Impact of potato psyllid (Hemiptera: Triozidae) feeding on free amino acid composition in potato

Abstract The impacts of potato psyllid (Bactericera cockerelli) feeding on potato foliage on the free amino acids (FAAs) composition in potato leaf and tubers were determined under greenhouse conditions. The free amino acids in plant extracts were separated by high‐performance liquid chromatography, and in both leaf and tuber samples, at least 17 FAAs were detected. Psyllid feeding significantly changed the levels of several FAAs in both leaf and tuber samples. The concentration of leucine increased 1.5‐fold, whereas that of serine and proline increased 2‐ and 3‐fold, respectively. In contrast, the concentrations of glutamic acid, aspartic acid and lyscine were significantly reduced by 42.0%, 52.1% and 27.5%, respectively. There were also significant changes in the levels of FAAs in the Zebra chip (ZC) infected tubers compared with the healthy tubers, and the levels of six of the FAAs increased, and the levels of nine of the FAAs decreased. The results from this study indicate that potato psyllid causes major changes in free amino acid composition of plant tissues, and this change in plant metabolism may contribute to the plant stress as indicated by increased levels of proline in the leaves and hence promoting the development of plant diseases such as ZC disease.     

Translocation of Assimilates Within and Between Potato Stems

Three aspects of translocation in potato were examined: (i)translocation within stems (ii) translocation between individualstems of a plant (iii) translocation between tubers followinginjection of ¹⁴C sucrose into a single daughter tuber. Assimilatesexported from single leaves of evenly illuminated potato stemsremained confined to the same side of the stem as the sourceleaf in a pattern consistent with the internal arrangement ofvascular bundles in the stem, and tubers borne on stolons verticallybelow the source leaf contained higher concentrations of ¹⁴Cthan those on the opposite side. Consequently ¹⁴C import intothe tubers bore little relationship to tuber growth rates. However,alteration of source/sink relations by pruning stems to a singlesouce leaf resulted in an even distribution of ¹⁴C throughoutthe vascular bundles of the stem and ¹⁴C import into the tubersbore a stronger relationship to tuber growth rates than to thephyllotactic relationship of the tubers with the source leaf. Labelling one stem of a potato plant resulted in little or nomovement of ¹⁴C into tubers on other unlabelled stems. However,removal of the unlabelled stems at ground level induced a significantmovement of ¹⁴C from the labelled stem to the tubers on unlabelledstems, this movement occurring via the mother tuber. Shadingthe unlabelled stems had less effect than stem removal. ¹⁴C sucrose injected into single daughter tubers was translocatedto other tubers on the same stem and also to tubers on a secondstem at the opposite end of the mother tuber. The sucrose wasconverted to starch in these tubers. The results favour the view that each potato stem functionsas an independent unit with potential for assimilate redistributionwithin a stem but with little or no carbon exchange occurringbetween stems, unless under severely altered source/sink patterns. Assimilates, ¹⁴C, autoradiography, potato (Solanum tuberosum L.), tuber growth     

Evidence for the complex relationship between free amino acid and sugar concentrations and acrylamide‐forming potential in potato

Free amino acids and reducing sugars participate in the Maillard reaction during high‐temperature cooking and processing. This results not only in the formation of colour, aroma and flavour compounds, but also undesirable contaminants, including acrylamide, which forms when the amino acid that participates in the reaction is asparagine. In this study, tubers of 13 varieties of potato (Solanum tuberosum), which had been produced in a field trial in 2010 and sampled immediately after harvest or after storage for 6 months, were analysed to show the relationship between the concentrations of free asparagine, other free amino acids, sugars and acrylamide‐forming potential. The varieties comprised five that are normally used for crisping, seven that are used for French fry production and one that is used for boiling. Acrylamide formation was measured in heated flour, and correlated with glucose and fructose concentration. In French fry varieties, which contain higher concentrations of sugars, acrylamide formation also correlated with free asparagine concentration, demonstrating the complex relationship between precursor concentration and acrylamide‐forming potential in potato. Storage of the potatoes for 6 months at 9°C had a significant, variety‐dependent impact on sugar and amino acid concentrations and acrylamide‐forming potential.     

