[Show abstract][Hide abstract] ABSTRACT: The lipid biopolymer suberin plays a major role as a barrier both at plant-environment interfaces and in internal tissues, restricting water and nutrient transport. In potato (Solanum tuberosum), tuber integrity is dependent on suberized periderm. Using microarray analyses, we identified ABCG1, encoding an ABC transporter, as a gene responsive to the pathogen-associated molecular pattern Pep-13. Further analyses revealed that ABCG1 is expressed in roots and tuber periderm, as well as in wounded leaves. Transgenic ABCG1-RNAi potato plants with downregulated expression of ABCG1 display major alterations in both root and tuber morphology, whereas the aerial part of the ABCG1-RNAi plants appear normal. The tuber periderm and root exodermis show reduced suberin staining and disorganized cell layers. Metabolite analyses revealed reduction of esterified suberin components and hyperaccumulation of putative suberin precursors in the tuber periderm of RNA interference plants, suggesting that ABCG1 is required for the export of suberin components.
[Show abstract][Hide abstract] ABSTRACT: As other bacteria, Mycobacterium smegmatis needs adaption mechanisms to cope with changing nitrogen sources and to survive situations of nitrogen starvation. In the study presented here, transcriptome analyses were used to characterize the response of the bacterium to nitrogen starvation and to elucidate the role of specific transcriptional regulators.
In response to nitrogen deprivation, a general starvation response is induced in M. smegmatis. This includes changes in the transcription of several hundred genes encoding e.g. transport proteins, proteins involved in nitrogen metabolism and regulation, energy generation and protein turnover. The specific nitrogen-related changes at the transcriptional level depend mainly on the presence of GlnR, while the AmtR protein controls only a small number of genes.
M. smegmatis is able to metabolize a number of different nitrogen sources and nitrogen control in M. smegmatis is similar to control mechanisms characterized in streptomycetes, while the master regulator of nitrogen control in corynebacteria, AmtR, is plays a minor role in this regulatory network.
[Show abstract][Hide abstract] ABSTRACT: Following tuber induction, potato tubers undergo a period of dormancy during which visible bud growth is inhibited. The length of the dormancy period is under environmental, physiological and hormonal control. Sucrose availability is one prerequisite for bud break. In the absence of sucrose, no bud break occurs. Thus, sucrose is likely to serve as nutrient and signal molecule at the same time. The mode of sucrose sensing is only vaguely understood, but most likely involves trehalose-6-phosphate and SnRK1 signalling networks. This conclusion is supported by the observation that ectopically manipulation of trehalose-6-phosphate levels influences the length of the dormancy period. Once physiological competence is achieved, sprouting is controlled by the level of phytohormones. Two phytohormones, ABA and ethylene, are supposed to suppress tuber sprouting; however, the exact role of ethylene remains to be elucidated. Cytokinins and gibberellins are required for bud break and sprout growth, respectively. The fifth classical phytohormone, auxin, seems to play a role in vascular development. During the dormancy period, buds are symplastically isolated, which changes during bud break. In parallel to the establishment of symplastic connectivity, vascular tissue develops below the growing bud most likely to support the outgrowing sprout with assimilates mobilised in parenchyma cells. Sprouting leads to major quality losses of stored potato tubers. Therefore, control of tuber sprouting is a major objective in potato breeding. Although comparative transcriptome analysis revealed a large number of genes differentially expressed in growing versus dormant buds, no master-regulator of potato tuber sprouting has been identified so far.
[Show abstract][Hide abstract] ABSTRACT: The interaction between Clavibacter michiganensis subsp. michiganensis with its host, the tomato plant (Solanum lycopersicum), is poorly understood and only few virulence factors are known. While studying of the bacteria in planta is time-consuming and difficult, the analysis in vitro would facilitate research. Therefore, a xylem mimicking medium (XMM) for C. michiganensis subsp. michiganensis was established in this study based on an apoplast medium for Xanthomonas campestris pv. vesicatoria. In contrast to the apoplast medium, XMM contains no sugars, but amino acids which serve as nitrogen and carbon source. As a result, growth in XMM induced transcriptional changes of genes encoding putative sugar, amino acid and iron uptake systems. In summary, mRNA levels of about 8% of all C. michiganensis subsp. michiganensis genes were changed when XMM-grown bacteria were compared to M9 minimal medium-grown cells. Almost no transcriptional changes of genes encoding hydrolytic enzymes were detected, leading to the idea that XMM reflects the situation in the beginning of infection and therefore allows the characterization of virulence factors in this early stage of infection. The addition of the plant wound substance acetosyringone to the XMM medium led to a change in transcript amount, including genes coding for proteins involved in protein transport, iron uptake and regulation processes.
