Accumulation of beta-Fructosidase in the Cell Walls of Tomato Roots following Infection by a Fungal Wilt Pathogen

Early leaf responses of cell physiological and sensor-based signatures reflect susceptibility of wheat seedlings to infection by leaf rust

Leaf rust caused by Puccinia triticina Erikss. can have devastating effects on wheat (Triticum aestivum L.), causing severe economic losses. This comprehensive study serves to facilitate our understanding of the impact of carbohydrate and antioxidant metabolism in association with sensor-based phenotyping and leaf rust stress responses in wheat seedlings. After 24 h of inoculation (hai) very susceptible variety to leaf rust (Ficko) increased cell-wall invertase (cwInv; EC 3.2.1.26), compared to other varieties that significantly increased cwInv later. This could mean that the Ficko variety cannot defend itself from leaf rust infections once symptoms have started to develop. Also, Ficko had significantly decreased amounts of cytoplasmic invertase (cytInv; EC 3.2.1.26) at 8 hai. The downregulation of cytInv in susceptible plants may facilitate the maintenance of elevated apoplastic sucrose availability favoring the pathogen. The significant role of vacuolar invertase (vacInv; EC 3.2.1.26) in moderately resistant varieties was recorded. Also, a significant decrease of glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) and UDP-glucose pyrophosphorylase (UGPase; EC 2.7.7.9) in moderately resistant varieties might restrict normal development of leaf rust due to reduced sugar. During plant-pathogen interaction, when the invader spreads systemically throughout the plant, the main role of ascorbate peroxidase (APX; EC 1.11.1.11) activity in one moderately resistant variety (Olimpija) and catalase (CAT; EC 1.11.1.6) activity in another moderately resistant variety (Alka) is to protect the plant against oxidative damage in the early stages of infection. Non-invasive phenotyping with a sensor-based technique could be used as a rapid method for pre-symptomatic determination of wheat leaf rust resistance or susceptibility.

Genome-wide transcriptome reveals mechanisms underlying Rlm1-mediated blackleg resistance on canola

Genetic resistance to blackleg (Leptosphaeria maculans, Lm) of canola (Brassica napus, Bn) has been extensively studied, but the mechanisms underlying the host-pathogen interaction are still not well understood. Here, a comparative transcriptome analysis was performed on a resistant doubled haploid Bn line carrying the resistance gene Rlm1 following inoculation with a virulent (avrLm1) or avirulent (AvrLm1) Lm isolate on cotyledons. A total of 6999 and 3015 differentially expressed genes (DEGs) were identified, respectively, in inoculated local tissues with compatible (susceptible) and incompatible (resistant) interactions. Functional enrichment analysis found several biological processes, including protein targeting to membrane, ribosome and negative regulation of programmed cell death, were over-represented exclusively among up-regulated DEGs in the resistant reaction, whereas significant enrichment of salicylic acid (SA) and jasmonic acid (JA) pathways observed for down-regulated DEGs occurred only in the susceptible reaction. A heat-map analysis showed that both biosynthesis and signaling of SA and JA were induced more significantly in the resistant reaction, implying that a threshold level of SA and JA signaling is required for the activation of Rlm1-mediated resistance. Co-expression network analysis revealed close correlation of a gene module with the resistance, involving DEGs regulating pathogen-associated molecular pattern recognition, JA signaling and transcriptional reprogramming. Substantially fewer DEGs were identified in mock-inoculated (control) cotyledons, relative to those in inoculated local tissues, including those involved in SA pathways potentially contributing to systemic acquired resistance (SAR). Pre-inoculation of cotyledon with either an avirulent or virulent Lm isolate, however, failed to induce SAR on remote tissues of same plant despite elevated SA and PR1 protein. This study provides insights into the molecular mechanism of Rlm1-mediated resistance to blackleg.

Cell Wall Invertase 3 Affects Cassava Productivity via Regulating Sugar Allocation From Source to Sink

Storage roots are the main sink for photo-assimilate accumulation and reflect cassava yield and productivity. Regulation of sugar partitioning from leaves to storage roots has not been elucidated. Cell wall invertases are involved in the hydrolysis of sugar during phloem unloading of vascular plants to control plant development and sink strength but have rarely been studied in root crops like cassava. MeCWINV3 encodes a typical cell wall invertase in cassava and is mainly expressed in vascular bundles. The gene is highly expressed in leaves, especially mature leaves, in response to diurnal rhythm. When MeCWINV3 was overexpressed in cassava, sugar export from leaves to storage roots was largely inhibited and sucrose hydrolysis in leaves was accelerated, leading to increased transient starch accumulation by blocking starch degradation and reduced overall plant growth. The progress of leaf senescence was promoted in the MeCWINV3 over-expressed cassava plants with increased expression of senescence-related genes. Storage root development was also delayed because of dramatically reduced sugar allocation from leaves. As a result, the transcriptional expression of starch biosynthetic genes such as small subunit ADP-glucose pyrophosphorylase, granule-bound starch synthase I, and starch branching enzyme I was reduced in accordance with insufficient sugar supply in the storage roots of the transgenic plants. These results show that MeCWINV3 regulates sugar allocation from source to sink and maintains sugar balance in cassava, thus affecting yield of cassava storage roots.

Genome-Wide Survey of Invertase Encoding Genes and Functional Characterization of an Extracellular Fungal Pathogen-Responsive Invertase in Glycine max

Invertases are essential enzymes that irreversibly catalyze the cleavage of sucrose into glucose and fructose. Cell wall invertase (CWI) and vacuolar invertase (VI) are glycosylated proteins and exert fundamental roles in plant growth as well as in response to environmental cues. As yet, comprehensive insight into invertase encoding genes are lacking in Glycine max. In the present study, the systematic survey of gene structures, coding regions, regulatory elements, conserved motifs, and phylogenies resulted in the identification of thirty⁻two putative invertase genes in soybean genome. Concomitantly, impacts on gene expression, enzyme activities, proteins, and soluble sugar accumulation were explored in specific tissues upon stress perturbation. In combination with the observation of subcellular compartmentation of the fluorescent fusion protein that indeed exported to apoplast, heterologous expression, and purification in using Pichia pastoris system revealed that GmCWI4 was a typical extracellular invertase. We postulated that GmCWI4 may play regulatory roles and be involved in pathogenic fungi defense. The experimental evaluation of physiological significance via phenotypic analysis of mutants under stress exposure has been initiated. Moreover, our paper provides theoretical basis for elucidating molecular mechanisms of invertase in association with inhibitors underlying the stress regime, and will contribute to the improvement of plant performance to a diverse range of stressors.

Cell wall invertase activity regulates the expression of the transfer cell-specific transcription factor ZmMRP-1

Studies in cell wall bound invertase mutants indicate that the promoter of the transfer cell-specific transcription factor, ZmMRP - 1 , is modulated by the carbohydrate balance. Transfer cells are highly specialized plant cells located at the surfaces that need to support an intensive exchange of nutrients, such as the entrance of fruits, seeds and nodules or the young branching points along the stem. ZmMRP-1 is a one-domain MYB-related transcription factor specifically expressed at the transfer cell layer of the maize endosperm. Previous studies demonstrated that this factor regulates the expression of a large number of transfer cell-specific genes, and suggested that ZmMRP-1 is a key regulator of the differentiation of this tissue. The expression of this gene is largely dominated by positional cues, but within the ZmMRP-1 expressing cells the promoter appears to be modulated by sugars. Here we have investigated in vivo this modulation. Using maize and Arabidopsis mutants for cell wall invertase genes, we found that the absence of cell wall invertase activity is a major inductive signal of the ZmMRP-1 expression.

