1 were unevenly dispersed throughout the pollen coat and wall. Moreover, a substantial amount of Zea m 1 was localized in the cytoplasm of the pollen interior. The presence of Zea m 1 in the pollen coat and wall suggests that Zea m 1 may play a potential role in pollen germination on the stigma.

The activation of metabolites of nitric oxide synthase by metals is both redox and oxygen dependent: a new feature of nitrogen oxide signaling

Nitrite (NO(2)-), N (G)-hydroxy-L-arginine (NOHA), and hydroxylamine (NH(2)OH) are products of nitric oxide synthase (NOS) activity and can also be formed by secondary reactions of nitric oxide (NO). These compounds are commonly considered to be rather stable and as such to be dosimeters of NO biosynthesis. However, each can be converted via metal-catalyzed reactions into either NO or other reactive nitrogen oxide species (RNOS), such as nitrogen dioxide (NO(2)) and nitroxyl (HNO), which have biologic activities distinct from those of the parent molecules. Consequently, certain aspects of tissue regulation controlled by RNOS may be dictated to a significant extent by metal-dependent reactions, thereby offering unique advantages for cellular and tissue regulation. For instance, because many metal-catalyzed reactions depend on the redox and oxygen status of the cellular environment, such reactions could serve as redox indicators. Formation of RNOS by metal-mediated pathways would confine the chemistry of these species to specific cellular sites. Additionally, such mechanisms would be independent both of NO and NOS, thus increasing the lifetime of RNOS that react with NO. Thus metal-mediated conversion of nitrite, NOHA, and NH(2)OH into biologically active agents may provide a unique signaling mechanism. In this review, we discuss the biochemistry of such reactions in the context of their pharmacologic and biologic implications.

A symbiont-independent endo-1,4-beta-xylanase from the plant-parasitic nematode Meloidogyne incognita

Substituted xylan polymers constitute a major part of the hemicellulose fraction of plant cell walls, especially in monocotyledons. Endo-1,4-beta-xylanases (EC 3.2.1.8) are capable of hydrolyzing substituted xylan polymers into fragments of random size. Many herbivorous animals have evolved intimate relationships with endosymbionts to exploit their enzyme complexes for the degradation of xylan. Here, we report the first finding of a functional endo-1,4-beta-xylanase gene from an animal. The gene (Mi-xyl1) was found in the obligate plant-parasitic root-knot nematode Meloidogyne incognita, and encodes a protein that is classified as a member of glycosyl hydrolase family 5. The expression of Mi-xyl1 is localized in the subventral esophageal gland cells of the nematode. Previous studies have shown that M. incognita has the ability to degrade cellulose and pectic polysaccharides in plant cell walls independent of endosymbionts. Including our current data on Mi-xyl1, we show that the endogenous enzyme complex in root-knot nematode secretions targets essentially all major cell wall carbohydrates to facilitate a stealthy intercellular migration in the host plant.

Identification and characterization of gene sequences expressed in wheat spikelets at the heading stage

Through differential analysis of transcripts (SDDM), 85 cDNA sequences specifically or preferentially expressed in wheat spikelets at heading time were identified and cloned; 54 of them had significant homology with genomic, cDNA and protein and 16 with EST sequences. Among these 54 clones, 44 matched genes with known functions, whereas 10 detected homology with putative genes encoding proteins whose functions have been deduced on the basis of bioinformatic comparisons. Seventeen clones corresponded to genes that had never been cloned in cereals, 5 were related to wheat genes with known functions, and the remaining 32 to genes cloned in other cereals. On the basis of their presumed functions, the 54 clones were assigned to seven groups. The first four of them contained 40 sequences likely involved in floral organ morphogenesis and gametogenesis, and precisely (i) sequences involved in the morphogenesis of floral organs; (ii) sequences expressed in pollen and/or anther tissues; (iii) sequences encoding transcription factors; (iv) sequences involved in signal perception and transduction (kinases and LRR proteins). The expression patterns of these 40 sequences have been studied by reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of transcripts from different tissues and spike organs of wheat.

