J. Insect Physiol. Vol. 43, No. 5, pp. 457464, 1997 Published by Elsevier Science Ltd Pergamon Printed in Great Britain PII: SOO22-1910(96)00124-2 0022-1910197 $17.00 + 0.00

Effect of Concentration on Carbohydrate Metabolism in Bemisia argentzfdii: Biochemical Mechanism and Physiological Role for Trehalulose Synthesis in the Silverleaf Whitefly MICHAEL E. SALVUCCI,*t GREGORY R. WOLFE,* DONALD L. HENDRIX*

Received 23 July 1996; revised 7 October 1996

Uptake and metabolism of sucrose by adult silverleaf whiteflies (Bern&a argentifolii) were investigated on defined diets containing sucrose concentrations from 3 to 30% (w/v). At an optimal pH of 7, the volume of liquid ingested decreased with increasing dietary sucrose concentration, but the amount of sucrose ingested showed a net increase. Above a dietary sucrose concentration of about lo%, a greater amount of the ingested carbon was excreted by the whiteflies than was retained, and the proportion that was excreted increased progress- ively with increasing dietary sucrose concentration. Carbohydrate analysis showed that the composition of excreted honeydew changed from predominantly and at low dietary sucrose concentrations to predominantly trehalulose at high concentrations, with little change in the proportion of larger oligosaccharides. Measurements of whitefly trehalulose synthase and sucrase activities revealed that the enzymatic potential for metabolizing sucrose shifted from favoring sucrose hydrolysis at low sucrose concentrations to sucrose isomerization at high sucrose concentrations. Thus, the amount of trehalulose synthesized by the silverleaf whitefly was directly related to the properties of trehalulose synthase and sucrase and the concentration of sucrose in the diet. We propose that trehalulose is synthesized for excretion when the carbon input from sucrose is in excess of metabolic needs. Published by Elsevier Science Ltd

Bemisia argentifolii Whitefly Carbohydrate metabolism Trehalulose

INTRODUCTION infested cotton becomes ‘sticky’ from deposits of - dew on the lint (Henneberry et al., 1995). ‘Stickiness’ The silverleaf whitefly, Bemisia argentzjblii Perring and Bellows (Homoptera: Aleyrodidae), is a serious pest reduces the market value of the crop because sticky lint world-wide (Byrne and Bellows, 1991; De Barro, 1995). is difficult or impossible to gin and process at textile The insect is a vector for numerous infectious plant mills, and is often discolored from microbial flora that viruses that damage a variety of crop species (Cock, grow on the honeydew sugars. 1993; Brown, 1994). Whiteflies can attain almost plague- The nutritional requirements of phloem-feeding insects like populations on cotton, causing severe reductions in have been examined extensively, but the vast majority of yield (Gerling et al., 1980; Bellows and Arakawa, 1988; these studies have focused on aphids (c.f. Auclair, 1963; Henneberry et al., 1995). In addition, fiber from whitefly- Walters and Mullin, 1988; Mittler and Meikle, 1991; Simpson et al., 1995; Rhodes et al., 1996). Although whiteflies, like aphids, are homopterans, whiteflies pos- sess Malpighian tubules and a well-developed filter *United States Department of Agriculture, Agricultural Research Ser- vice, Western Cotton Research Laboratory, 4135 E. Broadway chamber (Goodchild, 1966; Cicero et al., 1995), struc- Road, Phoenix, AZ 85040-8830, U.S.A. tures that are not present in most aphid species (Auclair, ~To whom all correspondence should be addressed. 1963 and references therein). Because of these anatom-

457 458 MICHAEL E. SALVUCCI rt id.