Journal of Biotechnology 09/2013; · 3.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Xanthomonas campestris pv. vesicatoria (Xcv) possess a type 3 secretion system (T3SS) to deliver effector proteins into its Solanaceous host plants. These proteins are involved in suppression of plant defense and in reprogramming of plant metabolism to favour bacterial propagation. There is increasing evidence that hexoses contribute to defense responses. They act as substrates for metabolic processes and as metabolic semaphores to regulate gene expression. Especially an increase in the apoplastic hexose-to-sucrose ratio has been suggested to strengthen plant defense. This shift is brought about by the activity of cell wall-bound invertase (cw-Inv). We examined the possibility that Xcv may employ type 3 effector (T3E) proteins to suppress cw-Inv activity during infection. Indeed, pepper leaves infected with a T3SS-deficient Xcv strain showed a higher level of cw-Inv mRNA and enzyme activity relative to Xcv wild type infected leaves. Higher cw-Inv activity was paralleled by an increase in hexoses and mRNA abundance for the pathogenesis-related gene PRQ. These results suggest that Xcv suppresses cw-Inv activity in a T3SS-dependent manner, most likely to prevent sugar-mediated defense signals. To identify Xcv T3Es that regulate cw-Inv activity, a screen was performed with eighteen Xcv strains, each deficient in an individual T3E. Seven Xcv T3E deletion strains caused a significant change in cw-Inv activity compared to Xcv wild type. Among them, Xcv lacking the xopB gene (Xcv ΔxopB) caused the most prominent increase in cw-Inv activity. Deletion of xopB increased the mRNA abundance of PRQ in Xcv ΔxopB-infected pepper leaves, but not of Pti5 and Acre31, two PAMP-triggered immunity markers. Inducible expression of XopB in transgenic tobacco inhibited Xcv-mediated induction of cw-Inv activity observed in wild type plants and resulted in severe developmental phenotypes. Together, these data suggest that XopB interferes with cw-Inv activity in planta to suppress sugar-enhanced defense responses during Xcv infection.
PLoS ONE 12/2012; 7(12):e51763. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Stomata are pores on the leaf surface, bounded by two guard cells, which control the uptake of CO(2) for photosynthesis and the concomitant loss of water vapor. In 1898, Francis Darwin  showed that stomata close in response to reduced atmospheric relative humidity (rh); however, our understanding of the signaling pathway responsible for coupling changes in rh to alterations in stomatal aperture is fragmentary. The results presented here highlight the primacy of abscisic acid (ABA) in the stomatal response to drying air. We show that guard cells possess the entire ABA biosynthesis pathway and that it appears upregulated by positive feedback by ABA. When wild-type Arabidopsis and the ABA-deficient mutant aba3-1 were exposed to reductions in rh, the aba3-1 mutant wilted, whereas the wild-type did not. However, when aba3-1 plants, in which ABA synthesis had been specifically rescued in guard cells, were challenged with dry air, they did not wilt. These data indicate that guard cell-autonomous ABA synthesis is required for and is sufficient for stomatal closure in response to low rh. Guard cell-autonomous ABA synthesis allows the plant to tailor leaf gas exchange exquisitely to suit the prevailing environmental conditions.
Current biology: CB 12/2012; · 10.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nitrogen catabolite repression (NCR) is a regulatory strategy found in microorganisms that restricts the utilization of complex and unfavored nitrogen sources in the presence of favored nitrogen sources. In fungi, this concept has been best studied in yeasts and filamentous ascomycetes, where the GATA transcription factors Gln3p and Gat1p (in yeasts) and Nit2/AreA (in ascomycetes) constitute the main positive regulators of NCR. The reason why functional Nit2 homologs of some phytopathogenic fungi are required for full virulence in their hosts has remained elusive. We have identified the Nit2 homolog in the basidiomycetous phytopathogen Ustilago maydis and show that it is a major, but not the exclusive, positive regulator of nitrogen utilization. By transcriptome analysis of sporidia grown on artificial media devoid of favored nitrogen sources, we show that only a subset of nitrogen-responsive genes are regulated by Nit2, including the Gal4-like transcription factor Ton1 (a target of Nit2). Ustilagic acid biosynthesis is not under the control of Nit2, while nitrogen starvation-induced filamentous growth is largely dependent on functional Nit2. nit2 deletion mutants show the delayed initiation of filamentous growth on maize leaves and exhibit strongly compromised virulence, demonstrating that Nit2 is required to efficiently initiate the pathogenicity program of U. maydis.