Role of brassinosteroids in alleviating toxin-induced stress of Verticillium dahliae on cotton callus growth

Brassinosteroids are well known to mitigate biotic stresses; however, their role to induce tolerance against Verticillium dahliae is unknown. The current study employed V. dahliae (Vd) toxin as pathogen-free model system to induce stress on cotton callus growth, and its amelioration was investigated using 24-epibrassinolide (EBR). Results revealed that EBR has ameliorative effects against Vd toxin with greater seen effect when callus was treated with EBR prior to its exposure to Vd toxin (pre-EBR treatment) than EBR applied along with Vd toxin simultaneously (co-EBR treatment). Pre-EBR-treated calli remained green, while 65 and 90% callus browning was observed in co-EBR- and Vd toxin-alone-treated callus, respectively. Likewise, the fresh weight of the pre-EBR-treated callus was 52% higher than Vd toxin-alone treatment, whereas this increase was only 23% in co-EBR-treated callus. Meanwhile, EBR treatment of the cotton callus has also increased the contents of chlorophylls a and b, carotenoids, total phenols, flavonoids, soluble sugars, and proteins and increased the activity of enzymes involved in secondary metabolism like polyphenol oxidase (PPO), phenylalanine ammonialyase (PAL), cinnamyl alchol dehydrogenase (CAD), and shikimate dehydrogenase (SKDH) over Vd toxin-alone treatment with higher increments being observed in pre-EBR-treated callus. Furthermore, EBR treatment mimicked the DNA damage and improved the structure of mitochondria, granum, stroma thylakoids, and the attachment of ribosomes with the endoplasmic reticulum. This EBR-mediated mitigation was primarily associated with substantially increased contents of photosynthetic pigments and regulation of secondary metabolism.

Genome-Wide Identification of the Invertase Gene Family in Populus

Invertase plays a crucial role in carbohydrate partitioning and plant development as it catalyses the irreversible hydrolysis of sucrose into glucose and fructose. The invertase family in plants is composed of two sub-families: acid invertases, which are targeted to the cell wall and vacuole; and neutral/alkaline invertases, which function in the cytosol. In this study, 5 cell wall invertase genes (PtCWINV1-5), 3 vacuolar invertase genes (PtVINV1-3) and 16 neutral/alkaline invertase genes (PtNINV1-16) were identified in the Populus genome and found to be distributed on 14 chromosomes. A comprehensive analysis of poplar invertase genes was performed, including structures, chromosome location, phylogeny, evolutionary pattern and expression profiles. Phylogenetic analysis indicated that the two sub-families were both divided into two clades. Segmental duplication is contributed to neutral/alkaline sub-family expansion. Furthermore, the Populus invertase genes displayed differential expression in roots, stems, leaves, leaf buds and in response to salt/cold stress and pathogen infection. In addition, the analysis of enzyme activity and sugar content revealed that invertase genes play key roles in the sucrose metabolism of various tissues and organs in poplar. This work lays the foundation for future functional analysis of the invertase genes in Populus and other woody perennials.

Pathogen associated molecular pattern (PAMP)-triggered immunity is compromised under C-limited growth

In the interaction between plants and pathogens, carbon (C) resources provide energy and C skeletons to maintain, among many functions, the plant immune system. However, variations in C availability on pathogen associated molecular pattern (PAMP) triggered immunity (PTI) have not been systematically examined. Here, three types of starch mutants with enhanced susceptibility to Pseudomonas syringae pv. tomato DC3000 hrcC were examined for PTI. In a dark period-dependent manner, the mutants showed compromised induction of a PTI marker, and callose accumulation in response to the bacterial PAMP flagellin, flg22. In combination with weakened PTI responses in wild type by inhibition of the TCA cycle, the experiments determined the necessity of C-derived energy in establishing PTI. Global gene expression analyses identified flg22 responsive genes displaying C supply-dependent patterns. Nutrient recycling-related genes were regulated similarly by C-limitation and flg22, indicating re-arrangements of expression programs to redirect resources that establish or strengthen PTI. Ethylene and NAC transcription factors appear to play roles in these processes. Under C-limitation, PTI appears compromised based on suppression of genes required for continued biosynthetic capacity and defenses through flg22. Our results provide a foundation for the intuitive perception of the interplay between plant nutrition status and pathogen defense.

Cell-wall invertases, key enzymes in the modulation of plant metabolism during defence responses

Most plant-pathogen interactions do not result in pathogenesis because of pre-formed defensive plant barriers or pathogen-triggered activation of effective plant immune responses. The mounting of defence reactions is accompanied by a profound modulation of plant metabolism. Common metabolic changes are the repression of photosynthesis, the increase in heterotrophic metabolism and the synthesis of secondary metabolites. This enhanced metabolic activity is accompanied by the reduced export of sucrose or enhanced import of hexoses at the site of infection, which is mediated by an induced activity of cell-wall invertase (Cw-Inv). Cw-Inv cleaves sucrose, the major transport sugar in plants, irreversibly yielding glucose and fructose, which can be taken up by plant cells via hexose transporters. These hexose sugars not only function in metabolism, but also act as signalling molecules. The picture of Cw-Inv regulation in plant-pathogen interactions has recently been broadened and is discussed in this review. An interesting emerging feature is the link between Cw-Inv and the circadian clock and new modes of Cw-Inv regulation at the post-translational level.

Sucrose and invertases, a part of the plant defense response to the biotic stresses

Sucrose is the main form of assimilated carbon which is produced during photosynthesis and then transported from source to sink tissues via the phloem. This disaccharide is known to have important roles as signaling molecule and it is involved in many metabolic processes in plants. Essential for plant growth and development, sucrose is engaged in plant defense by activating plant immune responses against pathogens. During infection, pathogens reallocate the plant sugars for their own needs forcing the plants to modify their sugar content and triggering their defense responses. Among enzymes that hydrolyze sucrose and alter carbohydrate partitioning, invertases have been reported to be affected during plant-pathogen interactions. Recent highlights on the role of invertases in the establishment of plant defense responses suggest a more complex regulation of sugar signaling in plant-pathogen interaction.

Altered invertase activities of symptomatic tissues on Beet severe curly top virus (BSCTV) infected Arabidopsis thaliana

Arabidopsis thaliana infected with Beet severe curly top virus (BSCTV) exhibits systemic symptoms such as stunting of plant growth, callus induction on shoot tips, and curling of leaves and shoot tips. The regulation of sucrose metabolism is essential for obtaining the energy required for viral replication and the development of symptoms in BSCTV-infected A. thaliana. We evaluated the changed transcript level and enzyme activity of invertases in the inflorescence stems of BSCTV-infected A. thaliana. These results were consistent with the increased pattern of ribulose-1,5-bisphosphate carboxylase/oxygenase activity and photosynthetic pigment concentration in virus-infected plants to supply more energy for BSCTV multiplication. The altered gene expression of invertases during symptom development was functionally correlated with the differential expression patterns of D-type cyclins, E2F isoforms, and invertase-related genes. Taken together, our results indicate that sucrose sensing by BSCTV infection may regulate the expression of sucrose metabolism and result in the subsequent development of viral symptoms in relation with activation of cell cycle regulation.

