Activation of tomato PR and wound-related genes by a mutagenized tomato MAP kinase kinase through divergent pathways

A mitogen-activated protein kinase kinase (MAPKK) gene, tMEK2, was isolated from tomato cv. Bonny Best. By mutagenesis, a permanently active variant, tMEK2^MUT, was created. Both wild-type tMEK2 and mutant tMEK2^MUT were driven by a newly described strong plant constitutive promoter, tCUP, in a tomato protoplast transient gene expression system. Pathogenesis-related genes, PR1b1, PR3 and Twi1, and a wound-inducible gene, ER5, were activated by tMEK2^MUT. Specific inhibitors of p38 class MAPK inhibited tMEK2^MUT-induced activation of PR3 and ER5 genes but not that of the PR1b1 or Twi1 gene. Arabidopsis dual-specificity protein tyrosine phosphatase1 (DsPTP1) and maize protein phosphatase 1 (PP1) inhibited tMEK2^MUT-induced activation of the ER5 gene and the Twi1 gene, respectively, whereas PR1b1 and PR3 were not affected by either AtDsPTP1, or maize PP1, or Arabidopsis protein phosphatase 2A (PP2A). We have demonstrated for the first time that a single MAPKK activates an array of PR and wound-related genes. Our observation indicates that the activation of the genes downstream of tMEK2 occurs through divergent pathways and that tMEK2 may play an important role in the interaction of signal transduction pathways that mediate responses to both biotic (e.g. disease) and abiotic stresses (e.g. wound responsiveness).     

Induction of oxidative stress and GPX-like protein activation in tomato plants after mechanical stimulation

In Lycopersicon esculentum Mill. (cv. VFN8), mechanical stimulation induced a rapid and transient increase of with hydrogen peroxide (H₂O₂), a part of an oxidative burst. The reaction was followed by an antioxidative response, with the involvement of phospholipid hydroperoxide glutathione peroxidase (PHGPX)-like protein (EC 1.11.1.9). Induction of expression of two putative PHGPX genes was observed in rubbed internodes. To characterize the importance of this antioxidant gene, enzymatic activities of glutathione peroxidase (GPX) and PHGPX were measured, respectively, H₂O₂ and hydroperoxide lipid as oxidant. Only PHGPX activities were induced by the mechanical treatment, suggesting a major role of PHGPX in the mechanisms of antioxidant defence in plant.     

Salt stress response in tomato beyond the salinity tolerance threshold

Crop salt tolerance is generally assessed as the relative yield response to increasing root zone salinity, expressed as soil (EC_e) or irrigation water (EC_w) electrical conductivity. Alternatively, the dynamic process of salt accumulation into the shoot relative to the shoot biomass has also been considered as a tolerance index. These relationships are graphically represented by two intersecting linear regions, which identify (1) a specific threshold tolerance, at which yield begins to decrease, and (2) a declining region, which defines the yield reduction rate. Although the salinity threshold is intuitively a critical parameter for establishing plant salt tolerance, we focused our interest on physiological modifications that may occur in the plant at salinity higher than the so-called tolerance threshold. For this purpose, we exposed hydroponically grown tomato plants to eight different salinity levels (EC = 2.5 (non-salinized control); 4.2; 6.0; 7.8; 9.6; 11.4; 13.2; 15.0 dS m⁻¹). Based on biomass production, water relations, leaf ions accumulation, leaf and root abscisic acid and stomatal conductance measurements, we were able to identify a specific EC value (approximately 9.6 dS m⁻¹) at which a sharp increase of the shoot and root ABA levels coincided with (1) a decreased sensitivity of stomatal response to ABA; (2) a different partitioning of Na⁺ ions between young and mature leaves; (3) a remarkable increase of the root-to-shoot ratio. The specificity and functional significance of this response in salt stress adaptation is discussed.     

Localization of proton-ATPase genes expressed in arbuscular mycorrhizal tomato plants

In arbuscular mycorrhizal symbioses, solutes such as phosphate are transferred to the plant in return for photoassimilates. The uptake mechanism is probably facilitated by a proton gradient generated by proton H⁺-ATPases. We investigated expression of Lycopersicon esculentum Mill. H⁺-ATPases in mycorrhizal and non-mycorrhizal plants to determine if any are specifically regulated in response to colonization. Tissue expression and cellular localization of H⁺-ATPases were determined by RNA gel blot analysis and in situ hybridization of mycorrhizal and non-mycorrhizal roots. LHA1, LHA2, and LHA4 had high levels of expression in roots and were expressed predominantly in epidermal cells. LHA1 and LHA4 were also expressed in cortical cells containing arbuscules. The presence of arbuscules in root sections was correlated with lower levels of expression of these two isoforms in the epidermis. These results suggest that LHA1 and LHA4 expression is decreased in epidermal cells located in regions of the root that contain arbuscules. This provides evidence of differential regulation between molecular mechanisms involved in proton-coupled nutrient transfer either from the soil or fungus to the plant.     