[Show abstract][Hide abstract] ABSTRACT: Arabidopsis thaliana contains 18 genes encoding Hsp70s. This heat shock protein superfamily is divided into two sub-families: DnaK and Hsp110/SSE. In order to functionally characterize members of the Hsp70 superfamily, loss-of-function mutants with reduced cytosolic Hsp70 expression were studied. AtHsp70-1 and AtHsp70-2 are constitutively expressed and represent the major cytosolic Hsp70 isoforms under ambient conditions. Analysis of single and double mutants did not reveal any difference compared to wild-type controls. In yeast, SSE protein has been shown to act as a nucleotide exchange factor, essential for Hsp70 function. To test whether members of the Hsp110/SSE sub-family serve essential functions in plants, two members of the sub-family, AtHsp70-14 and AtHsp70-15, were analysed. Both genes are highly homologous and constitutively expressed. Deficiency of AtHsp70-15 but not of AtHsp70-14 led to severe growth retardation. AtHsp70-15-deficient plants were smaller than wild-type and exhibited a slightly different leaf shape. Stomatal closure under ambient conditions and in response to ABA was impaired in the AtHsp70-15 transgenic plants, but ABA-dependent inhibition of germination was not affected. Heat treatment of AtHsp70-15-deficient plants resulted in drastically increased mortality, indicating that AtHsp70-15 plays an essential role during normal growth and in the heat response of Arabidopsis plants. AtHsp70-15-deficient plants are more tolerant to infection by turnip mosaic virus. Comparative transcriptome analysis revealed that AtHsp70-15-deficient plants display a constitutive stress response similar to the cytosolic protein response. Based on these results, AtHsp70-15 is likely to be a key factor in proper folding of cytosolic proteins, and may function as nucleotide exchange factor as proposed for yeast.
The Plant Journal 03/2011; 66(6):983-95. · 6.82 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Reactivation of dormant meristems is of central importance for plant fitness and survival. Due to their large meristem size, potato (Solanum tuberosum) tubers serve as a model system to study the underlying molecular processes. The phytohormones cytokinins (CK) and gibberellins (GA) play important roles in releasing potato tuber dormancy and promoting sprouting, but their mode of action in these processes is still obscure. Here, we established an in vitro assay using excised tuber buds to study the dormancy-releasing capacity of GA and CK and show that application of gibberellic acid (GA(3)) is sufficient to induce sprouting. In contrast, treatment with 6-benzylaminopurine induced bud break but did not support further sprout growth unless GA(3) was administered additionally. Transgenic potato plants expressing Arabidopsis (Arabidopsis thaliana) GA 20-oxidase or GA 2-oxidase to modify endogenous GA levels showed the expected phenotypical changes as well as slight effects on tuber sprouting. The isopentenyltransferase (IPT) from Agrobacterium tumefaciens and the Arabidopsis cytokinin oxidase/dehydrogenase1 (CKX) were exploited to modify the amounts of CK in transgenic potato plants. IPT expression promoted earlier sprouting in vitro. Strikingly, CKX-expressing tubers exhibited a prolonged dormancy period and did not respond to GA(3). This supports an essential role of CK in terminating tuber dormancy and indicates that GA is not sufficient to break dormancy in the absence of CK. GA(3)-treated wild-type and CKX-expressing tuber buds were subjected to a transcriptome analysis that revealed transcriptional changes in several functional groups, including cell wall metabolism, cell cycle, and auxin and ethylene signaling, denoting events associated with the reactivation of dormant meristems.