Growth and biomass production in potato grown in the hot tropics as influenced by paclobutrazol

Two similar field trials were carried out during 2003 in a hot tropical region of eastern Ethiopia to investigate the effect of leaf and soil applied paclobutrazol on the growth, dry matter production and assimilate partitioning in potato. A month after planting paclobutrazol was applied as a foliar spray or soil drench at rates of 0, 2, 3, and 4 kg a.i. paclobutrazol ha^–1. Plants were sampled during treatment application and subsequently 2, 4, 6 and 8 weeks after treatment application. The data was analyzed using standard growth analyses techniques. None of the growth parameters studied was affected by the method of paclobutrazol application. Paclobutrazol decreased leaf area index, crop growth rate, and total biomass production, and increased specific leaf weight, tuber growth rate, net assimilation rate, and partitioning coefficient of potato. At all harvesting stages, paclobutrazol reduced the partitioning of assimilate to the leaves, stems, and roots and stolons and increased allocation to the tubers. Although paclobutrazol decreased the total biomass production it improved tuber yield by partitioning more assimilates to the tubers. Paclobutrazol improved the productivity of potato under tropical conditions by redirecting assimilate allocation to the tubers.     

A second L-type isozyme of potato glucan phosphorylase: cloning,antisense inhibition and expression analysis

In potato tubers two starch phosphorylase isozymes, types L and H, have been described and are believed to be responsible for the complete starch breakdown in this tissue. Type L has been localized in amyloplasts, whereas type H is located within the cytosol. In order to investigate whether the same isozymes are also present in potato leaf tissue a cDNA expression library from potato leaves was screened using a monoclonal antibody recognizing both isozyme forms. Besides the already described tuber L-type isozyme a cDNA clone encoding a second L-type isozyme was isolated. The 3171 nucleotide long cDNA clone contains an uninterrupted open reading frame of 2922 nucleotides which encodes a polypeptide of 974 amino acids. Sequence comparison between both L-type isozymes on the amino acid level showed that the polypeptides are highly homologous to each other, reaching 81–84% identity over most parts of the polypeptide. However the regions containing the transit peptide (amino acids 1–81) and the insertion sequence (amino acids 463–570) are highly diverse, reaching identities of only 22.0% and 29.0% respectively.Northern analysis revealed that both forms are differentially expressed. The steady-state mRNA levels of the tuber L-type isozyme accumulates strongly in potato tubers and only weakly in leaf tissues, whereas the mRNA of the leaf L-type isozyme accumulates in both tissues to the same extent. Constitutive expression of an antisense RNA specific for the leaf L-type gene resulted in a strong reduction of starch phosphorylase L-type activity in leaf tissue, but had only sparse effects in potato tuber tissues. Determination of the leaf starch content revealed that antisense repression of the starch phosphorylase activity has no significant influence on starch accumulation in leaves of transgenic potato plants. This result indicated that different L-type genes are responsible for the starch phosphorylase activity in different tissues, but the function of the different enzymes remains unclear.     

Stable isotope distribution in the major metabolites of source and sink organs of Solanum tuberosum L.: a powerful tool in the study of metabolic partitioning in intact plants

A method was developed for the purification of main intermediates and storage products of leaves and tubers of potato for analysis of their ¹³C content. The method was tested for recovery of metabolites and carbon isotope discrimination during the purification process. Leaf metabolite δ¹³C values showed an enrichment of starch relative to sucrose and citrate. This result is in agreement with previous findings in other higher plants and indicates the existence of isotope discrimination steps during transport and metabolism of triose-phosphates in potato leaf mesophyll cells. Active anaplerotic replenishment of the tricarboxylic acid cycle in the leaves of the plants investigated was also deduced from the significant ¹³C enrichment of malate relative to citrate and asparagine/aspartate relative to glutamine/glutamate. Analysis of tuber metabolite δ¹³C values showed no difference between starch and sucrose. However, tuber sucrose appeared significantly enriched compared with leaf sucrose and also relative to tuber citrate and malate. This finding suggests the existence of sites of isotopic discrimination during sucrose processing in developing tubers. It also confirms that metabolic cycles of sucrose synthesis and breakdown and of hexose-phosphate/triose-phosphate interconversion, which have been described in excised tuber tissue, also occur in intact organs. The δ¹³C values were also used to estimate the metabolic rate of carbon oxidation in developing tubers on the assumption that pyruvate dehydrogenase is the main site of isotopic discrimination in the tuber cells. The result obtained was in agreement with the available literature, suggesting that analyses of natural isotopic distribution in plant products may be a useful tool for the study of metabolic processes and sink-source relationships in intact plants. Received: 21 May 1998 / Accepted: 10 July 1998     

Variations in the content of free and bound amino acids during dormancy and the first cell cycle in Helianthus tuberosus tuber explants

Abstract

Qualitative and quantitative analysis of free and bound amino acids and amides during dormancy and the most important phases of the first cell cycle was carried out in tubers of Helianthus tuberosus.

In the dormant tuber arginine was confirmed to be the most abundant amino acid. A high amount of asparagine was also present; on the contrary glutamine was found in very low concentrations. During the progression of dormancy, all the free amino acids and amides declined while aspartic and glutamic acid increased.

