Fruits of wild and semi-domesticated Diospyros tree species have contrasting phenological, metabolic, and antioxidant activity profiles.

BACKGROUND: In contrast to commercial Diospyros species, Mesoamerican fruit-producing species are scarcely known, particularly wild species that might harbor desirable traits suitable for breeding. Thus, metabolomic, chemical, and antioxidant profiles of fruits harvested from cultivated Diospyros digyna and wild Diospyros rekoi trees during consecutive winter seasons were obtained. Fruits were harvested in habitats having marked differences in soil quality, climate, and luminosity. RESULTS: D. digyna fruits were larger and less acid than D. rekoi fruits, whereas antioxidant activity tended to be higher in D. rekoi fruits. Phenolic, flavonoid, and sugar contents also varied significantly between species. Metabolomic analysis allowed the pre-identification of 519 and 1665 metabolites in negative and positive electrospray ionization (ESI) modes, respectively. Principal component analysis of the positive ESI data explained 51.8% of the variance and indicated clear metabolomic differences between D. rekoi and D. digyna fruits that were confirmed by direct-injection ESI mass spectrometry profiles. Twenty-one discriminating metabolites were detected in fruits of both species; D. digyna fruits differentially accumulated lysophospholipids, whereas discriminating metabolites in D. rekoi fruits were chemically more diverse than those in D. digyna fruits. CONCLUSION: Domesticated D. digyna fruits have improved physicochemical fruit traits compared with wild D. rekoi fruits, including larger size and lower acidity. The metabolomic and chemical composition of their respective fruits were also significantly different, which in D. rekoi was manifested as a notable season-dependent increase in antioxidant capacity. Therefore, wild D. rekoi can be considered as an important genetic resource for the improvement of commercial Diospyros fruit quality. © 2019 Society of Chemical Industry.

RNA Isolation Method in Marginal Crops with High Agronomic Potential.

Ribonucleic Acid (RNA) isolation is a basic technique in the field of molecular biology. The purpose of RNA isolation is to acquire pure and complete RNA that can be used to evaluate gene expression. Many methods can be used to perform RNA isolation, all of them based on the chemical properties of nucleic acids. However, some of them do not achieve high RNA yields and purity levels when used in a number of marginally studied crops of agronomic importance, such as grain and vegetable amaranth plants. In the method described here, the use of guanidinium thiocyanate and two additional precipitation steps with different reagents designed to obtain high yields and RNA purity levels from diverse plant species employed for plant functional genomics studies is described.

Trunk Injection of Citrus Trees with a Polymeric Nanobactericide Reduces Huanglongbing Severity Caused by Candidatus Liberibacter asiaticus.

Huanglongbing (HLB) is a disease caused by the phloem- limited Candidatus Liberibacter asiaticus (CLas) that affects the citrus industry worldwide. To date, only indirect strategies have been implemented to eradicate HLB. Included among these is the population control of the psyllid vector (Diaphorina citri), which usually provides inconsistent results. Even though strategies for direct CLas suppression seem a priori more promising, only a handful of reports have been focused on a confrontation of the pathogen. Recent developments in polymer chemistry have allowed the design of polycationic self-assembled block copolymers with outstanding antibacterial capabilities. Here, we report the use of polymeric nano-sized bactericide particles (PNB) to control CLas directly in the phloem vasculature. The field experiments were performed in Rioverde, San Luis Potosí, and is one of the most important citrusproducing regions in Mexico. An average 52% reduction in the bacterial population was produced when PNB was injected directly into the trunk of 20 infected trees, although, in some cases, reduction levels reached 97%. These results position PNB as a novel and promising nanotechnological tool for citrus crop protection against CLas and other related pathogens.

Systemic whitefly-induced metabolic responses in newly developed distal leaves of husk tomato plants (Physalis philadelphica) impairs whiteflies development.