Fungal dimorphism regulated gene expression in Ustilago maydis: II. Filament down-regulated genes

SUMMARY Ustilago maydis displays dimorphic growth alternating between a budding haploid form and a filamentous dikaryon resulting from mating of two haploid cells. This morphological switch plays a critical role in pathogenicity because only the filamentous dikaryon can infect corn plants. Previously, we identified a role for the cAMP signal transduction pathway in dimorphism and pathogenicity. The repression of a subset of genes in filamentous cells may be critical for programming virulence. To identify these filament down-regulated genes and to understand better the role of wild-type budding cells in the life and disease cycle of U. maydis in nature, we used suppression subtractive hybridization. We arrayed a library of approximately 5500 cDNA clones and showed by reverse Northern blot analysis that most, as expected, are down-regulated during filamentous growth. By an iterative sequencing and hybridization process to eliminate previously determined sequences, we showed that > 88% of the clones detected as differential in the reverse Northern blot screening harbour sequences corresponding to 48 different genes. Differential expression was confirmed for 37 of these genes by Northern blot analysis. For eight of these confirmed differential genes, expression could only be detected in budding cells. For genes expressed in both growth forms, levels of differential expression varied from as much as 65-fold to only two-fold higher levels in budding cells. Twenty-seven of the 37 genes confirmed to be differential had similarity to database sequences, and fell into several putative functional categories. In future studies we will produce deletion mutants in several highly differentially expressed genes to study their roles in morphogenesis and pathogenesis.

Primary cell wall metabolism: tracking the careers of wall polymers in living plant cells

Numerous examples have been presented of enzyme activities, assayed in vitro, that appear relevant to the synthesis of structural polysaccharides, and to their assembly and subsequent degradation in the primary cell walls (PCWs) of higher plants. The accumulation of the corresponding mRNAs, and of the (immunologically recognized) proteins, has often also (or instead) been reported. However, the presence of these mRNAs, antigens and enzymic activities has rarely been shown to correspond to enzyme action in the living plant cell. In some cases, apparent enzymic action is observed in vivo for which no enzyme activity can be detected in in-vitro assays; the converse also occurs. Methods are reviewed by which reactions involving structural wall polysaccharides can be tracked in vivo. Special attention is given to xyloglucan endotransglucosylase (XET), one of the two enzymic activities exhibited in vitro by xyloglucan endotransglucosylase/hydrolase (XTH) proteins, because of its probable importance in the construction and restructuring of the PCW's major hemicellulose. Attention is also given to the possibility that some reactions observed in the PCW in vivo are not directly enzymic, possibly involving the action of hydroxyl radicals. It is concluded that some proposed wall enzymes, for example XTHs, do act in vivo, but that for other enzymes this is not proven. Contents I. Primary cell walls: composition, deposition and roles 642 II. Reactions that have been proposed to occur in primary cell walls 645 III. Tracking the careers of wall components in vivo: evidence for action of enzymes in the walls of living plant cells 656 IV. Evidence for the occurrence of nonenzymic polymer scission in vivo? 666 VI. Conclusion 667 References 667.

Properties of TAXI-type endoxylanase inhibitors

Two types of proteinaceous endoxylanase inhibitors occur in different cereals, i.e. the TAXI [Triticum aestivum endoxylanase inhibitor]-type and XIP [endoxylanase inhibiting protein]-type inhibitors. The present paper focuses on the TAXI-type proteins and deals with their structural characteristics and the identification, characterisation and heterologous expression of a TAXI gene from wheat. In addition, to shed light on the mechanism by which TAXI-type endoxylanase inhibitors work, the enzyme specificity, the optimal conditions for maximal inhibition activity, the molar complexation ratio and the inhibition kinetics of the inhibitors are explained and the effect of mutations of an endoxylanase on the inhibition by TAXIs is discussed.