During the G₁ phase of the first cell cycle induced by 2,4-D, all the free amino acids and amides decreased with the exception of glutamic acid.

At 18, 20, 24 h of activation with 2,4-D, corresponding to the S phase and the beginning of mitosis, bound amino acids were also determined. In these phases of the cell cycle they increased reaching a maximum at 20 h; on the other hand the free amino acid and amide content, especially aspartic acid, asparagine and arginine, decreased with the exception of glutamic acid, alanine and phenylalanine.     

Improving the nutritive value of tubers: elevation of cysteine and glutathione contents in the potato cultivar White Lady by marker-free transformation

The amino acids that limit the nutritive value of potato are the sulfur containing amino acids methionine and cysteine. Manipulation of the targeted amino acid biosynthesis is a way to circumvent this problem. Cysteine is synthesised from O-acetyl-l-serine formed by serine acetyltransferase (SAT). To increase the cysteine content of the commercial potato cultivar White Lady the chimeric SAT-coding cysE gene from Escherichia coli under the control of the constitutive CaMV 35S promoter and fused to the chloroplast targeting rbcS 5'-transit peptide sequence was introduced into the White Lady genome. Novelty of the approach was the application of marker-free transformation. Two transgenic lines were obtained that accumulated the cysE mRNA in high amounts. Crude leaf extracts of these plants exhibited up to 80- and 20-fold higher SAT activity in leaves and tubers, respectively, than those prepared from non-transformed plants. Levels of cysteine and glutathione both in leaves and tubers were 1.5-fold higher in average than in control plants. The alterations observed had no effect on tuber yield and sprouting behaviour. Gas chromatography coupled to mass spectrometry showed that all other amino acids than cysteine were unaffected. Here we demonstrate for the first time that the cysteine content of tubers can be enhanced by metabolic engineering.     

Polyphenol oxidase in potato. A multigene family that exhibits differential expression patterns.

Polyphenol oxidase (PPO) activity in potato (Solanum tuberosum) plants was high in stolons, tubers, roots, and flowers but low in leaves and stems. PPO activity per tuber continued to increase throughout tuber development but was highest on a fresh weight basis in developing tubers. PPO activity was greatest at the tuber exterior, including the skin and cortex tissue 1 to 2 mm beneath the skin. Flowers had high PPO activity throughout development, particularly in the anthers and ovary. Five distinct cDNA clones encoding PPO were isolated from developing tuber RNA. POT32 was the major form expressed in tubers and was found in all parts of the tuber and at all stages of tuber development. It was also expressed in roots but not in photosynthetic tissues. POT33 was expressed in tubers but mainly in the tissue near the skin. POT72 was detected in roots and at low levels in developing tubers. NOR333 was identical with the P2 PPO clone previously isolated from potato leaves (M.D. Hunt, N.T. Eannetta, Y. Haifeng, S.M. Newman, J.C. Steffens [1993] Plant Mol Biol 21: 59-68) and was detected in young leaves and in tissue near the tuber skin but was highly expressed in flowers. The results indicate that PPO is present as a small multigene family in potato and that each gene has a specific temporal and spatial pattern of expression.     

Reduced amino acid content in transgenic potato tubers due to antisense inhibition of the leaf H+/amino acid symporter StAAP1

Transport processes across the plasma membrane of leaf vascular tissue are essential for transport and distribution of assimilates. In potato, leaves are the predominant sites for nitrate reduction and amino acid biosynthesis. From there, assimilated amino acids are exported through the phloem to supply tubers with organic nitrogen. To study the role of amino acid transporters in long-distance transport and allocation of organic nitrogen in potato plants, a gene encoding a functional, leaf-expressed amino acid permease StAAP1 was isolated. Similar to the sucrose transporter SUT1, StAAP1 expression was induced during the sink-to-source transition, indicating a role in phloem loading. To test the role of StAAP1, expression was inhibited by an antisense approach. Transgenic plants with reduced StAAP1 expression were phenotypically indistinguishable from wild type, as were photosynthetic capacity and tuber yield. However, tubers from antisense StAAP1 plants showed up to 50% reduction in free amino acid contents. In comparison, starch content was not affected or tended to increase relative to wild type. The reduction in all amino acids except aspartate in the antisense plants is consistent with the properties of amino acid permeases (AAPs) found in heterologous systems. The results demonstrate an important role for StAAP1 in long-distance transport of amino acids and highlight the importance of plasma membrane transport for nutrient distribution in plants.     