Understanding the role of defective invertases in plants: tobacco Nin88 fails to degrade sucrose

Cell wall invertases (cwINVs), with a high affinity for the cell wall, are fundamental enzymes in the control of plant growth, development, and carbon partitioning. Most interestingly, defective cwINVs have been described in several plant species. Their highly attenuated sucrose (Suc)-hydrolyzing capacity is due to the absence of aspartate-239 (Asp-239) and tryptophan-47 (Trp-47) homologs, crucial players for stable binding in the active site and subsequent hydrolysis. However, so far, the precise roles of such defective cwINVs remain unclear. In this paper, we report on the functional characterization of tobacco (Nicotiana tabacum) Nin88, a presumed fully active cwINV playing a crucial role during pollen development. It is demonstrated here that Nin88, lacking both Asp-239 and Trp-47 homologs, has no invertase activity. This was further supported by modeling studies and site-directed mutagenesis experiments, introducing both Asp-239 and Trp-47 homologs, leading to an enzyme with a distinct Suc-hydrolyzing capacity. In vitro experiments suggest that the addition of Nin88 counteracts the unproductive and rather aspecific binding of tobacco cwINV1 to the wall, leading to higher activities in the presence of Suc and a more efficient interaction with its cell wall inhibitor. A working model is presented based on these findings, allowing speculation on the putative role of Nin88 in muro. The results presented in this work are an important first step toward unraveling the specific roles of plant defective cwINVs.

Purification and biochemical characterization of insoluble acid invertase (INAC-INV) from pea seedlings

Invertase (EC 3.2.1.26) catalyzes the hydrolysis of sucrose into D-glucose and D-fructose. Insoluble acid invertase (INAC-INV) was purified from pea (Pisum sativum L.) by sequential procedures entailing ammonium sulfate precipitation, ion exchange chromatography, absorption chromatography, reactive green-19 affinity chromatography, and gel filtration. The purified INAC-INV had a pH optimum of 4.0 and a temperature optimum of 45 °C. The effects of various concentrations of Tris-HCl, HgCl(2), and CuSO(4) on the activities of the purified invertase were examined. INAC-INV was not affected by Tris-HCl and HgCl(2). INAC-INV activity was inhibited by 6.2 mM CuSO(4) up to 50%. The enzymes display typical hyperbolic saturation kinetics for sucrose hydrolysis. The K(m) and V(max) values of INAC-INV were determined to be 4.41 mM and 8.41 U (mg protein)(-1) min(-1), respectively. INAC-INV is a true member of the β-fructofuranosidases, which can react with sucrose and raffinose as substrates. SDS-PAGE and immunoblotting were used to determine the molecular mass of INAC-INV to be 69 kDa. The isoelectric point of INAC-INV was estimated to be about pH 8.0. Taken together, INAC-INV is a pea seedling invertase with a stable and optimum activity at lower acid pH and at higher temperature than other invertases.

Changes in carbohydrate metabolism in Plasmopara viticola-infected grapevine leaves

The oomycete Plasmopara viticola is responsible for downy mildew, a severe grapevine disease. In infected grapevine leaves, we have observed an abnormal starch accumulation at the end of the dark period, suggesting modifications in starch metabolism. Therefore, several complementary approaches, including transcriptomic analyses, measurements of enzyme activities, and sugar quantification, were performed in order to investigate and to understand the effects of P. viticola infection on leaf starch and-to a larger extent-carbohydrate metabolism. Our results indicate that starch accumulation is associated with an increase in ADP-glucose pyrophosphorylase (AGPase) activity and modifications in the starch degradation pathway, especially an increased α-amylase activity. Together with these alterations in starch metabolism, we have observed an accumulation of hexoses, an increase in invertase activity, and a reduction of photosynthesis, indicating a source-to-sink transition in infected leaf tissue. Additionally, we have measured an accumulation of the disaccharide trehalose correlated to an increased trehalase gene expression and enzyme activity. Altogether, these results highlight a dramatic alteration of carbohydrate metabolism correlated with later stages of P. viticola development in leaves.

Post-translational derepression of invertase activity in source leaves via down-regulation of invertase inhibitor expression is part of the plant defense response

There is increasing evidence that pathogens do not only elicit direct defense responses, but also cause pronounced changes in primary carbohydrate metabolism. Cell-wall-bound invertases belong to the key regulators of carbohydrate partitioning and source-sink relations. Whereas studies have focused so far only on the transcriptional induction of invertase genes in response to pathogen infection, the role of post-translational regulation of invertase activity has been neglected and was the focus of the present study. Expression analyses revealed that the high mRNA level of one out of three proteinaceous invertase inhibitors in source leaves of Arabidopsis thaliana is strongly repressed upon infection by a virulent strain of Pseudomonas syringae pv. tomato DC3000. This repression is paralleled by a decrease in invertase inhibitor activity. The physiological role of this regulatory mechanism is revealed by the finding that in situ invertase activity was detectable only upon infection by P. syringae. In contrast, a high invertase activity could be measured in vitro in crude and cell wall extracts prepared from both infected and non-infected leaves. The discrepancy between the in situ and in vitro invertase activity of control leaves and the high in situ invertase activity in infected leaves can be explained by the pathogen-dependent repression of invertase inhibitor expression and a concomitant reduction in invertase inhibitor activity. The functional importance of the release of invertase from post-translational inhibition for the defense response was substantiated by the application of the competitive chemical invertase inhibitor acarbose. Post-translational inhibition of extracellular invertase activity by infiltration of acarbose in leaves was shown to increase the susceptibility to P. syringae. The impact of invertase inhibition on spatial and temporal dynamics of the repression of photosynthesis and promotion of bacterial growth during pathogen infection supports a role for extracellular invertase in plant defense. The acarbose-mediated increase in susceptibility was also detectable in sid2 and cpr6 mutants and resulted in slightly elevated levels of salicylic acid, demonstrating that the effect is independent of the salicylic acid-regulated defense pathway. These findings provide an explanation for high extractable invertase activity found in source leaves that is kept inhibited in situ by post-translational interaction between invertase and the invertase inhibitor proteins. Upon pathogen infection, the invertase activity is released by repression of invertase inhibitor expression, thus linking the local induction of sink strength to the plant defense response.

General and species-specific transcriptional responses to downy mildew infection in a susceptible (Vitis vinifera) and a resistant (V. riparia) grapevine species

Background

Downy mildew is a destructive grapevine disease caused by Plasmopara viticola (Berk. and Curt.) Berl. and de Toni, which can only be controlled by intensive fungicide treatments. Natural sources of resistance from wild grapevine (Vitis) species are used in conventional breeding approaches, but the signals and effectors involved in resistance in this important crop species are not well understood.