Cell wall reactive proteins in the coat and wall of maize pollen: potential role in pollen tube growth on the stigma and through the style

The surface of a pollen grain consists of an outermost coat and an underlying wall. In maize (Zea mays L.), the pollen coat contains two major proteins derived from the adjacent tapetum cells in the anthers. One of the proteins is a 35-kDa endoxylanase (Wu, S. S. H., Suen, D. F., Chang, H. C., and Huang, A. H. C. (2002) J. Biol. Chem. 277, 49055-49064). The other protein of 70 kDa was purified to homogeneity and shown to be a beta-glucanase. Its gene sequence and the developmental pattern of its mRNA differ from those of the known beta-glucanases that hydrolyze the callose wall of the microspore tetrad. Mature pollen placed in a liquid medium released about nine major proteins. These proteins were partially sequenced and identified via GenBank trade mark data bases, and some had not been previously reported to be in pollen. They appear to have wall-loosening, structural, and enzymatic functions. A novel pollen wall-bound protein of 17 kDa has a unique pattern of cysteine distribution in its sequence (six tandem repeats of CX3CX10-15) that could chelate cations and form signal-receiving finger motifs. These pollen-released proteins were synthesized in the pollen interior, and their mRNA increased during pollen maturation and germination. They were localized mainly in the pollen tube wall. The pollen shell was isolated and found to contain no detectable proteins. We suggest that the pollen-coat beta-glucanase and xylanase hydrolyze the stigma wall for pollen tube entry and that the pollen secrete proteins to loosen or become new wall constituents of the tube and to break the wall of the transmitting track for tube advance.

The mechanisms of pollination and fertilization in plants

In flowering plants, pollen grains germinate to form pollen tubes that transport male gametes (sperm cells) to the egg cell in the embryo sac during sexual reproduction. Pollen tube biology is complex, presenting parallels with axon guidance and moving cell systems in animals. Pollen tube cells elongate on an active extracellular matrix in the style, ultimately guided by stylar and embryo sac signals. A well-documented recognition system occurs between pollen grains and the stigma in sporophytic self-incompatibility, where both receptor kinases in the stigma and their peptide ligands from pollen are now known. Complex mechanisms act to precisely target the sperm cells into the embryo sac. These events initiate double fertilization in which the two sperm cells from one pollen tube fuse to produce distinctly different products: one with the egg to produce the zygote and embryo and the other with the central cell to produce the endosperm.

Adhesion and guidance in compatible pollination

The mechanisms of compatible pollination are less studied than those of incompatible pollination and yet most of the angiosperms show self-compatibility. From the release of pollen from anthers to the penetration of the micropyle by the pollen tube tip, there are numerous steps where the interaction between pollen and the pistil can be regulated. Recent studies have documented some diverse ways in which pollen tubes carrying sperm cells are guided to the ovules through the pistil extracellular matrices of the transmitting tract. What is still missing is an understanding of pollen tube cell biology in vivo. A recent finding supports the role of the synergids in the crucial guidance cue for the pollen tube tip at the micropyle, but experimental evidence for other 'guidepost' cells in the pistil is still lacking. The fact that the pollen tube must first travel through the matrices of the stigma and style before it can respond to the cue from the ovule makes it likely that there is a hierarchy of signalling events in pollen-pistil interactions starting at the stigma and ending at the micropyle. On the pistil side, several model systems have been used in the discovery of molecules implicated in either physical or chemical guidance. In lily, which has a hollow style, adhesion molecules (pectin and SCA) are implicated in guidance. SCA alone is also capable of inducing pollen chemotropism in an in vitro assay, suggesting that this peptide plays a dual role in lily pollination: chemotactic in the stigma and haptotactic (adhesion mediated) in the style.