[Show abstract][Hide abstract] ABSTRACT: Gene silencing of Lyc e 1 leads to reduced allergenicity of tomato fruits but impaired growth of transgenic tomato plants. The aim of the study was to restore growth of Lyc e 1-deficient tomato plants while retaining reduced allergenicity by simultaneous complementation of profilin deficiency by expression of nonallergenic yeast profilin. Transgenic plants were generated and tested by RT-PCR and immunoblotting; allergenicity of yeast profilin and transgenic fruits was investigated by IgE binding, basophil activation, and skin-prick tests. Lyc e 1 content of transgenic tomato fruits was <5% of that of wild-type plants, causing significantly reduced IgE antibody binding. Simultaneous coexpression of yeast profilin restored growth and biomass production almost to wild-type levels. Yeast profilin, sharing 32.6% amino acid sequence identity with Lyc e 1, displayed low IgE-binding capacity and allergenic potency. Among 16 tomato-allergic patients preselected for sensitization to Lyc e 1, none showed significant reactivity to yeast profilin. Yeast profilin did not induce mediator release, and coexpression of yeast profilin did not enhance the allergenicity of Lyc e 1-reduced fruits. Simultanous coexpression of yeast profilin allows silencing of tomato profilin and generation of viable plants with Lyc e 1-deficient tomato fruits. Therefore, a novel approach to allergen avoidance, genetically modified foods with reduced allergen accumulation, can be generated even if the allergen fulfills an essential cellular function in the plant. In summary, our findings of efficiently complementing profilin-deficient tomato plants by coexpression of low allergenic yeast profilin demonstrate the feasibility of creating low-allergenic food even if the allergen fulfills essential cellular functions.
The FASEB Journal 12/2010; 24(12):4939-47. · 5.48 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Identification of molecular markers defining
the end of tuber dormancy prior to visible
sprouting is of agronomic interest for potato growers
and the potato processing industry. In potato tubers,
breakage of dormancy is associated with the reactivation
of meristem function. In dormant meristems,
cells are arrested in the G1/G0 phase of the cell cycle
and re-entry into the G1 phase followed by DNA
replication during the S phase enables bud outgrowth.
Deoxyuridine triphosphatase (dUTPase) is essential
for DNA replication and was therefore tested as a
potential marker for meristem reactivation in tuber
buds. The corresponding cDNA clone was isolated
from potato by PCR. The deduced amino acid
sequence showed 94% similarity to the tomato
homologue. By employing different potato cultivars,
a positive correlation between dUTPase expression
and onset of tuber sprouting could be confirmed.
Moreover, gene expression analysis of tuber buds
during storage time revealed an up-regulation of the
dUTPase 1 week before visible sprouting occurred.
Further analysis using an in vitro sprout assay
supported the assumption that dUTPase is a good
molecular marker to define the transition from
dormant to active potato tuber meristems.
[Show abstract][Hide abstract] ABSTRACT: The aim of the present study was to assess possible adverse effects of transgene expression in leaves of field-grown barley relative to the influence of genetic background and the effect of plant interaction with arbuscular mycorrhizal fungi. We conducted transcript profiling, metabolome profiling, and metabolic fingerprinting of wild-type accessions and barley transgenics with seed-specific expression of (1,3-1, 4)-beta-glucanase (GluB) in Baronesse (B) as well as of transgenics in Golden Promise (GP) background with ubiquitous expression of codon-optimized Trichoderma harzianum endochitinase (ChGP). We found more than 1,600 differential transcripts between varieties GP and B, with defense genes being strongly overrepresented in B, indicating a divergent response to subclinical pathogen challenge in the field. In contrast, no statistically significant differences between ChGP and GP could be detected based on transcriptome or metabolome analysis, although 22 genes and 4 metabolites were differentially abundant when comparing GluB and B, leading to the distinction of these two genotypes in principle component analysis. The coregulation of most of these genes in GluB and GP, as well as simple sequence repeat-marker analysis, suggests that the distinctive alleles in GluB are inherited from GP. Thus, the effect of the two investigated transgenes on the global transcript profile is substantially lower than the effect of a minor number of alleles that differ as a consequence of crop breeding. Exposing roots to the spores of the mycorrhizal Glomus sp. had little effect on the leaf transcriptome, but central leaf metabolism was consistently altered in all genotypes.
Proceedings of the National Academy of Sciences 03/2010; 107(14):6198-203. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Even though the process of potato tuber starch biosynthesis is well understood, mechanisms regulating biosynthesis are still unclear. Transcriptome analysis provides valuable information as to how genes are regulated. Therefore, this work aimed at investigating transcriptional regulation of starch biosynthetic genes in leaves and tubers of potato plants under various conditions. More specifically we looked at gene expression diurnally in leaves and tubers, during tuber induction and in tubers growing at different velocities. To determine velocity of potato tuber growth a new method based on X-ray Computed Tomography (X-ray CT) was established.
Comparative transcriptome analysis between leaves and tubers revealed striking similarities with the same genes being differentially expressed in both tissues. In tubers, oscillation of granule bound starch synthase (GBSS) expression) was observed which could be linked to sucrose supply from source leaves. X-ray CT was used to determine time-dependent changes in tuber volume and the growth velocity was calculated. Although there is not a linear correlation between growth velocity and expression of starch biosynthetic genes, there are significant differences between growing and non-growing tubers. Co-expression analysis was used to identify transcription factors positively correlating with starch biosynthetic genes possibly regulating starch biosynthesis.