Results

Early transcriptional changes associated with P. viticola infection in susceptible V. vinifera and resistant V. riparia plants were analyzed using the Combimatrix microarray platform. Transcript levels were measured 12 and 24 h post-inoculation, reflecting the time points immediately preceding the onset of resistance in V. riparia, as determined by microscopic analysis. Our data indicate that resistance in V. riparia is induced after infection, and is not based on differences in basal gene expression between the two species. The strong and rapid transcriptional reprogramming involves the induction of pathogenesis-related proteins and enzymes required for the synthesis of phenylpropanoid-derived compounds, many of which are also induced, albeit to a lesser extent, in V. vinifera. More interestingly, resistance in V. riparia also involves the specific modulation of numerous transcripts encoding components of signal transduction cascades, hypersensitive reaction markers and genes involved in jasmonate biosynthesis. The limited transcriptional modulation in V. vinifera represents a weak attempted defense response rather than the activation of compatibility-specific pathways.

Conclusions

Several candidate resistance genes were identified that could be exploited in future biotechnological approaches to increase disease resistance in susceptible grapevine species. Measurements of jasmonic acid and methyl jasmonate in infected leaves suggest that this hormone may also be involved in V. riparia resistance to P. viticola.

Structural insights into glycoside hydrolase family 32 and 68 enzymes: functional implications

Glycoside hydrolases (GH) have been shown to play unique roles in various biological processes like the biosynthesis of glycans, cell wall metabolism, plant defence, signalling, and the mobilization of storage reserves. To date, GH are divided into more than 100 families based upon their overall structure. GH32 and GH68 are combined in clan GH-J, not only harbouring typical hydrolases but also non-Leloir type transferases (fructosyltransferases), involved in fructan biosynthesis. This review summarizes the recent structure-function research progress on plant GH32 enzymes, and highlights the similarities and differences compared with the microbial GH32 and GH68 enzymes. A profound analysis of ligand-bound structures and site-directed mutagenesis experiments identified key residues in substrate (or inhibitor) binding and recognition. In particular, sucrose can bind as inhibitor in Cichorium intybus 1-FEH IIa, whereas it binds as substrate in Bacillus subtilis levansucrase and Arabidopsis thaliana cell wall invertase (AtcwINV1). In plant GH32, a single residue, the equivalent of Asp239 in AtcwINV1, appears to be important for sucrose stabilization in the active site and essential in determining sucrose donor specificity.

Metabolic regulation of leaf senescence: interactions of sugar signalling with biotic and abiotic stress responses

Sugars are important signals in the regulation of plant metabolism and development. During stress and in senescing leaves, sugars often accumulate. In addition, both sugar accumulation and stress can induce leaf senescence. Infection by bacterial and fungal pathogens and attack by herbivores and gall-forming insects may influence leaf senescence via modulation of the sugar status, either by directly affecting primary carbon metabolism or by regulating steady state levels of plant hormones. Many types of biotic interactions involve the induction of extracellular invertase as the key enzyme of an apoplasmic phloem unloading pathway, resulting in a source-sink transition and an increased hexose/sucrose ratio. Induction of the levels of the phytohormones ethylene and jasmonate in biotic interactions results in accelerated senescence, whereas an increase in plant- or pathogen-derived cytokinins delays senescence and results in the formation of green islands within senescing leaves. Interactions between sugar and hormone signalling also play a role in response to abiotic stress. For example, interactions between sugar and abscisic acid (ABA) signalling may be responsible for the induction of senescence during drought stress. Cold treatment, on the other hand, can result in delayed senescence, despite sugar and ABA accumulation. Moreover, natural variation can be found in senescence regulation by sugars and in response to stress: in response to drought stress, both drought escape and dehydration avoidance strategies have been described in different Arabidopsis accessions. The regulation of senescence by sugars may be key to these different strategies in response to stress.

Jasmonic acid influences mycorrhizal colonization in tomato plants by modifying the expression of genes involved in carbohydrate partitioning

The role of jasmonic acid (JA) on mycorrhizal colonization by Glomus fasciculatum in tomato plants was examined using mutant plants overexpressing prosystemin (PS) or affected in the synthesis of JA (suppressor of prosystemin-mediated responses 2, spr2). The degree of mycorrhizal colonization was determined by measuring frequency (F%) and intensity (M%) of colonization and arbuscule abundance (A%). Gene expression and biochemical analyses were also performed in roots to detect changes in carbon (C) partitioning. Colonization was similar in mycorrhizal PS and wild-type roots, except for a higher A% in the former. Conversely, colonization was severely reduced in roots of spr2 mutants. No association was found between levels of expression of genes coding for systemic wound responsive proteins (or SWRPs) and other defense-related proteins in roots and mycorrhization levels in these plants. On the other hand, the degree of mycorrhizal colonization correlated with changes in the transcriptional regulation of a number of genes involved in sucrose hydrolysis and transport, cell wall invertase activity and mycorrhizal-specific fatty acid content in roots. The results obtained suggest that one of the mechanisms by which JA might operate to modulate the mycorrhization process could be through its influence on the regulation of C partitioning in the plant. The significant colonization increase observed in mycorrhizal spr2 plants supplied with exogenous methyl jasmonate supports its role as a positive regulator of the symbiosis.

Unraveling the difference between invertases and fructan exohydrolases: a single amino acid (Asp-239) substitution transforms Arabidopsis cell wall invertase1 into a fructan 1-exohydrolase

Plant cell wall invertases and fructan exohydrolases (FEHs) are very closely related enzymes at the molecular and structural level (family 32 of glycoside hydrolases), but they are functionally different and are believed to fulfill distinct roles in plants. Invertases preferentially hydrolyze the glucose (Glc)-fructose (Fru) linkage in sucrose (Suc), whereas plant FEHs have no invertase activity and only split terminal Fru-Fru linkages in fructans. Recently, the three-dimensional structures of Arabidopsis (Arabidopsis thaliana) cell wall Invertase1 (AtcwINV1) and chicory (Cichorium intybus) 1-FEH IIa were resolved. Until now, it remained unknown which amino acid residues determine whether Suc or fructan is used as a donor substrate in the hydrolysis reaction of the glycosidic bond. In this article, we present site-directed mutagenesis-based data on AtcwINV1 showing that the aspartate (Asp)-239 residue fulfills an important role in both binding and hydrolysis of Suc. Moreover, it was found that the presence of a hydrophobic zone at the rim of the active site is important for optimal and stable binding of Suc. Surprisingly, a D239A mutant acted as a 1-FEH, preferentially degrading 1-kestose, indicating that plant FEHs lacking invertase activity could have evolved from a cell wall invertase-type ancestor by a few mutational changes. In general, family 32 and 68 enzymes containing an Asp-239 functional homolog have Suc as a preferential substrate, whereas enzymes lacking this homolog use fructans as a donor substrate. The presence or absence of such an Asp-239 homolog is proposed as a reliable determinant to discriminate between real invertases and defective invertases/FEHs.

