Diurnal nitrate uptake in young tomato (Lycopersicon esculentum Mill.) plants: test of a feedback-based model

A simple model is proposed to describe diurnal net nitrate uptake rate patterns observed experimentally on young plants grown under constant non-limiting nutrition. It rests on two hypotheses: net uptake rate is under negative feedback control by internal plant nitrate content, and nitrogen metabolism occurs only during the light period. The model parameters were determined from the results of three independent experiments performed under non-disturbing conditions in a growth room at constant air and solution temperatures. Net hourly nitrate uptake rate was measured through a diurnal cycle and after an extended 28 h period of darkness. It increased continuously during the light period and decreased during the dark period. Under prolonged darkness, net uptake declined to an asymptotic positive uptake rate of about 10^-5 mol h^-1 g^-1 total plant dry weight. The measured hourly nitrate uptake rate values were consistent with independent determinations of long-term nitrate and total N accumulations in the plant. Realistic simulations of experimental data are achieved with the proposed model. Furthermore, the maintenance of a positive net uptake rate, measured in non-growing plants subjected to prolonged darkness, is explained in the model by the continuous increase of plant water content. The importance of the diurnal variations of plant water content for nitrate uptake rate is emphasized and gives consistency to the homeostasis hypothesis of the model. The diurnal changes in nitrate uptake predicted by the model are strongly dependent on the assumption made for diurnal changes in nitrate assimilation. While the purely photosynthetic assumption is convenient, a more realistic metabolism sub-model is needed.     

Physiological response and yield of paclobutrazol treated tomato plants (Lycopersicon esculentum Mill.)

Experiments were conducted to study the physiologicaleffect of the plant growth retardant paclobutrazol(PBZ) and its impact on the yield of tomato plants(cv. Precador). Seedlings were treated at the time of prickingout with soil and foliar applications of PBZ atconcentrations of 1.0 and 25.0 mg l^-1respectively. The results established that:-- The reduced height and the increased thickness ofthe young plant stem, as well as the accelerated rootformation are a significant advantage of the PBZtreatment, contributing to the improvement of seedlingquality at planting.-- Soil treatment (1 mg l^-1) and foliar treatment(25 mg l^-1) with PBZ improves the photosyntheticactivity and water balance of tomato cv. Precador.-- PBZ accelerates fruit formation and increases earlyfruit yield.-- The concentrations of the retardant used and themode of its application ensure the production offruits without any residual retardant and harmless tohuman health from a phytosanitary point of view.     

Phosphorus use efficiency in anthocyanin-free tomato (Lycopersicon esculentum Mill.)

An anthocyanin-free tomato plant, H957, and its parental wild type, H883, were hydroponically grown to test for tolerance to a low phosphorus (P) in H957. The tolerance was evaluated by comparing growth and metabolism of H957 vs. H883 at different P concentrations ranging 25–400 μM. Fresh weights were measured weekly. Dry weight, mineral contents, photosynthetic rate, and P utilization ratios of the plants were measured after five weeks of growth in the hydroponic culture. Although the growth of both varieties was severely impaired at 25 μM P, H957 showed a greater fresh weight and dry weight at 50–400 μM P. H957 showed a higher net photosynthetic rate on older leaves while both varieties showed similar photosynthetic rate on young leaves. H957 tissue contains an overall lower P concentration in its tissue than H883. These observations together indicate that the anthocyaninless mutant H957 tolerate to lower P concentration. It does so by utilizing internal P with better efficiency rather than by absorbing external P better.     

Phytochrome-mediated induction of phenylalanine ammonia-lyase in the cotyledons of tomato (Lycopersicon esculentum Mill.) plants