Maize tapetum xylanase is synthesized as a precursor, processed and activated by a serine protease, and deposited on the pollen

Pollen coat contains ingredients that interact with the stigma surface during sexual reproduction. In maize (Zea mays L.) pollen coat, the predominant protein is a 35-kDa endoxylanase, whose mRNA is located in the tapetum cells enclosing the maturing pollen in the anthers. This 2.0-kb mRNA was found to have an open reading frame of 1,635 nucleotides encoding a 60-kDa pre-xylanase. In developing anthers, the pre-xylanase protein appeared prior to the 35-kDa xylanase protein and enzyme activity and then peaked and declined, whereas the 35-kDa xylanase protein and activity continued to increase until anther maturation. An acid protease in the anther extract converted the inactive pre-xylanase to the active 35-kDa xylanase in vitro. The protease activity was inhibited by inhibitors of serine proteases but unaffected by inhibitors of cysteine, aspartic, or metallic proteases. Sequence analysis revealed that the 60-kDa pre-xylanase was converted to the 35-kDa xylanase with the removal of 198 and 48 residues from the N and C termini, respectively. During in vitro and in vivo conversions, no intermediates of 60-35 kDa were observed, and the 35-kDa xylanase was highly stable. The pre-xylanase was localized in the tapetum-containing anther wall, whereas the 35-kDa xylanase was found in the pollen coat. The significance of having a large non-active pre-xylanase and the mode of transfer of the xylanase to the pollen coat are discussed. A gene encoding the barley (Hordeum vulgare L.) tapetum xylanase was cloned; this gene and the gene encoding the seed aleurone-layer xylanase had strict tissue-specific expressions.

Modifying the pollen coat protein composition in Brassica

The interactions between pollen and stigma are essential for plant reproduction and are made possible by compounds, such as proteins and lipids, located on their surfaces. The pollen coat is formed in part by compounds synthesized in, and released from, the tapetum, which become transferred to the pollen coat late in pollen development. In the Brassicaceae the predominant proteins of the mature pollen coat are the tapetal oleosin-like proteins, which are highly expressed in, and ultimately transferred from, the tapetum. Here we report the modification of the protein composition of the pollen coat by the addition of an active enzyme which was synthesized in the tapetum. The marker enzyme beta-glucuronidase (GUS) was successfully targeted to the pollen coat in transgenic Brassica carinata plants expressing GUS translationally fused to a B. napus tapetal oleosin-like protein (BnOlnB;4). To our knowledge this is the first demonstration of the targeting of an enzyme to the pollen coat.

A xylanase, AtXyn1, is predominantly expressed in vascular bundles, and four putative xylanase genes were identified in the Arabidopsis thaliana genome

The cDNA clone RXF12, which encodes a xylanase (EC 3.2.1.8), was isolated from Arabidopsis thaliana. The C-terminal half of the amino acid sequence of the deduced protein, named AtXyn1, showed similarity with the catalytic domain of barley xylanase X-1. The N-terminal half of AtXyn1 also contained three regions with sequences similar to cellulose-binding domains (CBDs). A xylanase assay revealed that transgenic A. thaliana plants expressing exogenous AtXyn1 fused with enhanced green fluorescent protein (EGFP) possessed approximately twice as much xylanase activity as wild-type plants. Observation by fluorescence microscopy of transgenic A. thaliana plants expressing a fusion protein of AtXyn1 and EGFP suggested that AtXyn1 is a cell wall protein. Analysis of the localization of beta-glucuronidase (GUS) activity in transgenic A. thaliana plants containing a chimeric gene with the upstream sequence of the AtXyn1 gene and the GUS gene demonstrated that the AtXyn1 gene is predominantly expressed in vascular bundles, but not in vessel cells. These data suggest that AtXyn1 is involved in the secondary cell wall metabolism of vascular bundle cells. A database search revealed that four putative xylanase genes exist in the A. thaliana genome, besides the AtXyn1 gene. Of these, two also contain several regions with sequences similar to CBDs in their N-terminal regions. Comparison of the amino acid sequences of the five xylanases suggests a possible process for their molecular evolution.