Metabolic processes and carbon nutrient exchanges between host and pathogen sustain the disease development during sunflower infection by Sclerotinia sclerotiorum

Interactions between the necrotrophic fungus Sclerotinia sclerotiorum and one of its hosts, Helianthus annuus L., were analyzed during fungal colonization of plant tissues. Metabolomic analysis, based on (13)C- and (31)P-NMR spectroscopy, was used to draw up the profiles of soluble metabolites of the two partners before interaction, and to trace the fate of metabolites specific of each partner during colonization. In sunflower cotyledons, the main soluble carbohydrates were glucose, fructose, sucrose and glutamate. In S. sclerotiorum extracts, glucose, trehalose and mannitol were the predominant soluble carbon stores. During infection, a decline in sugars and amino acids was observed in the plant and fungus total content. Sucrose and fructose, initially present almost exclusively in plant, were reduced by 85%. We used a biochemical approach to correlate the disappearance of sucrose with the expression and the activity of fungal invertase. The expression of two hexose transporters, Sshxt1 and Sshxt2, was enhanced during infection. A database search for hexose transporters homologues in the S. sclerotiorum genome revealed a multigenic sugar transport system. Furthermore, the composition of the pool of reserve sugars and polyols during infection was investigated. Whereas mannitol was produced in vitro and accumulated in planta, glycerol was exclusively produced in infected tissues and increased during colonization. The hypothesis that the induction of glycerol synthesis in S. sclerotiorum exerts a positive effect on osmotic protection of fungal cells and favors fungal growth in plant tissues is discussed. Taken together, our data revealed the importance of carbon-nutrient exchanges during the necrotrophic pathogenesis of S. sclerotiorum.

Regulation of invertase: a 'suite' of transcriptional and post-transcriptional mechanisms

Recent evidence indicates that several mechanisms can alter invertase activity and, thus, affect sucrose metabolism and resource allocation in plants. One of these mechanisms is the compartmentalisation of at least some vacuolar invertases in precursor protease vesicles (PPV), where their retention could control timing of delivery to vacuoles and hence activity. PPV are small, ER-derived bodies that sequester a subset of vacuolar-bound proteins (such as invertases and protease precursors) releasing them to acid vacuoles in response to developmental or environmental signals. Another newly-identified effector of invertases is wall-associated kinase 2 (WAK2), which can regulate a specific vacuolar invertase in Arabidopsis (AtvacINV1) and alter root growth when osmolyte supplies are limiting. WAKs are ideally positioned to sense changes in the interface between the cell wall and plasma membrane (such as turgor), because the N-terminus of each WAK extends into the cell wall matrix (where a pectin association is hypothesised) and the C-terminus has a cytoplasmic serine/threonine kinase domain (signalling). Still other avenues of invertase control are provided by a diverse group of kinases and phosphatases, consistent with input from multiple sensing systems for sugars, pathogens, ABA and other hormones. Mechanisms of regulation may also vary for the contrasting sugar responses of different acid invertase transcripts. Some degree of hexokinase involvement and distinctive kinetics have been observed for the sugar-repressed invertases, but not for the more common, sugar-induced forms examined thus far. An additional means of regulation for invertase gene expression lies in the multiple DST (Down STream) elements of the 3' untranslated region for the most rapidly repressed invertases. Similar sequences were initially identified in small auxin-up RNAs (SAUR) where they mediate rapid mRNA turnover. Finally, the invertase inhibitors, cell wall- and vacuolar inhibitors of fructosidase (CIF and VIF, respectively) are indistinguishable by sequence alone from pectin methylesterase inhibitors (PMEI); however, recent evidence suggests binding specificity may be determined by flexibility of a short, N-terminal region. These recently characterised processes increase the suite of regulatory mechanisms by which invertase - and, thus, sucrose metabolism and resource partitioning - can be altered in plants.

Metabolic consequences of susceptibility and resistance (race-specific and broad-spectrum) in barley leaves challenged with powdery mildew

In a compatible interaction biotrophic fungi often lower the yield of their hosts by reducing photosynthesis and altering the fluxes of carbon within the infected leaf. In contrast, comparatively little is known about the metabolic consequences of activating resistance responses. In this study we investigated the hypothesis that the activation of both race-specific (Mla12) and broad-spectrum (mlo) resistance pathways in barley leaves infected with Blumeria graminis represents a cost to the plant in terms of carbon production and utilization. We have shown, using quantitative imaging of chlorophyll fluorescence, that during a susceptible interaction, photosynthesis was progressively reduced both in cells directly below fungal colonies and in adjacent cells when compared with uninoculated leaves. The lower rate of photosynthesis was associated with an increase in invertase activity, an accumulation of hexoses and a down-regulation of photosynthetic gene expression. During both Mla12- and mlo-mediated resistance, photosynthesis was also reduced, most severely inhibited in cells directly associated with attempted penetration of the fungus but also in surrounding cells. These cells displayed intense autofluorescence under ultraviolet illumination indicative of the accumulation of phenolic compounds and/or callose deposition. The depression in photosynthesis was not due only to cell death but also to an alteration in source-sink relations and carbon utilization. Apoplastic (cell wall-bound) invertase activity increased more rapidly and to a much greater extent than in infected susceptible leaves and was accompanied by an accumulation of hexoses that was localized to areas of the leaf actively exhibiting resistance responses. The accumulation of hexoses was accompanied by a down-regulation in the expression of Rubisco (rbcS) and chlorophyll a/b binding protein (cab) genes (although to a lesser extent than in a compatible interaction) and with an up-regulation in the expression of the pathogenesis-related protein 1 (PR-1). These results are consistent with a role for invertase in the generation of hexoses, which may supply energy for defence reactions and/or act as signals inducing defence gene expression.

Cloning and characterization of a novel invertase from the obligate biotroph Uromyces fabae and analysis of expression patterns of host and pathogen invertases in the course of infection

Invertases are key enzymes in carbon partitioning in higher plants. They gain additional importance in the distribution of carbohydrates in the event of wounding or pathogen attack. Although many researchers have found an increase in invertase activity upon infection, only a few studies were able to determine whether the source of this activity was host or parasite. This article analyzes the role of invertases involved in the biotrophic interaction of the rust fungus Uromyces fabae and its host plant, Vicia faba. We have identified a fungal gene, Uf-INV1, with homology to invertases and assessed its contribution to pathogenesis. Expression analysis indicated that transcription began upon penetration of the fungus into the leaf, with high expression levels in haustoria. Heterologous expression of Uf-INV1 in Saccharomyces cerevisiae and Pichia pastoris allowed a biochemical characterization of the enzymatic activity associated with the secreted gene product INV1p. Expression analysis of the known vacuolar and cell-wall-bound invertase isoforms of V. faba indicated a decrease in the expression of a vacuolar invertase, whereas one cell-wall-associated invertase exhibited increased expression. These changes were not confined to the infected tissue, and effects also were observed in remote plant organs, such as roots. These findings hint at systemic effects of pathogen infection. Our results support the hypothesis that pathogen infection establishes new sinks which compete with physiological sink organs.

Crystallization and preliminary X-ray diffraction study of a cell-wall invertase from Arabidopsis thaliana

Cell-wall invertase 1 (AtcwINV1), a plant protein from Arabidopsis thaliana which is involved in the breakdown of sucrose, has been crystallized in two different crystal forms. Crystal form I grows in space group P3(1) or P3(2), whereas crystal form II grows in space group C222(1). Data sets were collected for crystal forms I and II to resolution limits of 2.40 and 2.15 A, respectively.

Molecular cloning and expression analysis of the cell-wall invertase gene family in rice (Oryza sativa L.)