Oxidative stress injury in tomato plants induced by supplemental UV-B radiation

Tomato (Lycopersicon esculentum Mill. cv. PKM 1) plants growing under field conditions were exposed for 15 d to solar radiation with UV-B component (280 - 320 nm) enhanced to 6.3 kJ m-2 d-1. This simulated a 15% stratospheric ozone depletion over Madurai (9° 50′ N latitude). Lipid peroxidation in the leaves of UV-B treated plants was 32% higher compared to the control. Superoxide dismutase (SOD) and catalase activities registered parallel promotion by 126 and 50 %, respectively, in the UV-B treated plants. Further, the contents of total phenols and anthocyanins in the leaves have also been enhanced by 40 and 156%, respectively. On the contrary, polyphenol oxidase activity demonstrated a 58 % inhibition in the leaves of UV-B treated plants. While anthocyanins and phenols are proposed to act as antioxidants, the reduction in polyphenol oxidase activity may maintain the turnover of phenols in the UV-B treated plants. This revised version was published online in July 2006 with corrections to the Cover Date.     

Inhibition of ethylene synthesis in tomato plants subjected to anaerobic root stress

Enhanced ethylene production and leaf epinasty are characteristic responses of tomato (Lycopersicon esculentum Mill.) to waterlogging. It has been proposed (Bradford, Yang 1980 Plant Physiol 65: 322-326) that this results from the synthesis of the immediate precursor of ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC), in the waterlogged roots, its export in the transpiration stream to the shoot, and its rapid conversion to ethylene. Inhibitors of the ethylene biosynthetic pathway are available for further testing of this ACC transport hypothesis: aminooxyacetic acid (AOA) or aminoethoxyvinylglycine (AVG) block the synthesis of ACC, whereas CO²⁺ prevents its conversion to ethylene. AOA and AVG, supplied in the nutrient solution, were found to inhibit the synthesis and export of ACC from anaerobic roots, whereas Co²⁺ had no effect, as predicted from their respective sites of action. Transport of the inhibitors to the shoot was demonstrated by their ability to block wound ethylene synthesis in excised petioles. All three inhibitors reduced petiolar ethylene production and epinasty in anaerobically stressed tomato plants. With AOA and AVG, this was due to the prevention of ACC import from the roots as well as inhibition of ACC synthesis in the petioles. With Co²⁺, conversion of both root- and petiole-synthesized ACC to ethylene was blocked. Collectively, these data support the hypothesis that the export of ACC from low O₂ roots to the shoot is an important factor in the ethylene physiology of waterlogged tomato plants.     

MicroRNAs in tomato plants

MicroRNAs (miRNAs) are a specialized class of small silencing RNAs that regulate gene expression in eukaryotes. In plants, miRNAs negatively regulate target mRNAs containing a highly complementary sequence by either mRNA cleavage or translational repression. As a model plant to study fleshy fruit ripening, miRNA studies in tomato have made great progress recently. MiRNAs were predicted to be involved in nearly all biological processes in tomato, particularly development, differentiation, and biotic and abiotic stress responses. Surprisingly, several miRNAs were verified to be involved in tomato fruit ripening and senescence. Recent studies suggest that miRNAs are related to host-virus interactions, which raises the possibility that miRNAs can be used as diagnostic markers for response to virus infection in tomato plants. In this review, we summarize our current knowledge systematically and advance future directions for miRNA research in tomato.     

Early responses of drought-resistant and -susceptible tomato plants subjected to water stress

Three-week-old seedlings of one drought-susceptible tomato cultivar (Lycopersicon esculentum cv. “New Yorker”) and two drought-resistant species of tomato (Solanum pennellii andLycopersicon chilense) were subjected to various degrees of PEG 8000-induced water stress from ?0.017 to ?1.0 MPa for a duration of 24 h so that their early responses to water stress could be compared. Such a comparison would determine if there was a relationship to root cytokinin levels following sudden induction of water stress in the drought-resistant species. Transpiration rates of leaves were monitored throughout the 24-h period, shoots were evaluated for leaf water potential (LWP), and roots were extracted for levels oft-zeatin riboside (t-ZR) and dihydrozeatin riboside (DHZR) using a monoclonal antibody enzyme immunoassay. Transpiration rates were evaluated gravimetrically by difference every 6 h up to 24 h. Transpiration rate decreased with increasing PEG levels and passage of time in all three species, measured at 6 and 12 h, logarithmically in the case of the twoLycopersicon species and linearly in the case ofSolanum. From 12–18 h (while plants were in darkness), transpiration rate was a function of the level of PEG only and not time in all three species. When light resumed from 18–24 h, only 5.pennellii showed no further decrease in transpiration rate over time with increasing PEG. Drought-susceptibleL. esculentum had a stronger linear decrease in LWP with increasing PEG 8000 concentration than the other two species.L. esculentum also had a higher initial transpiration rate than did either of the drought-resistant species. The two drought-resistant species showed less change in LWP with 5.pennellii having a small decrease andL. chilense having little change. OnlyS. pennellii exhibited a decrease in roott-ZR levels, which may imply a role for root cytokinin within the first 24-h exposure to water stress in this species.L. esculentum exhibited no change in roott-ZR. The levels oft-ZR inL. chilense were less than that ofL. esculentum but showed only a slight decrease with increasing PEG.S. pennellii andL. chilense, although both drought-resistant tomato species, showed different patterns of response with respect to pattern of decline in transpiration rate, LWP, and roott-ZR levels.     