Phenylalanine ammonia-lyase (PAL; EC 4.3.1.5.) induction in cotyledons from 96-h dark-grown Lycopersicon esculentum Mill. was studied in response to continuous light and hourly light pulses (blue, red, far red). The increases of PAL promoted by blue and red pulses are reversed completely by immediately following 758 nm irradiations. The response to continuous red light could be substituted for by hourly 6-min red light pulses. The effect of continuous red treatments is mainly due to a multiple induction effect of phytochrome. In contrast to red light, hourly light pulses with far red and blue, light can only partially substitute for continuous irradiation. The continuous blue response could be due to a combination of a multiple induction response and of a high irradiance response of phytochrome. The continuous far red response, could represent a high irradiance response of phytochrome. Dichromatic irradiations indicate that phytochrome is the photoreceptor controlling the light response (PAL) in tomato seedlings.Abbreviations Norflurazon NF-4-chloro-5-(methylamino)-2-(,,,-trifluoro-m-tolyl)-3 (2H) pyridazinone - PAL phenylalanine ammonia-lyase - phytochrome photoequilibrium P_fr/P_tot - P_fr far-red absorbing form of phytochrome - P_r red absorbing form of phytochrome - P_tot total phytochrome: P_r+P_fr     

Identification and distribution of profilin in tomato (Lycopersicon esculentum Mill.)

Profilin is a G-actin monomer-binding protein which has been shown to participate in actin-based tipgrowth of animal cells. The abundance of profilin in pollen and its occurrence in several vegetable foods raises the question of the role of profilin in plants. First, its distribution throughout various parts of the plant needs to be determined. This paper describes observations on the presence of profilin in the tomato plant (Lycopersicon esculentum Mill.). The distribution of profilin in flower buds, stems, leaves, roots, and fruits of tomato was determined by immunoblotting and by tissue printing, showing that profilin is present in most if not all parts of the tomato plant.We gratefully acknowledge the help provided by Dr. A.T. Jagendorf and the donation of tomato seeds and maize pollen by N. Eanetta and Dr. M. Smith, respectively. The use of Dr. R. Wayne's SZH ILLD dissecting microscope is gratefully acknowledged. This work was aided by helpful discussions with C.S. Combs, Dr. C.A. Conley, and Dr. J. Andersland. This work was supported by a Hatch grant and NRI Competitive Grants Program/USDA 94-37304-1046 to MVP. This material is based upon work supported under a National Science Foundation Graduate Research Fellowship to DWD.     

Isolation and characterization of nitrate reductase-deficient mutants in tomato (Lycopersicon esculentum Mill.).

Summary Five nitrate reductase-deficient mutants of tomato were isolated from an M2 population after ethyl-methanesulphonate (EMS) seed treatment by means of selection for chlorate resistance. All mutations were monogenic and recessive and complementation analysis revealed that they were non-allelic. Biochemical and molecular characterization of these mutants showed that four of them are cofactor mutants while one is an apoenzyme mutant.     

The histogenesis of somaclones from tomato (Lycopersicon esculentum Mill.) cotyledons

Summary A histological study ofin vitro cultured cotyledonary expiants of tomato (Lycopersicon esculentum) was performed in order to determine the site (differentiated tissue or developing callus) and the mode of plant regeneration.Results have shown that callus develops at the excision sites of cotyledonary expiants and that shoots are formed exclusively within the unorganized callus: excision areas are the only morphogenetic sites and the proximal excision is the preferred site for plant regeneration.Shoots differentiate by organogenesis within the superficial region of the callus. Few neocambial cells cooperate in the neoformation. Origin from a single cell is highly unlikely since rarely observed single activated cells never developed into shoots.Regenerated plants may be chimeras if invitro culture induces genetic diversity in the initial cells.Abbreviations IAA Indole-3-acetic acid - c callus - d vegetative dome - s shoot - ad adaxial - ab abaxial - t tracheid - p parenchyma - S sieve tube     

Characterization of the subtilase gene family in tomato (Lycopersicon esculentum Mill.)

The gene family of subtilisin-like serine proteases (subtilases, SBTs) in tomato (Lycopersicon esculentum Mill.) comprises at least 15 members, 12 of which have been characterized in this study. Sequence comparison revealed that tomato subtilases fall into 5 distinct subfamilies. Single genes were shown to exist for LeSBT1, LeSBT2 and tmp, while 5 and 6 genes were found in the LeSBT3/4 and P69 subfamilies, respectively. With the exception of tmp, tomato subtilase genes were found to lack introns. Expression of subtilase genes was confirmed at the mRNA level by northern blot analysis and/or by primer extension experiments. For each of the 5 subtilase subfamilies, a distinctive pattern of expression was observed in tomato organs. At least one of the subtilases was found to be expressed in each organ analysed. Structural features evident from deduced amino acid sequences are discussed with reference to the related mammalian proprotein convertases.     