Cell-wall invertase (CIN) catalyzes the hydrolysis of sucrose into glucose and fructose for the supply of carbohydrates to sink organs via an apoplastic pathway. To study the CIN genes in rice (Oryza sativa L.), we isolated cDNA clones showing amino acid similarity to the plant cell wall invertase proteins from a search of rice sequence databases. Profile analyses revealed that the cloned genes are expressed in unique patterns in various organs. For example, transcripts of OsCIN1, OsCIN2, OsCIN4, and OsCIN7 were detected in immature seeds whereas OsCIN3 gene expression was flower-specific. Further transcript analysis of these genes expressed in developing seeds indicated that OsCIN1, OsCIN2, and OsCIN7 might play an important role involving sucrose partitioning to the embryo and endosperm. Sucrose, a substrate of CINs, induced the accumulation of OsCIN1 transcripts in excised leaves and OsCIN2 in immature seeds, while the level of OsCIN5 was significantly down-regulated in excised leaves treated with sucrose. Infecting the tissues with rice blast (Magnaporthe grisea) as a biotic stressor increased the expression of OsCIN1, OsCIN4, and OsCIN5, suggesting that these genes may participate in a switch in metabolism to resist pathogen invasion. These results demonstrate that OsCIN genes play diverse roles involving the regulation of metabolism, growth, development, and stress responses.

Calystegines in Calystegia sepium do not inhibit fungal growth and invertase activity but interact with plant invertase

Calystegines are alkaloidal glycosidase inhibitors. They accumulate predominantly in young and meristemic parts of Calystegia sepium (Convolvulaceae). C. sepium, bindweed, infests meadows and cereal fields and is difficult to control chemically. Fungal pathogens against C. sepium are established as mycoherbicides. Stagonospora convolvuli LA39 attacks C. sepium and does not affect crop plants, but young plants of C. sepium are less susceptible to the fungus. The interaction of Stagonospora convolvuli with calystegines was investigated. Further, endophytic fungi of several classes were isolated from wild-grown Calystegia sepium leaves, and selected strains were tested for interaction with calystegines. Fungal growth on agar containing calystegines was not affected considerably. Plants in climate chambers were infected with an endophyte, Phomopsis, and with the fungal pathogen, Stagonospora convolvuli. Calystegine levels were measured in infected and non-infected plant tissues. Accumulation depended on developmental stage of the plant tissue and was not influenced by infection. Acid invertase was measured from fungal mycelia and from infected and non-infected plant tissues. Fungal acid invertase activity was not inhibited by 10 mM calystegine B (2), while invertase from C. sepium leaves was inhibited. It is concluded that calystegines do not inhibit fungal development and sucrose consumption under the conditions of the present investigation, but may act by redirection of plant carbohydrate metabolism.

Unexpected presence of fructan 6-exohydrolases (6-FEHs) in non-fructan plants: characterization, cloning, mass mapping and functional analysis of a novel "cell-wall invertase-like" specific 6-FEH from sugar beet (Beta vulgaris L.)

About 15% of flowering plant species synthesize fructans. Fructans serve mainly as reserve carbohydrates and are subject to breakdown by plant fructan exohydrolases (FEHs), among which 1-FEHs (inulinases) and 6-FEHs (levanases) can be differentiated. This paper describes the unexpected finding that 6-FEHs also occur in plants that do not synthesize fructans. The purification, characterization, cloning and functional analysis of sugar beet (Beta vulgaris L.) 6-FEH are described. Enzyme activity measurements during sugar beet development suggest a constitutive expression of the gene in sugar beet roots. Classical enzyme purification followed by in-gel trypsin digestion and mass spectrometry (quadruple-time-of-flight mass spectrometry (Q-TOF) MS) led to peptide sequence information used in subsequent RT-PCR based cloning. Levan-type fructans (beta-2,6) are the best substrates for the enzyme, while inulin-type fructans (beta-2,1) and sucrose are poorly or not degraded. Sugar beet 6-FEH is more related to cell wall invertases than to vacuolar invertases and has a low iso-electric point (pI), clearly different from typical high pI cell wall invertases. Poor sequence homology to bacterial or fungal FEHs makes an endophytic origin highly unlikely. The functionality of the 6-FEH cDNA was further demonstrated by heterologous expression in Pichia pastoris. As fructans are absent in sugar beet, the role of 6-FEH in planta is not obvious. Like chitinases and beta-glucanases hydrolysing cell-surface components of fungal plant pathogens, a straightforward working hypothesis for further research might be that plant 6-FEHs participate in hydrolysis (or prevent the formation) of levan-containing slime surrounding endophytic or phytopathogenic bacteria.

Extracellular invertase: key metabolic enzyme and PR protein

Extracellular invertase is the key enzyme of an apoplasmic phloem unloading pathway and catalyses the hydrolytic cleavage of the transport sugar sucrose released into the apoplast. This mechanism contributes to long-distance assimilate transport, provides the substrate to sustain heterotrophic growth and generates metabolic signals known to effect various processes of primary metabolism and defence responses. The essential function of extracellular invertase for supplying carbohydrates to sink organs was demonstrated by the finding that antisense repression of an anther-specific isoenzyme provides an efficient method for metabolic engineering of male sterility. The regulation of extracellular invertase by all classes of phytohormones indicates an essential link between the molecular mechanism of phytohormone action and primary metabolism. The up-regulation of extracellular invertase appears to be a common response to various biotic and abiotic stress-related stimuli such as pathogen infection and salt stress, in addition to specific stress-related reactions. Based on the observed co-ordinated regulation of source/sink relations and defence responses by sugars and stress-related stimuli, the identified activation of distinct subsets of MAP kinases provides a mechanism for signal integration and distribution within such complex networks. Sucrose derivatives not synthesized by higher plants, such as turanose, were shown to elicit responses distinctly different from metabolizable sugars and are rather perceived as stress-related stimuli.

Transcripts for genes encoding soluble acid invertase and sucrose synthase accumulate in root tip and cortical cells containing mycorrhizal arbuscules

Arbuscule formation by the arbuscular mycorrhizal fungus Glomus intraradices (Schenck & Smith) was limited to cortical cells immediately adjacent to the endodermis. Because these cortical cells are the first to intercept photosynthate exiting the vascular cylinder, transcript levels for sucrose metabolizing-enzymes were compared between mycorrhizal and non-mycorrhizal roots. The probes corresponded to genes encoding a soluble acid invertase with potential vacuolar targeting, which we generated from Phaseolus vulgaris roots, a Rhizobium-responsive sucrose synthase of soybean and a cell wall acid invertase of carrot. Transcripts in non-mycorrhizal roots were developmentally regulated and abundant in the root tips for all three probes but in differentiated roots of P. vulgaris they were predominantly located in phloem tissues for sucrose synthase or the endodermis and phloem for soluble acid invertase. In mycorrhizal roots increased accumulations of transcripts for sucrose synthase and vacuolar invertase were both observed in the same cortical cells bearing arbuscules that fluoresce. There was no effect on the expression of the cell wall invertase gene in fluorescent carrot cells containing arbuscules. Thus, it appears that presence of the fungal hyphae in the fluorescent arbusculated cell stimulates discrete alterations in expression of sucrose metabolizing enzymes to increase the sink potential of the cell.