Purification and characterization of the 22-kilodalton potato tuber proteins

Three abundant proteins of approximate molecular masses of 22, 23, and 24 kilodaltons were purified from potato (Solanum tuberosum L.) tubers by DEAE cellulose and CM-52 cellulose ion exchange column chromatography, electroelution, and high-pressure liquid chromatography (HPLC). Antibodies specific to the gel-purified 22-kilodalton protein were prepared. Immunoblot analysis showed that the 22-, 23-, and 24-kilodalton proteins are immunologically related and that these proteins are present in tubers and as higher molecular mass forms in leaves, but not in stems, roots, and stolons. The ratios of amino acid composition were compared among the three purified proteins, and the aminoterminal amino acid sequences were determined for these three proteins. All three proteins have identical amino-terminal sequences that match the deduced amino acid sequence of an abundant tuber protein cDNA.     

Shoot, Stolon, and Tuber Formation on Potato (Solanum tuberosum L.) Cuttings in Response to Photoperiod

The intensity of “tuberization stimulus” in potato shoots (Solanum tuberosum L.) can be assessed from cuttings containing one or more leaves. Cuttings maintained in a mist chamber under long days will form tubers from underground buds if prior to taking the cutting the leaves received sufficient exposure to photoperiods less than the critical photoperiod. The greatest tendency to tuberize was found in cuttings that consisted of a single, fully expanded leaf and its subtended bud. Grafts showed that genetical differences in critical photoperiod resided in properties of the leaf. Short days before cutting tended to shift growth from above ground buds of two-node cuttings to below ground buds, even if the number of short days was insufficient for tuber induction. As few as 6 short days reduced growth of shoots at the upper bud and increased underground growth of shoots and stolons.     

Metabolism and Transport of [2-14C]({+/-}) Jasmonic Acid in the Potato Plant

Metabolism and transport of [2-¹⁴C](±) jasmonic acid(JA) were studied using 2-week-old potato plants. Plants grownunder tuber inducing conditions (short days) and non-inducingconditions (long days) were assayed 1, 5, 10, 15, 20, and 25days after substance application. The incorporated [2-¹⁴C](±)JAwas metabolized to tuberonic acid glucoside (TAG) within 2 weeks.The radioactivity of the ethanol extract from each plant partwas measured. More than half of the total radioactivity wasstill associated with the treated leaves up to 25 days afterthe application of [2-¹⁴C](±)JA. In the other parts ofthe plants, the upper leaves contained the most total radioactivity,followed by the stems and lower leaves. Under long days no tuberswere produced, and the radioactivity of the stolons was observed.In contrast, under short days tuber were produced, and the radioactivitieswere present in stolons and tubers. Flower buds and flowerswere produced more under long days and accumulated high radioactivity.It is concluded from the combined data that JA is metabolizedto TAG and transported to all parts of the plant. A high accumulationof TAG in tubers and flower buds may induce the formation oftubers and flower buds. (Received January 22, 1996; Accepted April 15, 1996)     

Effects of storage temperature on the contents of sugars and free amino acids in tubers from different potato cultivars and acrylamide in chips

To clarify the effects of storage temperature on potato components and acrylamide in chips, tubers from five cultivars were stored at various temperatures (2, 6, 8, 10, and 18 degrees C) for 18 weeks, and the contents of sugars, free amino acids in tubers, and acrylamide in chips after frying were analyzed. At temperatures lower than 8 degrees C, the contents of reducing sugars increased markedly in all cultivars, with similar increases in the acrylamide level and dark brown chip color. Free amino acids showed little change at the storage temperatures tested and varied within certain ranges characteristic of each cultivar. The contents of reducing sugars correlated well with the acrylamide level when the fructose/asparagine molar ratio in the tubers was <2. When the fructose/asparagine ratio was >2 by low-temperature storage, the asparagine content, rather than the reducing sugar content, was found to be the limiting factor for acrylamide formation.     

Tuberization in Potato at High Temperatures: Gibberellin Content and Transport from Buds

Warm temperatures (35°C day/30°C night) which inhibittuberization in potato (Solanum tuberosum L., cv. Sebago) increasedgibberellin activity in crude extracts from buds, but not frommature leaves, as determined by the lettuce hypocotyl bioassay.Changes in the growth of tubers and stolons indicate the occurrenceof basipetal movement of GA₃ applied to the terminal bud ora mature leaf. ¹⁴C labelling from GA₃ or mevalonic acid injectedjust below the terminal bud was recovered in the lower shoot,stolons and tubers, but the amount transported was greater atcool temperatures (20/15°C). It is concluded that high temperaturespromote the synthesis of gibberellin in the buds rather thantransport to the stolons. Solanum tuberosum L., potato, tuberization, gibberellin