BACKGROUND: Metabolic reconfiguration in plants is a hallmark response to insect herbivory that occurs in the attack site and systemically in undamaged tissues. Metabolomic systemic responses can occur rapidly while the herbivore is still present and may persist in newly developed tissue to counterattack future herbivore attacks. This study analyzed the metabolic profile of local and newly developed distal (systemic) leaves of husk tomato (Physalis philadelphica) plants after whitefly Trialeurodes vaporariorum infestation. In addition, the effect of these metabolomic adjustments on whitefly oviposition and development was evaluated. RESULTS: Our results indicate that T. vaporariorum infestation induced significant changes in husk tomato metabolic profiles, not only locally in infested leaves, but also systemically in distal leaves that developed after infestation. The distinctive metabolic profile produced in newly developed leaves affected whitefly nymphal development but did not affect female oviposition, suggesting that changes driven by whitefly herbivory persist in the young leaves that developed after the infestation event to avoid future herbivore attacks. CONCLUSIONS: This report contributes to further understanding the plant responses to sucking insects by describing the metabolic reconfiguration in newly developed, undamaged systemic leaf tissues of husk tomato plants after whitefly infestation. © 2022 Society of Chemical Industry.

Metabolic and enzymatic changes associated with carbon mobilization, utilization and replenishment triggered in grain amaranth (Amaranthus cruentus) in response to partial defoliation by mechanical injury or insect herbivory.

BACKGROUND: Amaranthus cruentus and A. hypochondriacus are crop plants grown for grain production in subtropical countries. Recently, the generation of large-scale transcriptomic data opened the possibility to study representative genes of primary metabolism to gain a better understanding of the biochemical mechanisms underlying tolerance to defoliation in these species. A multi-level approach was followed involving gene expression analysis, enzyme activity and metabolite measurements. RESULTS: Defoliation by insect herbivory (HD) or mechanical damage (MD) led to a rapid and transient reduction of non-structural carbohydrates (NSC) in all tissues examined. This correlated with a short-term induction of foliar sucrolytic activity, differential gene expression of a vacuolar invertase and its inhibitor, and induction of a sucrose transporter gene. Leaf starch in defoliated plants correlated negatively with amylolytic activity and expression of a ß-amylase-1 gene and positively with a soluble starch synthase gene. Fatty-acid accumulation in roots coincided with a high expression of a phosphoenolpyruvate/phosphate transporter gene. In all tissues there was a long-term replenishment of most metabolite pools, which allowed damaged plants to maintain unaltered growth and grain yield. Promoter analysis of ADP-glucose pyrophosphorylase and vacuolar invertase genes indicated the presence of cis-regulatory elements that supported their responsiveness to defoliation. HD and MD had differential effects on transcripts, enzyme activities and metabolites. However, the correlation between transcript abundance and enzymatic activities was very limited. A better correlation was found between enzymes, metabolite levels and growth and reproductive parameters. CONCLUSIONS: It is concluded that a rapid reduction of NSC reserves in leaves, stems and roots followed by their long-term recovery underlies tolerance to defoliation in grain amaranth. This requires the coordinate action of genes/enzymes that are differentially affected by the way leaf damage is performed. Defoliation tolerance in grain is a complex process that can't be fully explained at the transcriptomic level only.

Cross-kingdom effects of plant-plant signaling via volatile organic compounds emitted by tomato (Solanum lycopersicum) plants infested by the greenhouse whitefly (Trialeurodes vaporariorum).

Volatile organic compounds (VOCs) emitted from plants in response to insect infestation can function as signals for the attraction of predatory/parasitic insects and/or repulsion of herbivores. VOCs also may play a role in intra- and inter-plant communication. In this work, the kinetics and composition of VOC emissions produced by tomato (Solanum lycopersicum) plants infested with the greenhouse whitefly Trialeurodes vaporariorum was determined within a 14 days period. The VOC emission profiles varied concomitantly with the duration of whitefly infestation. A total of 36 different VOCs were detected during the experiment, 26 of which could be identified: 23 terpenoids, plus decanal, decane, and methyl salicylate (MeSA). Many VOCs were emitted exclusively by infested plants, including MeSA and 10 terpenoids. In general, individual VOC emissions increased as the infestation progressed, particularly at 7 days post-infestation (dpi). Additional tunnel experiments showed that a 3 days exposure to VOC emissions from whitefly-infested plants significantly reduced infection by a biotrophic bacterial pathogen. Infection of VOC-exposed plants induced the expression of a likely tomato homolog of a methyl salicylate esterase gene, which preceded the expression of pathogenesis-related protein genes. This expression pattern correlated with reduced susceptibility in VOC-exposed plants. The observed cross-kingdom effect of plant-plant signaling via VOCs probably represents a generalized defensive response that contributes to increased plant fitness, considering that resistance responses to whiteflies and biotrophic bacterial pathogens in tomato share many common elements.