Infection of Arabidopsis thaliana leaves with Albugo candida (white blister rust) causes a reprogramming of host metabolism

Abstract Albugo candida (Pers.) (O.) Kunze is a biotrophic pathogen which infects the crucifer Arabidopsis thaliana (L.) Heynh forming discrete areas of infection. Eight days after inoculation of leaves, white blisters became visible on the under surface of the leaf although no symptoms were apparent on the upper surface. By day 14, the region of leaf invaded by fungal mycelium had become chlorotic. Recently it has been hypothesized that an accumulation of soluble carbohydrates, following an increase in invertase activity, may trigger sugar signal transduction pathways leading to the repression of photosynthetic gene expression and to the induction of defence proteins. This hypothesis was investigated by quantifying localized changes in carbohydrate and photosynthetic metabolism and the expression of genes encoding photosynthetic and defence proteins. Quantitative imaging of chlorophyll fluorescence revealed that the rate of photosynthesis declined progressively in the invaded regions of the leaf. However, in uninfected regions of the infected leaf the rate of photosynthesis was similar to that measured in the control leaf until late on during the infection cycle when it declined. Images of nonphotochemical fluorescence quenching (NPQ) suggested that the capacity of the Calvin cycle had been reduced in infected regions and that there was a complex metabolic heterogeneity within the infected leaf. A. candida also caused localized changes in the carbohydrate metabolism of the leaf; soluble carbohydrates accumulated in the infected region whereas the amount of starch declined. The reverse was seen in uninfected regions of the infected leaf; carbohydrates did not accumulate until late on during infection and the amount of starch increased as the infection progressed. There was an increase in the activity of invertases which was confined to regions of the leaf invaded by the fungal mycelium. The increase in apoplastic invertase activity was of host origin, as mRNA levels of the ATbetaFRUCT1 gene (measured by semiquantitative RT-PCR) increased 40-fold in the infected region. The increase in soluble invertase activity resulted from the appearance of a new isoform in the invaded region of the leaf. Current evidence suggests that this was of fungal origin. Northern blot analysis of cab and rbcS showed that photosynthetic gene expression was repressed in the infected leaf from 6 days after inoculation (DAI) when compared to control leaves. In contrast, there was no detectable induction of defence proteins in the infected leaf. These data are discussed in the context of the sugar-sensing hypothesis presented above.

The sucrose-cleaving enzymes of plants are crucial for development, growth and carbon partitioning

Sink organs of most plant species are supplied with carbon and energy in the form of sucrose. The channeling of sucrose into sink metabolism requires its cleavage by several isoforms of invertase and sucrose synthase, which are localized in different subcellular compartments. These activities regulate the entry of sucrose into distinct biochemical pathways, such as respiration or biosynthesis of cell wall polysaccharides and storage reserves. Other vital roles for the sucrose-cleaving enzymes include invertase activity at the site of phloem unloading and vacuolar invertase and sucrose synthase in sink organs, which drives the long-distance transport of sucrose. In addition, invertases have been implicated in the defense response and in turgor-driven cell expansion, and sucrose synthase expression is associated with low temperature and anaerobiosis responses. Finally, because sugars also regulate gene expression, the sucrose-cleaving enzymes play a fundamental role in controlling cell differentiation and development.

Antisense repression of vacuolar and cell wall invertase in transgenic carrot alters early plant development and sucrose partitioning

To unravel the functions of cell wall and vacuolar invertases in carrot, we used an antisense technique to generate transgenic carrot plants with reduced enzyme activity. Phenotypic alterations appeared at very early stages of development; indeed, the morphology of cotyledon-stage embryos was markedly changed. At the stage at which control plantlets had two to three leaves and one primary root, shoots of transgenic plantlets did not separate into individual leaves but consisted of stunted, interconnected green structures. When transgenic plantlets were grown on media containing a mixture of sucrose, glucose, and fructose rather than sucrose alone, the malformation was alleviated, and plantlets looked normal. Plantlets from hexose-containing media produced mature plants when transferred to soil. Plants expressing antisense mRNA for cell wall invertase had a bushy appearance due to the development of extra leaves, which accumulated elevated levels of sucrose and starch. Simultaneously, tap root development was markedly reduced, and the resulting smaller organs contained lower levels of carbohydrates. Compared with control plants, the dry weight leaf-to-root ratio of cell wall invertase antisense plants was shifted from 1:3 to 17:1. Plants expressing antisense mRNA for vacuolar invertase also had more leaves than did control plants, but tap roots developed normally, although they were smaller, and the leaf-to-root ratio was 1.5:1. Again, the carbohydrate content of leaves was elevated, and that of roots was reduced. Our data suggest that acid invertases play an important role in early plant development, most likely via control of sugar composition and metabolic fluxes. Later in plant development, both isoenzymes seem to have important functions in sucrose partitioning.

Association of Binucleate Rhizoctonia with Soybean and Mechanism of Biocontrol of Rhizoctonia solani

ABSTRACT The association of binucleate Rhizoctonia (BNR) AG-K with soybean and the interaction of BNR, R. solani AG-4, and soybean seedlings were investigated to elucidate the mechanism of biocontrol of R. solani by BNR. Sixty-hour-old seedlings were inoculated and incubated in a growth chamber at 24 degrees C; plants were examined with light microscopy and with scanning and transmission electron microscopy at various times following inoculation. BNR grew over hypocotyls, roots, and root hairs, but only colonized epidermal cells. Hyphae of BNR appeared to attach to the epidermis and, 5.5 h following inoculation, began penetrating cells by means of penetration pegs without forming distinct appressoria or infection cushions. There was evidence of cuticle degradation at the point of penetration. Infection hyphae moved to adjacent epidermal cells by direct penetration of epidermal radial walls. There were epidermal and cortical cell necrosis, beginning with the fragmentation of the tonoplast and followed by the disintegration of cytoplasm, organelles, and plasma membranes. Cell necrosis was also observed in adjacent cells where there was no evidence of BNR hyphae. Cell walls were not destroyed. After 144 h, there was noevidence of BNR hyphae in cortical cells. Attempted penetrations were observed, but papillae formed on the inside of cortical cell walls. Pre-inoculation of soybean seedlings with BNR 24 or 48 h before inoculation with R. solani (1 cm between inocula) affected the growth of R. solani on soybean tissue. There were fewer hyphae of R. solani, the hyphae branched sparingly, and infection cushions were rare when compared with hyphal growth on soybean inoculated only with R. solani. These effects were observed before the BNR hyphae began to intermingle with the hyphae of R. solani on the surface of the inoculated host. Preinoculation of soybean seedlings 24 h before inoculation with R. solani significantly (P = 0.05) reduced disease incidence and severity caused by R. solani AG-4. The lesions caused by R. solani always appeared distally, not proximally, to the BNR inoculum. The interactions of intermingling hyphae of BNR and R. solani were examined in vitro and on the surface of the host. There was no evidence of lysis, mycoparasitism, inhibition of growth, or any other form of antagonism between hyphae. The results of these studies strongly suggest that induced resistance is the mechanism of biocontrol of R. solani on soybean by BNR. The inhibition of hyphal growth of R. solani on the surface of soybean tissue preinoculated with BNR appears to be a novel characteristic of induced resistance.