Most starch biosynthetic enzymes are encoded by gene families. Co-expression analysis revealed that the same members of these gene families are co-regulated in leaves and tubers. This suggests that regulation of transitory and storage starch biosynthesis in leaves and tubers, respectively, is surprisingly similar. X-ray CT can be used to monitor growth and development of belowground organs and allows to link tuber growth to changes in gene expression. Comparative transcriptome analysis provides a useful tool to identify transcription factors possibly involved in the regulation of starch biosynthesis.
[Show abstract][Hide abstract] ABSTRACT: The phytopathogenic bacterium Xanthomonas campestris pv. vesicatoria uses the type III secretion system (T3SS) to inject effector proteins into cells of its Solanaceous host plants. It is generally assumed that these effectors manipulate host pathways to favor bacterial replication and survival. However, the molecular mechanisms by which type III effectors suppress host defense responses are far from being understood. Based on sequence similarity, Xanthomonas outer protein J (XopJ) is a member of the YopJ/AvrRxv family of SUMO peptidases and acetyltranferases, although its biochemical activity has not yet been demonstrated. Confocal laser scanning microscopy revealed that green fluorescent protein (GFP) fusions of XopJ are targeted to the plasma membrane when expressed in plant cells, which most likely involves N-myristoylation. In contrast to a XopJ(C235A) mutant disrupted in the catalytic triad sequence, the wild-type effector GFP fusion protein was also localized in vesicle-like structures colocalizing together with a Golgi marker protein, suggesting an effect of XopJ on vesicle trafficking. To explore an effect of XopJ on protein secretion, we used a GFP-based secretion assay. When a secreted (sec)GFP marker was coexpressed with XopJ in leaves of Nicotiana benthamiana, GFP fluorescence was retained in reticulate structures. In contrast, in plant cells expressing secGFP alone or along with the XopJ(C235A) mutant, no GFP fluorescence accumulated within the cells. Moreover, coexpressing secGFP together with XopJ led to a reduced accumulation of secGFP within the apoplastic fluid of N. benthamiana leaves, further showing that XopJ affects protein secretion. Transgenic expression of XopJ in Arabidopsis suppressed callose deposition elicited by a T3SS-negative mutant of Pseudomonas syringae pv. tomato DC3000. A role of XopJ in the inhibition of cell wall-based defense responses is discussed.
[Show abstract][Hide abstract] ABSTRACT: Piriformospora indica is a mutualistic root-colonising basidiomycete that tranfers various benefits to colonized host plants including growth promotion, yield increases as well as abiotic and biotic stress tolerance. The fungus is characterized by a broad host spectrum encompassing various monocots and dicots. Our recent microarray-based studies indicate a general plant defense suppression by P. indica and significant changes in the GA biosynthesis pathway. Furthermore, barley plants impaired in GA synthesis and perception showed a significant reduction in mutualistic colonization, which was associated with an elevated expression of defense-related genes. Here, we discuss the importance of plant hormones for compatibility in plant root-P. indica associations. Our data might provide a first explanation for the colonization success of the fungus in a wide range of higher plants.
[Show abstract][Hide abstract] ABSTRACT: Fungi of the order Sebacinales (Basidiomycota) are involved in a wide spectrum of mutualistic symbioses with various plants, thereby exhibiting unique potential for biocontrol strategies. Piriformospora indica, a model organism of this fungal order, is able to increase the biomass and grain yield of crop plants, and induces local and systemic resistance to fungal diseases and tolerance to abiotic stress. To elucidate the molecular basis for root colonization, we characterized the interaction of P. indica with barley roots by combining global gene expression profiling, metabolic profiling, and genetic studies. At the metabolic level, we show that fungal colonization reduces the availability of free sugars and amino acids to the root tip. At the transcriptional level, consecutive interaction stages covering pre-penetration-associated events and progressing through to root colonization showed differential regulation of signal perception and transduction components, secondary metabolism, and genes associated with membrane transport. Moreover, we observed stage-specific up-regulation of genes involved in phytohormone metabolism, mainly encompassing gibberellin, auxin and abscisic acid, but salicylic acid-associated gene expression was suppressed. The changes in hormone homoeostasis were accompanied with a general suppression of the plant innate immune system. Further genetic studies showed reduced fungal colonization in mutants that are impaired in gibberellin synthesis as well as perception, and implicate gibberellin as a modulator of the root's basal defence. Our data further reveal the complexity of compatibility mechanisms in host-microbe interactions, and identify gibberellin signaling as potential target for successful fungi.