Characterization of the tomato prosystemin promoter: organ-specific expression, hormone specificity and methyl jasmonate responsiveness by deletion analysis in transgenic tobacco plants.

Tomato systemin is a bioactive peptide that regulates the systemic activation of wound-responsive genes. It is released from its 200 amino acid precursor called prosystemin. Initial tissue-localization and hormone-induced expression assays indicated that the tomato prosystemin gene (SlPS) accumulates mainly in floral tissues and in response to exogenous abscisic acid and methyl jasmonate (MeJA) treatments, respectively. Later, the promoter regions of the PS gene in tomato (Solanum lycopersicum L. cv. Castlemart), pepper (Capsicum annuum) and potato (Solanum tuberosum) were isolated and an in silico analysis of the SlPS promoter revealed an over-representation of stress- and MeJA-responsive motifs. A subsequent 5' deletion analysis of the SlPS promoter fused to the ß-glucuronidase reporter (GUS) gene showed that the -221 to +40 bp proximal SlPS promoter region was sufficient to direct the stigma, vascular bundle-specific and MeJA-responsive expression of GUS in transgenic tobacco plants. Important vascular-tissue-specific, light- and MeJA-responsive cis-elements were also present in this region. These findings provide relevant information regarding the transcriptional regulation mechanisms of the SlPS promoter operating in transgenic tobacco plants. They also suggest that its tissue-specificity and inducible nature could have wide applicability in plant biotechnology.

Tomato Prosystemin Is Much More than a Simple Systemin Precursor.

Systemin (Sys) is an octadecapeptide, which upon wounding, is released from the carboxy terminus of its precursor, Prosystemin (ProSys), to promote plant defenses. Recent findings on the disordered structure of ProSys prompted us to investigate a putative biological role of the whole precursor deprived of the Sys peptide. We produced transgenic tomato plants expressing a truncated ProSys gene in which the exon coding for Sys was removed and compared their defense response with that induced by the exogenous application of the recombinant truncated ProSys (ProSys(1-178), the Prosystemin sequence devoid of Sys region). By combining protein structure analyses, transcriptomic analysis, gene expression profiling and bioassays with different pests, we demonstrate that truncated ProSys promotes defense barriers in tomato plants through a hormone-independent defense pathway, likely associated with the production of oligogalacturonides (OGs). Both transgenic and plants treated with the recombinant protein showed the modulation of the expression of genes linked with defense responses and resulted in protection against the lepidopteran pest Spodoptera littoralis and the fungus Botrytis cinerea. Our results suggest that the overall function of the wild-type ProSys is more complex than previously shown, as it might activate at least two tomato defense pathways: the well-known Sys-dependent pathway connected with the induction of jasmonic acid biosynthesis and the successive activation of a set of defense-related genes, and the ProSys(1-178)-dependent pathway associated with OGs production leading to the OGs mediate plant immunity.

Transformed tobacco (Nicotiana tabacum) plants over-expressing a peroxisome proliferator-activated receptor gene from Xenopus laevis (xPPARα) show increased susceptibility to infection by virulent Pseudomonas syringae pathogens.