Overexpression of GlyI and GlyII genes in transgenic tomato (Solanum lycopersicum Mill.) plants confers salt tolerance by decreasing oxidative stress

The glyoxalase system plays an important role in various physiological processes in plants, including salt stress tolerance. We report the effects of overexpressing glyoxalase I and glyoxalase II genes in transgenic tomato (Solanum lycopersicum Mill.) cv. Ailsa Craig. Stable expression of both transgenes was detected in the transformed tomato plants under salt stress. The transgenic lines overexpressing GlyI and GlyII under a high NaCl concentration (800 mM) showed reduced lipid peroxidation and the production of H₂O₂ in leaf tissues. A greater decrease in the chlorophyll a+b content in wild-type (WT) compared with transgenic lines was also observed. These results suggest that the over expression of two genes, GlyI and GlyII, may enhance salt stress tolerance by decreasing oxidative stress in transformed tomato plants. This work will help our understanding of the putative role of the glyoxalase system in the tolerance to abiotic stress in tomato plants.     

Salt stress increases ferredoxin-dependent glutamate synthase activity and protein level in the leaves of tomato

Ferredoxin-dependent glutamate synthase (EC 1.4.7.1) catalyzes an essential step in the pathway of glutamate biosynthesis. Exposing detached tomato ( Lycopersicon esculentum ) leaves for 6 h to 12 g l⁻¹ NaCl resulted in a significant two-fold increase in the activity of ferredoxin-dependent glutamate synthase extracted from the leaves. Western blot studies demonstrated that salt treatment also increased the ferredoxin-dependent glutamate synthase content of the leaves. A similar effect of salt on the concentration of this enzyme was found in the leaves of hydroponically-grown tomato plants. The induction of ferredoxin-dependent glutamate synthase under salt stress may provide the glutamate required for the proline synthesis which is a common response to salt stress.     

Response of tomato plants to chilling stress in association with nutrient or phosphorus starvation

The experiments were conducted on two tomato cultivars: Garbo and Robin. Mineral starvation due to plant growth in 20-fold diluted nutrient solution (DNS) combined with chilling reduced the rate of photosynthesis (P _N) and stomatal conductance (g) to a greater extent than in plants grown in full nutrient solution (FNS). In phosphate-starved tomato plants the P _N rate and stomatal conductance decreased more after chilling than in plants grown on FNS. In low-P plants even 2 days after chilling the recovery of CO₂ assimilation rate and stomatal conductance was low. A resupply of phosphorus to low-P plants (low P + P) did not improve the rate of photosynthesis in non-chilled plants (NCh) but prevented P_N inhibition in chilled (Ch) plants. The greatest effect of P resupply was expressed as a better recovery of photosynthesis and stomatal conductance, especially in non-chilled low P + P plants. The F _v/F _m (ratio of variable to maximal chlorophyll fluorescence) decreased more during P starvation than as an effect of chilling. Supplying phosphorus to low-P plants caused the slight increase in the F _v/F _mratio. In conclusion, after a short-term chilling in darkness a much more drastic inhibition of photosynthesis was observed in nutrient-starved or P-insufficient tomato plants than in plants from FNS. This inhibition was caused by the decrease in both photochemical efficiency of photosystems and the reduction of stomatal conductance. The presented results support the hypothesis that tomato plants with limited supply of mineral nutrients or phosphorus are more susceptible to chilling.     

Brassinosteroid-induced epinasty in tomato plants

The effects of root treatments of brassinosteroid (BR) on the growth and development of hydroponically grown tomato plants (Lycopersicon esculentum Mill cv Heinz 1350) were evaluated. There was a dramatic increase in petiole bending when the plants were treated with 0.5 to 1.0 micromolar BR. The leaf angle of the treated plants was almost three times that of untreated controls. BR-induced epinasty appeared to be due to stimulation of ethylene production. Excised petioles from BR-treated plants produced more than twice as much ethylene as did untreated controls. As ethylene production increased, the degree of petiole bending also increased, and inhibition of ethylene production by AOA or CoCl₂ also inhibited epinasty. BR-treated plants had increased levels of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) in the leaf tissue. ACC appeared to accumulate primarily in the petioles with the greatest amount of ACC accumulating in the youngest petioles. Time course evaluations revealed that BR treatment stimulated ACC production. As ACC accumulated, ethylene increased, resulting in epinasty. Little or no ACC was found in the xylem sap, indicating that there was a signal transported from the roots which stimulated ACC synthesis in the leaf tissue.     