Cloning of cDNA for a cell wall-bound acid invertase from tomato (Lycopersicon esculentum) and expression of soluble and cell wall-bound invertases in plants and wounded leaves of L. esculentum and L. peruvianum

A cDNA, Wiv-1, for an isozyme of acid invertase (EC 3.2.1.26) was cloned from wounded leaves of tomato (Lycopersicon esculentum). The encoded protein had a basic isoelectric point and strong similarity to the amino acid sequences of plant cell wall-bound invertases. The conserved sequence WECPD that is found in all plant cell wall-bound invertases was also found in the deduced protein. These results suggested that Wiv-1 encoded a cell wall-bound acid invertase of tomato. Wounding increased the levels of mRNAs for soluble and cell wall-bound invertases and the activities of these invertases in leaves of L. esculentum and of a related species, L. peruvianum. The induction of Aiv-1 mRNA for the soluble enzyme in wounded leaves was not very strong, while that of Wiv-1 mRNA for the wall-bound enzyme was prominent. The level of Aiv-1 mRNA reached a maximum 48 h after wounding while that of Wiv-1 mRNA continued to rise for up to 96 h. These findings suggested that the genes for the two isozymes responded independently to wounding. The levels in various organs of Aiv-1 and Wiv-1 mRNAs were higher in L. esculentum than in L. peruvianum. Possible roles of cell wall-bound acid invertase in wound response and in developing plant are discussed.

Purification and characterisation of soluble invertases from leaves of Arabidopsis thaliana

Multiple isoforms of beta-fructofuranosidase (invertase, EC 3.2.1.26) were identified in mature green leaves of the cruciferous plant Arabidopsis thaliana (L.) Heynh. There were four major and one minor isoforms of soluble acid invertase and an additional activity which could be released from the cell wall by buffers of high ionic strength. This study reports the separation and characterisation of three soluble isoforms following ammonium sulphate and polyethylene glycol 6000 precipitations, Concanavalin A, MonoQ ion exchange, Superose 12 size-exclusion chromatography and chromatofocusing. These isoforms, designated INV1, INV2 and INV3, had isoelectric points of 4.75, 4.70 and 4.65 and a Km for sucrose of 5, 12 and 5 mM, respectively. Each had a pH optimum of 5.5, exhibited optimal activity at 45 degrees C and used sucrose as the preferred substrate. All fractions containing these isoforms contained a 52-kDa polypeptide which was specifically detected by immunoblotting with an antibody raised against deglycosylated wheat invertase. The N-terminal amino-acid sequence of this polypeptide was homologous to acid invertases isolated from other plant species. The possible origin of isoforms of soluble acid invertase is discussed.

Immunocytochemistry of plant defense mechanisms induced upon microbial attack

During the past few years, cyto- and immunocytochemical techniques have been developed and widely used for locating and identifying various molecules in plant cell compartments. The last decade has witnessed tremendous improvements in molecular cytology, thus allowing an accurate in situ detection of various components thought to play important biological functions in the plant metabolism. The use of immunocytochemistry to investigate resistance mechanisms of plants upon pathogen attack has provided key information on the defense strategy that plants elaborate during a host-pathogen interaction. Of the various proteins induced in response to infection, chitinases and beta-1,3-glucanases have been the focus of particular attention due to their believed antimicrobial activity through the hydrolysis of the main fungal wall components, chitin and beta-1,3-glucans. Attention has also been paid to beta-fructosidase, the enzyme that hydrolyzes sucrose into glucose and fructoside. The marked accumulation of this enzyme upon pathogen infection has led to the consideration that infection may greatly influence the metabolic activity of colonized tissues by creating alterations of source-sink relationships. Another facet of the plant's defense strategy that has been the focus of considerable interest is related to the accumulation of structural compounds, such as hydroxyproline-rich glycoproteins and callose, to reinforce the wall architecture, thus decreasing vulnerability to microbial enzymes. A number of alternatives designed to improve plant protection towards pathogen invasion have been suggested. Among these, the production of transgenic plants expressing constitutively a foreign resistance gene and the pretreatment of plants with elicitors of defense reactions have been the subject of intensive studies at the molecular, biochemical, and cytological levels. Results of such studies clearly demonstrate the important contribution that cyto- and immunocytochemical approaches can make to our knowledge of how plants defend themselves and how plant disease resistance can be directly enhanced. These approaches will undoubtedly be active areas for future research in the development of biological control alternatives in which the mode of action of the product used is of key importance.

Purification and characterization of three soluble invertases from barley (Hordeum vulgare L.) leaves

Three soluble isoforms of invertase (beta-fructofuranosidase; EC 3.2.1.26) were purified from 7-d-old primary leaves of barley (Hordeum vulgare L.). Invertase I, a monomeric protein of 64 kD, was purified to apparent homogeneity as shown by sodium dodecylsulfate-polyacrylamide gel electrophoresis. Invertases IIA and IIB, multimeric proteins with molecular masses of the 116 and 155 kD, were purified 780- and 1370-fold, respectively, but were not yet homogeneous. Extracts of epidermal strips of leaves contained only invertase IIB. The specific activity of invertase was more than 100-fold higher in the epidermis than in the mesophyll. All three isoforms were acidic invertases, with pH optima of around 5.0 and little activity in the alkaline range. Invertase I had a Km for sucrose of 8.1 mM, and invertases IIA and IIB had much lower values of 1.0 and 1.7 mM, respectively. Invertase I was more than 2-fold more resistant than the other two invertases to the inhibitors HgCl2 and pyridoxal. All three constitutive invertases were found to act also as sucrose-sucrose fructosyltransferases when supplied with high concentrations of sucrose, forming 1-kestose as principal product. However, the fructosyltransferase activity of all three enzymes was inhibited by pyridoxal in the same way as their invertase activity. This characteristic clearly differentiates them from the inducible sucrose-sucrose fructosyltransferase of barley leaves, the activity responsible for the initial steps of fructan biosynthesis, which has previously been shown to be insensitive to pyridoxal.

cDNA cloning of an extracellular dermal glycoprotein of carrot and its expression in response to wounding

Suspension-cultured cells of carrot (Daucus carota L.) synthesize and secrete a glycoprotein that is normally found only in dermal tissues (epidermis, endodermis and periderm). This protein, previously called GP57, is now referred to as EDGP (E xtracellular D ermal G lyco P rotein). We purified sufficient quantities of EDGP to obtain amino-acid sequences on two internal tryptic peptides and screened a cDNA library of young carrot roots with antiserum to EDGP and with oligonucleotides corresponding to the peptides. Here we report the derived amino-acid sequence of EDGP. Sequence comparisons show that it has 40% amino-acid sequence identity with 7S basic globulin, a protein that is released when soybean seeds are soaked in hot water for a few hours. We suggest that these two proteins belong to a new family of dermal proteins. As far as we know, this is the first reported derived amino-acid sequence for protein that is specific to the epidermis and other dermal tissues. The level of EDGP mRNA is low in dry seeds, but increases rapidly in growing seedlings as they develop dermal tissues. The level of mRNA is low in storage roots, but increases rapidly in response to wounding. The presence of EDGP in dermal tissues and its up-regulation in response to wounding indicate a role in the response of plants to biotic and-or abiotic stresses. An unusual feature of the amino-acid sequence of EDGP is that it contains a short motif, which is present at the active site of aspartyl proteases such as pepsin and chymosin.