Transgenic tobacco plants capable of over-expressing Xenopus PPARα (xPPARα), a transcription factor known to be required for peroxisome proliferation in animals, were recently generated. These plants (herewith referred to as PPAR-OE) were found to have increased peroxisome abundance, higher peroxisomal acyl-CoA oxidase and catalase activity and modified fatty acid metabolism. Further characterization of PPAR-OE plants revealed a higher susceptibility to virulent and a partial loss of resistance to avirulent Pseudomonas syringae pathogens, whereas the basal resistance response remained unaffected. Biochemical- and defense-related gene expression analyses showed that increased susceptibility to bacterial invasion coincided with the generalized reduction in H(2)O(2) and salicylic acid (SA) levels observed within the first 24 h of bacterial contact. Decreased H(2)O(2) levels were correlated with modified activity levels of catalase and other antioxidant enzymes. A correspondence between a rapid (within 1-24 hpi; ACCO and AOC) and sustained increase (up to 6 days pi; ACCO) in the expression levels of ethylene (ACCO) and jasmonic acid (AOC) biosynthetic genes and a higher susceptibility to virulent bacterial invasion was also observed in PPAR-OE plants. Conversely, no apparent differences in the short- and/or long-term expression levels of markers for the hypersensitive-response, oxidative burst and systemic-acquired resistance were observed between wild type and PPAR-OE plants. The results suggest that peroxisome proliferation could lead to increased susceptibility to bacterial pathogens in tobacco by altering the redox balance of the plant and the expression pattern of key defense signaling pathway genes.

Highest Defoliation Tolerance in Amaranthus cruentus Plants at Panicle Development Is Associated With Sugar Starvation Responses.

Defoliation tolerance (DT) in Amaranthus cruentus is known to reach its apex at the panicle emergence (PE) phase and to decline to minimal levels at flowering (FL). In this study, defoliation-induced changes were recorded in the content of non-structural carbohydrates and raffinose family oligosaccharides (RFOs), and in the expression and/or activity of sugar starvation response-associated genes in plants defoliated at different vegetative and reproductive stages. This strategy identified sugar-starvation-related factors that explained the opposite DT observed at these key developmental stages. Peak DT at PE was associated with increased cytosolic invertase (CI) activity in all organs and with the extensive induction of various class II trehalose-phosphate synthase (TPS) genes. Contrariwise, least DT at FL coincided with a sharp depletion of starch reserves and with sucrose (Suc) accumulation, in leaves and stems, the latter of which was consistent with very low levels of CI and vacuolar invertase activities that were not further modified by defoliation. Increased Suc suggested growth-inhibiting conditions associated with altered cytosolic Suc-to-hexose ratios in plants defoliated at FL. Augmented cell wall invertase activity in leaves and roots, probably acting in a regulatory rather than hydrolytic role, was also associated with minimal DT observed at FL. The widespread contrast in gene expression patterns in panicles also matched the opposite DT observed at PE and FL. These results reinforce the concept that a localized sugar starvation response caused by C partitioning is crucial for DT in grain amaranth.

Contrasting Metabolic Fingerprints and Seed Protein Profiles of Cucurbita foetidissima and C. radicans Fruits from Feral Plants Sampled in Central Mexico.

Cucurbita foetidissima and C. radicans are scarcely studied wild pumpkin species that grow in arid and semi-arid areas of Mexico and the United States. This study describes the morphological, proximal composition, metabolic finger-prints and seed protein profiles of C. foetidissima and C. radicans fruits collected in the wild during a one-year period in different locations of central-western Mexico. The results obtained complement the limited information concerning the fruit composition of C. foetidissima and greatly expand information in this respect regarding C. radicans. Morphology and proximal composition of their fruits varied significantly. Different metabolic fingerprints and seed protein profiles were detected between them and also with the chemical composition of domesticated Cucurbita fruits. The neutral lipids in seed, pulp and peels were rich in wax content and in unsaturated compounds, probably carotenoids and tocopherols, in addition to tri-, di- and mono-acylglycerols. The tri- and diacylglycerol profiles of their seed oils were different from commercial seed oils and between each other. They also showed unusual fatty acid compositions. Evidence of a possible alkaloid in the pulp and peel of both species was obtained in addition to several putative cucurbitacins. An abundance of phenolic acids was found in all fruit parts, whereas flavonoids were only detected in the peels. Unlike most cucurbits, globulins were not the main protein fraction in the seeds of C. radicans, whereas the non-structural carbohydrate and raffinose oligosaccharide content in their fruit parts was lower than in other wild cucurbit species. These results emphasize the significantly different chemical composition of these two marginally studied Cucurbita species, which was more discrepant in C. radicans, despite the notion regarding C. foetidissima as an aberrant species with no affinity to any other Cucurbita species.