Responses of antioxidant systems to lanthanum nitrate treatments in tomato plants during drought stress

Abstract

Lanthanum is one of the most abundant elements in rare earths enriched fertilizers and is supposed to be one of the main responsible of the effects of such fertilizers on crops. In this work, the effect of lanthanum nitrate on H₂O₂ production, lipid peroxidation, ascorbate and glutathione content, and on the activity of cytosolic ascorbate peroxidase, dehydroascorbate reductase, monodehydroascorbate reductase, and glutathione reductase in Lycopersicon esculentum L. cv. Marmande during drought stress was evaluated. The results confirmed that treatments of tomato plants with lanthanum nitrate affect the antioxidant cellular defences and that lanthanum toxicity is dependent on the way of treatment. The stimulation of antioxidant systems did not induce any improvement in drought stress responses in tomato but seemed to be only a consequence of the unbalance in cell metabolism due to the treatment with lanthanum nitrate.     

Distribution of calcium in tomato plants in response to heat stress and plant growth regulators

Two cultivars of tomato with contrasting response to elevated temperature were compared: sensitive-Roma and tolerant-Robin. Experiments were done on fruit explants and on rooted cuttings with small fruits. In both cases⁴⁵Ca was poorly transported to the fruits. Nevertheless in fruit explants elevated temperature (40°C) increased⁴⁵Ca import into the fruits in both cultivars. In the compared cuttings, treated or not treated with growth regulators and at various temperatures, the greatest differences were observed in the amount of⁴⁵Ca transported to the fruits. Sensitive Roma cuttings scantily supplied their fruits with⁴⁵Ca both under optimal temperature and heat stress. In plants previously treated with NOA+GA₃ high temperature increased⁴⁵Ca transport to the fruits. Robin cuttings inversely responded to heat stress by transporting a much higher portion of⁴⁵Ca to the fruits, both in control and NOA+GA₃ cuttings.The diversity of⁴⁵Ca distribution during elevated temperature in cuttings, but not in fruit explants of both cultivars seems to be connected with an ability to control calcium supply to fruit or at least to prevent its decrease; this mechanism is perhaps located outside the cluster.     

Exchange of oxygen and its role in energy dissipation during drought stress in tomato plants

To elucidate how excess light energy is dissipated during water deficit, net photosynthesis (P_N), stomatal conductance (g_s), intercellular CO₂ concentration (c_i) and Chl a fluorescence were investigated in control and drought-stressed tomato plants ( Lycopersicon esculentum ). Gross O₂ evolution (E_o) and gross O₂ uptake (U_o) were determined by a mass spectrometric ¹⁶O/¹⁸O₂ isotope technique. Under drought stress P_N, g_s, c_i and U_o decline. While photochemical fluorescence quenching decreases under water deficit, non-photochemical quenching rises. The maximal efficiency of PSII measured in the dark is not affected by drought; however, in the light, E_o decreases under water deficit. The ratio P_N/E_o falls under stress while the ratio U_o/E_o increases. We conclude that tomato plants follow a double strategy to avoid photodamage under drought stress conditions: (1) a substantial portion of light energy is emitted as heat and PSII activity is downregulated. This results in a decrease in E_o as well as P_N and U_o. Despite reduced charge separation at PSII, the decline of CO₂ assimilation because of lowered stomatal conductance and metabolic changes results in the need of degrading excessive photosynthetic electrons. (2) Oxygen is used as an alternative electron acceptor in photorespiration or Mehler reaction and U_o rises relative to E_o.     

Salt tolerance of four tomato hybrids

Summary A greenhouse lysimeter experiment was conducted to evaluate the response of tomato hybrids to varying levels of salinity. Four tomato hybrids F-172, F-150, Bornia and Diego were grown at four salinity levels. The soil was salinized prior to transplanting by irrigating with waters that were prepared by adding NaCl to the tap water. The electrical conductivities of the irrigation waters were 1.8, 4.5, 7.0, and 9.5 dS/m at 25°C. Yield, fruit quality, and leaf mineral composition were measured. Fifty percent fruint yield reduction for all hybrids was associated with a soil salinity of 5.1 dS/m. Each unit increase in salinity above 2 dS/m reduced yield by 14%. This indicates that these tomato hybrids are more salt sensitive than the older varieties. Fruit quality and leaf mineral composition were also affected by salinity and hybrid.