Role of extensin peroxidase in tomato (Lycopersicon esculentum Mill.) seedling growth

 It is proposed that inhibition of extensin peroxidase activity leads to a less rigid cell wall and thus promotes cell expansion and plant growth. A low-molecular-weight inhibitor derived from the cell walls of suspension-cultured tomato cells was found to completely inhibit extensin peroxidase-mediated extensin cross-linking in vitro at a concentration of 260 μg/ml. The inhibitor had no effect upon guaiacol oxidation catalyzed by extensin peroxidase or horseradish peroxidase. We have demonstrated that the light-irradiated inhibition of plant growth may be partially offset by inhibition of endogenous extensin peroxidase activity. Overall plant growth was enhanced by up to 15% in the presence of inhibitor relative to control plants. Inhibitor-treated and illuminated tomato hypocotyls grew up to 15% taller than untreated controls. The inhibitor had no effect upon etiolated plants over a 15-d period, suggesting that only low levels of peroxidase-mediated cross-linking can be found in the cell walls of etiolated plants. SDS-PAGE/Western blots of ionically bound protein from both etiolated and illuminated hypocotyls identified a doublet at 57/58.5 kDa which is immuno-reactive with antibodies raised to tomato extensin peroxidase. Levels of the 58.5-kDa protein, determined by SDS-PAGE, were at least threefold higher in illuminated tomato hypocotyls than in etiolated hypocotyls. Three fold higher levels of extensin peroxidase, elevated in-vitro extensin cross-linking activity and 15% higher levels of cross-linked, non-extractable extensin were observed in illuminated tomato hypocotyls compared with etiolated tomato hypocotyls. This suggests that white-light inhibition of tomato hypocotyl growth appears to be mediated, at least partially, by deposition of cell wall extensin, a process regulated by M_r-58,500 extensin peroxidase. Our results indicate that the contribution of peroxidase-mediated extensin deposition to plant cell wall architecture may have an important role in plant growth. Received: 22 July 1999 / Accepted: 11 October 1999     

Purification and characterization of a glucokinase from young tomato (Lycopersicon esculentum L. Mill.) fruit

In order to clearly establish the properties of the enzymes responsible for hexose phosphorylation we have undertaken the separation and characterization of these enzymes present in tomato fruit (Martinez-Barajas and Randall 1996). This report describes the partial purification and characterization of glucokinase (EC. 2.7.1.1) from young green tomato fruit. The procedure yielded a 360-fold enrichment of glucokinase. Tomato fruit glucokinase is a monomer with a molecular mass of 53 kDa. Glucokinase activity was optimal between pH 7.5 and 8.5, preferred ATP as the phosphate donor (K _m = 0.223 mM) and exhibited low activity with GTP or UTP. The tomato fruit glucokinase showed highest affinity for glucose (K _m = 65 μM). Activity observed with glucose was 4-fold greater than with mannose and 50-fold greater than with fructose. The tomato fruit glucokinase was sensitive to product inhibition by ADP (K _i = 36 μM). Little inhibition was observed with glucose 6-phosphate (up to 15 mM) at pH 8.0; however, at pH 7.0 glucokinase activity was inhibited 30–50% by physiological concentrations of glucose 6-phosphate. Received: 4 October 1997 / Accepted: 10 January 1998     

New cases of somatic conversion (paramutation) in tomato (Lycopersicon esculentum Mill.)

Summary The genetic behaviour of three chlorophyll variegated F₁ tomato plants, derived from irradiated gametophytes, was analyzed over several generations of selfing and backcrossing. The results suggest that irradiation has put genes, different in all three mutants, into a labile state, n*, remaining so after fertilization. This state had the power of converting the associated wild allele N into a deficient form.Somatic conversion was soon followed, in Plant C₁₁ always and nearly always in Plant C₁₂, by stabilization of both alleles in a conversion-inactive recessive state, genetically similar, and stable except in special conditions.In the other type, found seldom in C₁₂, always in C₆, the n* state was permanent and transmissible. Conversion occurred with a certain frequency determined by developmental and genotypic influences, and the converted allele also acquired conversion power, so that gametes from an N n* plant were of three kinds: N, n* and n*. This process corresponds to paramutation (Brink 1958).Results were compared and contrasted with other published data.Contribution nr. 748 of the Biology Division of Euratom.     