Inoculation of tomato plants (Solanum lycopersicum) with growth-promoting Bacillus subtilis retards whitefly Bemisia tabaci development.

Root inoculation of tomato (Solanum lycopersicum) plants with a Bacillus subtilis strain BEB-DN (BsDN) isolated from the rhizosphere of cultivated potato plants was able to promote growth and to generate an induced systemic resistance (ISR) response against virus-free Bemisia tabaci. Growth promotion was evident 3 weeks after inoculation. No changes in oviposition density, preference and nymphal number in the early stages of B. tabaci development were observed between BsDN-treated plants and control plants inoculated with a non-growth promoting Bs strain (PY-79), growth medium or water. However, a long-term ISR response was manifested by a significantly reduced number of B. tabaci pupae developing into adults in BsDN-treated plants. The observed resistance response appeared to be a combination of jasmonic acid (JA) dependent and JA-independent responses, since the BsDN-related retardation effect on B. tabaci development was still effective in the highly susceptible spr2 tomato mutants with an impaired capacity for JA biosynthesis. A screening of 244 genes, 169 of which were previously obtained from subtractive-suppressive-hybridization libraries generated from B. tabaci-infested plants suggested that the BsDN JA-dependent ISR depended on an anti-nutritive effect produced by the simultaneous expression of genes coding principally for proteases and proteinase inhibitors, whereas the JA-independent ISR observed in the spr2 background curiously involved the up-regulation of several photosynthetic genes, key components of the phenyl-propanoid and terpenoid biosynthetic pathways and of the Hsp90 chaperonin, which probably mediated pest resistance response(s), in addition to the down-regulation of pathogenesis and hypersensitive response genes.

Differential gene expression in whitefly Bemisia tabaci-infested tomato (Solanum lycopersicum) plants at progressing developmental stages of the insect's life cycle.

A suppression-subtractive-hybridization (SSH) strategy was used to identify genes whose expression was modified in response to virus-free whitefly Bemisia tabaci (Bt, biotype A) infestation in tomato (Solanum lycopersicum) plants. Thus, forward and reverse SSH gene libraries were generated at four points in the whitefly's life cycle, namely at (1) 2 days (adult feeding and oviposition: phase I); (2) 7 days (mobile crawler stage: phase II); (3) 12 days (second to third instar nymphal transition: phase III) and (4) 18 days (fourth instar nymphal stage: phase IV). The 169 genes with altered expression (up and downregulated) that were identified in the eight generated SSH libraries, together with 75 additional genes that were selected on the basis of their involvement in resistance responses against phytofagous insects and pathogens, were printed on a Nexterion(®) Slide MPX 16 to monitor their pattern of expression at the above phases. The results indicated that Bt infestation in tomato led to distinctive phase-specific expression/repression patterns of several genes associated predominantly with photosynthesis, senescence, secondary metabolism and (a)biotic stress. Most of the gene expression modifications were detected in phase III, coinciding with intense larval feeding, whereas fewer changes were detected in phases I and IV. These results complement previously reported gene expression profiles in Bt-infested tomato and Arabidopisis, and support and expand the opinion that Bt infestation leads to the downregulation of specific defense responses in addition to those controlled by jasmonic acid.

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.

Modulation of steroidal glycoalkaloid biosynthesis in tomato (Solanum lycopersicum) by jasmonic acid.