The Plant Journal 05/2009; 59(3):461-74. · 6.82 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Food allergies are a major health concern in industrialized countries. Since a specific immunotherapy for food allergies is not available in clinical routine praxis till now, reduction of allergens in foods, either by food processing or genetic engineering are strategies to minimize the risk of adverse reactions for food allergic patients. This review summarizes biotechnological approaches, especially the RNA interference (RNAi) technology, for the reduction of selected allergens in plant foods. So far, only a limited number of reports showing proof-of-concept of this methodology are available. Using RNAi an impressive reduction of allergen accumulation was obtained which was stable in the next generations of plants. Since threshold doses for most food allergens are not known, the beneficial effect has to be evaluated by oral challenge tests in the future. The article critically addresses the potential and limitations of genetic engineering, as well as of alternative strategies to generate "low allergic" foods.
Frontiers in Bioscience 02/2009; 14:59-71. · 4.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cell wall-bound invertase (cw-Inv) plays an important role in carbohydrate partitioning and regulation of sink-source interaction. There is increasing evidence that pathogens interfere with sink-source interaction, and induction of cw-Inv activity has frequently been shown in response to pathogen infection. To investigate the role of cw-Inv, transgenic tomato (Solanum lycopersicum) plants silenced for the major leaf cw-Inv isoforms were generated and analyzed during normal growth and during the compatible interaction with Xanthomonas campestris pv vesicatoria. Under normal growth conditions, activities of sucrolytic enzymes as well as photosynthesis and respiration were unaltered in the transgenic plants compared with wild-type plants. However, starch levels of source leaves were strongly reduced, which was most likely caused by an enhanced sucrose exudation rate. Following X. campestris pv vesicatoria infection, cw-Inv-silenced plants showed an increased sucrose to hexose ratio in the apoplast of leaves. Symptom development, inhibition of photosynthesis, and expression of photosynthetic genes were clearly delayed in transgenic plants compared with wild-type plants. In addition, induction of senescence-associated and pathogenesis-related genes observed in infected wild-type plants was abolished in cw-Inv-silenced tomato lines. These changes were not associated with decreased bacterial growth. In conclusion, cw-Inv restricts carbon export from source leaves and regulates the sucrose to hexose ratio in the apoplast. Furthermore, an increased apoplastic hexose to sucrose ratio can be linked to inhibition of photosynthesis and induction of pathogenesis-related gene expression but does not significantly influence bacterial growth. Indirectly, bacteria may benefit from low invertase activity, since the longevity of host cells is raised and basal defense might be dampened.
[Show abstract][Hide abstract] ABSTRACT: The significance of cell wall invertase (cwINV) for plant defense was investigated by comparing wild-type tobacco (Nicotiana tabacum) Samsun NN (SNN) with plants with RNA interference (RNAi)-mediated repression of cwINV (SNNcwINV). In source leaves of SNNcwINV, the activity of cwINV was repressed by about 90%. Sucrose export and apoplastic carbohydrate levels were significantly reduced, while photosynthesis and dark respiration exhibited little or no change. Activities of sucrose synthase and phosphofructokinase were depressed moderately, while ADP-glucose pyrophosphorylase was diminished greatly. Yet, the content of cytosolic/vacuolar carbohydrates was not significantly lower, which correlated with the absence of phenotypic effects in SNNcwINV under normal growing conditions. By contrast, defense-related processes in primary metabolism and hypersensitive cell death were impaired and delayed in correlation with repression of cwINV. The increase in cwINV observed in source leaves of the resistant wild type following infection with Phytophthora nicotianae was absent in SNNcwINV. Also, defense-related callose deposition at cell-to-cell interfaces, the related decline in sugar export, and accumulation of apoplastic carbohydrates were reduced and delayed. Expression of pathogenesis-related proteins and increase in phenylalanine ammonia-lyase and glucose-6-phosphate dehydrogenase activities were alleviated. Formation of hydrogen peroxide and development of hypersensitive lesions were weak and heterogeneous, and the pathogen was able to sporulate. We conclude that in photosynthetically active leaves of the apoplastic phloem loader, tobacco cwINV plays an essential role for acquisition of carbohydrates during plant-pathogen interactions and that the availability of these carbohydrates supports the onset of the hypersensitive reaction and ensures successful defense.