Unravelling cell wall formation in the woody dicot stem

Populus is presented as a model system for the study of wood formation (xylogenesis). The formation of wood (secondary xylem) is an ordered developmental process involving cell division, cell expansion, secondary wall deposition, lignification and programmed cell death. Because wood is formed in a variable environment and subject to developmental control, xylem cells are produced that differ in size, shape, cell wall structure, texture and composition. Hormones mediate some of the variability observed and control the process of xylogenesis. High-resolution analysis of auxin distribution across cambial region tissues, combined with the analysis of transgenic plants with modified auxin distribution, suggests that auxin provides positional information for the exit of cells from the meristem and probably also for the duration of cell expansion. Poplar sequencing projects have provided access to genes involved in cell wall formation. Genes involved in the biosynthesis of the carbohydrate skeleton of the cell wall are briefly reviewed. Most progress has been made in characterizing pectin methyl esterases that modify pectins in the cambial region. Specific expression patterns have also been found for expansins, xyloglucan endotransglycosylases and cellulose synthases, pointing to their role in wood cell wall formation and modification. Finally, by studying transgenic plants modified in various steps of the monolignol biosynthetic pathway and by localizing the expression of various enzymes, new insight into the lignin biosynthesis in planta has been gained.

Isolation of a type 2 metallothionein-like gene preferentially expressed in the tapetum in Zea mays

A Zea mays cDNA, MZm3-4, was isolated by differential screening of a cDNA library obtained from meiotic stage anthers against a cDNA of 3-week-old seedlings. Northern blot analysis of RNA from different maize tissues and from male reproductive organs at various developmental stages demonstrated expression of a single transcript in anthers, from the pollen mother cell stage through the uninucleated microspore stage. In situ hybridization to anther sections resulted in a distinct signal only in the tapetum. The MZm3-4 cDNA is 743 nucleotides in length and has an open reading frame encoding a protein of 75 amino acids. Sequence comparisons with various databases revealed that MZm3-4 exhibits high similarities with type 2 plant metallothioneins at both the nucleotide and the amino-acid level. Primer extension analysis indicated that MZm3-4 cDNA is deleted of 13bp at the 5' end. Southern blot analysis showed that the MZm3-4 gene may be present in one or two copies in a Z. mays inbred line genome. This is the first report of the isolation of a type 2 metallothionein-like protein in maize. Moreover, the expression of this type 2 metallothionein-like gene is high in the male reproductive organs engaged in microsporogenesis.

Chromosome nondisjunction and instabilities in tapetal cells are affected by B chromosomes in maize

Abnormal mitosis occurs in maize tapetum, producing binucleate cells that later disintegrate, following a pattern of programmed cell death. FISH allowed us to observe chromosome nondisjunction and micronucleus formation in binucleate cells, using DNA probes specific to B chromosomes (B's), knobbed chromosomes, and the chromosome 6 (NOR) of maize. All chromosome types seem to be involved in micronucleus formation, but the B's form more micronuclei than do knobbed chromosomes and knobbed chromosomes form more than do chromosomes without knobs. Micronuclei were more frequent in 1B plants and in a genotype selected for low B transmission rate. Nondisjunction was observed in all types of FISH-labeled chromosomes. In addition, unlabeled bridges and delayed chromatids were observed in the last telophase before binucleate cell formation, suggesting that nondisjunction might occur in all chromosomes of the maize complement. B nondisjunction is known to occur in the second pollen mitosis and in the endosperm, but it was not previously reported in other tissues. This is also a new report of nondisjunction of chromosomes of the normal set (A's) in tapetal cells. Our results support the conclusion that nondisjunction and micronucleus formation are regular events in the process of the tapetal cell death program, but B's strongly increase A chromosome instability.