Jasmonic acid (JA) is a phytohormone involved in plant development and defense. A major role of JA is the enhancement of secondary metabolite production, such as response to herbivory. Systemin is a bioactive plant peptide of 18 amino acids that contributes to the induction of local and systemic defense responses in tomato (Solanum lycopersicum) through JA biosynthesis. The overexpression of systemin (PS-OE) results in constitutive JA accumulation and enhances pest resistance in plants. Conversely, mutant plants affected in linolenic acid synthesis (spr2) are negatively compromised in the production of JA which favors damage and oviposition by insect herbivores. With undirected mass fingerprinting analyses, we found global metabolic differences between genotypes with modified jasmonic acid production. The spr2 mutants were enriched in di-unsaturated fatty acids and generally showed more changes. The PS-OE genotype produced an unidentified compound with a mass-to-charge ratio of 695 (MZ695). Most strikingly, the steroidal glycoalkaloid biosynthesis was negatively affected in the spr2 genotype. Complementation with jasmonic acid could restore the tomatine pathway, which strongly suggests the control of steroidal glycoalkaloid biosynthesis by jasmonic acid. spr2 plants were more susceptible to fungal infection with Fusarium oxysporum f.sp. ciceris, but not to bacterial infection with Clavibacter michiganensis subsp. michiganensis which supports the involvement of steroidal glycoalkaloids in the plant response against fungi.

The tolerance of grain amaranth (Amaranthus cruentus L.) to defoliation during vegetative growth is compromised during flowering.

The biochemical processes underlying variations of tolerance are often accompanied by source-sink transitions affecting carbon (C) metabolism. We investigated the tolerance of Amaranthus cruentus L. to total mechanical defoliation through development and in different growing seasons. Defoliated A. cruentus recovered ∼80% of their above-ground biomass and ∼100% of grain yield compared to intact plants if defoliation occurred early during ontogeny, but could not compensate when defoliation occurred during flowering. Tolerance index was higher in the summer season (-0.3) than in the winter season (-0.7). Overall, defoliation tolerance was closely related to phosphoenolpyruvate carboxylase (PEPC) activity in leaves and the subsequent accumulation of starch (∼500 µmol/gDW) and sucrose (∼140 µmol/gDW) in stems and roots. Thus, A. cruentus accumulated sufficient C in roots and stem to allow branching and shoot re-growth after defoliation, but it only possessed sufficient C reserves to maintain <19% seed yield in the absence of new vegetative tissue. Seed size was larger during the warm season but it was not affected by foliar damage. Seed chemical composition was altered by defoliation at flowering. We conclude that A. cruentus defoliation tolerance depends on both, the re-allocation of starch from stem and roots, and the activation of dormant meristems before flowering to generate new photosynthetic capacity to sustain seed filling.

Grain amaranths are defoliation tolerant crop species capable of utilizing stem and root carbohydrate reserves to sustain vegetative and reproductive growth after leaf loss.

Tolerance to defoliation can be defined as the degree to which productivity is affected by photosynthetic area reduction. This trait was studied in grain amaranth (Amaranthus cruentus and A. hypochondriacus), which are considered to be a highly defoliation-tolerant species. The physiological and biochemical responses to increasing levels of mechanical leaf removal up to total defoliation were quantified. Tolerance appeared to be dependent on various factors: ( i) amount of lost tissue; (ii) mechanics of leaf tissue removal; (iii) environment, and (iv) species tested. Thus, grain amaranth was found to be a highly tolerant species under green-house conditions when leaf tissue loss was performed by gradual perforation. However, tolerance was compromised under similar conditions when defoliation was done by gradual cutting of the leaf. Also tolerance in completely defoliated plants tended to decrease under field conditions, where differences between A. cruentus and A. hypochondriacus were observed. All non-structural carbohydrate (NSC) levels were reduced in stems and roots of totally defoliated amaranths one day after treatment. Such depletion probably provided the carbon (C) resources needed to sustain the early recovery process in the absence of photosynthetic capacity. This was corroborated by shading of intact plants, which produced the same rapid and drastic reduction of NSC levels in these tissues. These results emphasize the role of stored NSCs, particularly starch, in buffering the impact of severe defoliation in amaranth. The fall in sucrose synthase and cell wall invertase activity observed in stems and roots soon after defoliation was consistent with their predicted shift from sink to source tissues. It is concluded that mobilization of C stores in stems and roots, is a physiologically important trait underlying tolerance to defoliation in grain amaranth.