Purification and characterization of fructokinase from developing tomato (Lycopersicon esculentum Mill.) fruits

A procedure is described which allows the purification of fructokinase (EC 2.7.1.4) from young tomato fruit. The procedure yielded a 400-fold purification and two isoenzymes designated fructokinase I and II (FKI and FKII) were separated by anion-exchange chromatography. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) the molecular mass was estimated to be 35 kDa. Gel filtration on Sepharose-12 indicated that for both fructokinases the functional form is a dimer. Two dimensional isoelectric focusing/SDS-PAGE combined with immunoblotting showed that FKI has two components with isoelectric points (pIs) of 6.42 and 6.55, while four components with pIs from 6.07 to 6.55 were detected for FKII. A mixture of both fructokinases showed that the components of FKI match the more alkaline components of FKII. The activity of both fructokinases increased with increasing pH to around 8.0 and equal activity was observed from 8.0 to 9.5. Both fructokinases were specific for fructose with K _m values for fructose of 0.131 and 0.201 mM for FKI and FKII, respectively. At high concentrations (> 0.5 mM), fructose was also a strong inhibitor with inhibition constants (K _i) of 1.82 and 1.39 mM for FKI and FKII, respectively. The preferred phosphate donor for both isoforms was ATP, and K _m values of 0.11 and 0.15 mM were observed for FKI and FKII. At low concentrations (0.05–0.2 mM), fructose exhibited noncompetitive inhibition with respect to ATP for both fructokinases. This inhibition pattern changed to uncompetitive when higher fructose concentrations (0.5–10 mM) were used. These data indicated that substrate addition is ordered, with ATP adding first. Inhibition by ADP was also affected by the fructose concentrations. At 0.5 mM fructose, FKI showed non-competitive inhibition by ADP with respect to ATP and this inhibition changed to uncompetitive when 3 mM fructose was used. The isoform FKII showed a competitive inhibition pattern for ADP at 0.5 mM fructose which also changed to uncompetitive when 3 mM fructose was used. The features of the regulation of both fructokinases suggest that this enzyme might have a relevant role in carbon metabolism during tomato fruit development.     

Effect of salinity on tomato (Lycopersicon esculentum Mill.) during seed germination stage

A study was conducted using ten genetically diverse genotypes along with their 45F₁ (generated by diallel mating) under normal and salt stress conditions. Although, tomato (Lycopersicon esculentum Mill.) is moderately sensitive to salinity but more attention to salinity is yet to be required in the production of tomato. In present study, germination rate, speed of germination, dry weight ratio and Na⁺/K⁺ ratio in root and shoot, were the parameters assayed on three salinity levels; control, 1.0 % NaCl and 3.0 % NaCl with Hoagland’s solution. Increasing salt stress negatively affected growth and development of tomato. When salt concentration increased, germination of tomato seed was reduced and the time needed to complete germination lengthened, root/shoot dry weight ratio was higher and Na⁺ content increased but K⁺ content decreased. Among the varieties, Sel-7 followed by Arka Vikas and crosses involving them as a parent were found to be the more tolerant genotypes in the present study on the basis of studied parameters.     

Chromoplast-Targeted Proteins in Tomato (Lycopersicon esculentum Mill.) Fruit

The chloroplast to chromoplast transition during tomato (Lycopersicon esculentum Mill.) fruit ripening is characterized by a dramatic change in plastid structure and function. We have asked whether this process is mediated by an increase in the steady-state level of RNA for plastid targeted proteins. Assays for import of radiolabeled translation products into isolated pea (Pisum sativum L.) chloroplasts were used to monitor levels of chromoplast-targeted proteins at four stages of tomato fruit development. We have found striking increases during development in levels of translatable RNA for two such proteins. Additionally, the import of in vitro translation products was examined for seven individual cDNA clones known to encode RNA that increase during fruit ripening. Three of these clones produced in vitro translation products that were imported into pea chloroplasts. This implies that there is synthesis and import of new proteins during the transition from chloroplast to chromoplast and that the plastid conversion is an active developmental program rather than a simple decline in synthesis of the photosynthetic apparatus. Furthermore, our results demonstrate the utility of this method for identification of structural genes involved in plastid morphogenesis.