Bemisia tabaci-infested tomato plants show a phase-specific pattern of photosynthetic gene expression indicative of disrupted electron flow leading to photo-oxidation and plant death.

A suppression-subtractive-hybridization (SSH) strategy led to the identification of several genes whose expression was differentially modified in response to different larval phases present during the infestation process of tomato plants (Solanum lycopersicum) by virus-free whitefly Bemisia tabaci (Bt). The findings regarding photosynthetic gene expression were in accordance to previous studies reporting altered patterns of expression as a result of insect herbivory. However, the examination, in this study, of four stages of larval Bt development permitted the detection of phase-dependent changes in gene expression which appeared to target specific photosynthetic complexes. Thus, an upregulation of photosystem II genes in the latter two phases of Bt development contrasted with a general repression of genes belonging to the three other photosynthetic complexes, in addition to a number of genes coding for proteins associated with the oxygen evolving complex and the Calvin cycle. We propose that the contrasting pattern of expression led to an over-excitation of PSII and consequent oxidative damage, as suggested by the concomitant upregulation of oxidative stress genes, and could have contributed to the wide-spread necrosis observed in Bt-infested tomato plants at late stages of the plant-insect interaction.

Unraveling the network: Novel developments in the understanding of signaling and nutrient exchange mechanisms in the arbuscular mycorrhizal symbiosis.

The arbuscular mycorrhhiza (AM) symbiosis involves an intricate network of signaling and biochemical pathways designed to ensure that a beneficial relationship is established between the plant and fungal partners as a result of a mutual nutrient exchange. Emerging data has been recently published to explain why the relationship is not always fair, as observed in prevalent parasitic AM relationships in which the plant host receives no phosphorus (P) in exchange for carbon (C) delivered to the fungus. The theory behind this unorthodox view of the AM relationship, together with the description of other recent developments in nutrient mobilization as well as in key aspects of the bi-directional signaling that culminates in the symbiotic association, is the subject of this review.

The expression of the hydroxyproline-rich glycopeptide systemin precursor A in response to (a)biotic stress and elicitors is indicative of its role in the regulation of the wound response in tobacco (Nicotiana tabacum L.).

Two hydroxyproline-rich glycopeptide systemin (TobHS) precursor proteins known as preproTobHypSys-A and B were recently discovered in tobacco (Nicotiana tabacum L.) [Pearce et al. in Nature 411:817-820, 2001]. In this work, the effect of elicitors, insect damage, and abiotic stress on the expression of preproTobHypSys-A ppTobHS-A) in tobacco plants was evaluated. Foliar application of methyl jasmonate preferentially induced the systemic expression of ppTobHS-A in leaves phyllotactically one position above-treated leaves. Abscisic acid strongly induced ppTobHS-A, but water-stress did not. Mechanical wound-induction of ppTobHS-A in young plantlets was rapidly (1 h) and simultaneously detected in wounded and upper unwounded leaves, whereas in older plants induction was slow (12 h) and localized. ppTobHS-A was induced in plants infested with Bemisia tabaci or damaged by herbivory with Manduca sexta larvae. Compared to mechanical wounding, larval herbivory induced a stronger and more stable expression of ppTobHS-A. Moreover, exposure to Manduca-damaged plants induced its expression in neighboring intact plants. In most treatments, the expression patterns of ppTobHS-A coincided with those of selected wound-responsive (WR) genes (e.g., PIOX, NtPI-I, TPI). This correlation was tighter in the wounded and MeJA-treated leaves, whereas in distal, undamaged leaves, it appeared to depend on the type of WR gene examined and on the type of damage sustained by the plant. These results are consistent with the perceived role of the TobHS in defense signaling.