ical differences, the nutritional biochemisty of whiteflies shortened to a length of 1.5 cm. These tubes also served and aphids may differ substantially. as collectors for the honeydew that was produced during An aspect of Bemisia carbohydrate metabolism that is the feeding experiments. unique among whiteflies (Hendrix et al., 1992) and that Except where indicated, feeding solutions contained has not been reported in other insects is the synthesis of 0.1 M potassium phosphate, pH 7, the indicated concen- the sugar trehalulose (Bates et al., 1990; Byrne and trations of sucrose, 1-14 &i of radioactive tracer in the Miller, 1990). Trehalulose is a that often form of inulin-[‘Y]-carboxylate, [“S]Met/Cys or [U- occurs as a major component of Bemisia honeydew “C]sucrose, and 1 ~1 of yellow food coloring (Byrne and Miller, 1990; Tarcznski et ul., 1992; Hendrix (DecACake, Bums Phillips Food Inc., San Francisco, and Wei, 1994). Trehalulose is formed by rearrangement CA) in a total volume of 100 ~1. Adult silverleaf white- of the of sucrose from the two to the flies were freshly collected by aspiration, immobilized by one position of fructose, i.e. cr-D-glucose (1,2) p-D-fmc- chilling for 5 min at 4°C and then transferred to the tose+cy-D-glucose (1,l) D-fructose. The enzyme that wells of a solid polypropylene microfuge rack catalyzes this isomerization, trehalulose synthase, has (approximately 50 insects well -I). Feeding devices were been identified and described in several bacterial species inserted into the insect-containing wells in the light at (Cheetham, 1984; Park et al., 1992). Trehalulose syn- 24°C to initiate feeding. After 1 h, approximately 25 thase also occurs in B. argent$Ai (Davidson et al., whiteflies were attached to the parafilm bottom of each 1994), but the Bemisia enzyme has not been charac- feeding device and appeared to be actively feeding. At terized. this time, the feeding devices were carefully removed In the present study, we examine the effect of dietary from the microfuge rack and inserted into collection sucrose concentration on the uptake and metabolism of tubes. After 4 h in the light at 24°C the feeding devices sucrose by the silverleaf whitefly. The results show that with collection tubes attached were chilled to 4°C to metabolism of sucrose by B. argentzyolii is remarkably immobilize the whiteflies. Whiteflies were collected from consistent with the kinetic parameters of the enzymes the parafilm and collection tube, and the honeydew was that isomerize and hydrolyze sucrose, and suggest a poss- recovered by rinsing the collection tube with water. ible physiological role for trehalulose. Radioactivity was determined by liquid scintillation spec- troscopy using a Packard 2200CA liquid scintillation counter (Packard Instrument Company, Downers Grove, MATERIALS AND METHODS IL) after dissolving the whiteflies and honeydew from each of the feeders in BioSafe II liquid scintillation fluid Chemicals (Research Products International Corp., Mount Prospect, Inulin-[‘4C]-carboxylate (9 mCi mmol-‘), [U-‘4C]suc- IL). Four to eight separate feeders were used for each rose (677 mCi mmol-‘) and [35S]methionine/cysteine treatment. The results presented are the means plus stan- (75:15, 1000 Ci mmol-‘) were purchased from Amer- dard error of the feeders, each containing 25 whiteflies. sham Life Science (Arlington Heights, IL). Unless indi- cated otherwise, all other reagents were from Sigma Curhohydrate analysis Chemical Co. (St Louis, MO) The carbohydrate composition of the honeydew was determined by high-performance liquid chromatography Plant and insect material (HPLC) as described previously (Hendrix and Wei, Silverleaf whiteflies were reared on cotton plants 1994). The pooled contents of four collection tubes rep- (Gossypium hirsutum L., var. Coker 1OOA glandless). resenting each treatment were lyophilized in a Speed-Vat Plants were grown in screened boxes within a glasshouse concentrator. Dried material was resuspended in 50 ~1 under a 12 h day/night temperature regime of 35/25”C H,O and 25 ~1 was chromatographed using HPLC. and natural photoperiod. Enzyme assays Feeding experiments Approximately 100 freshly collected adult whiteflies Micro spin-filtration devices (Lida Manufacturing Co., were homogenized at 4°C in a Ten Broeck glass homo- Kenosha, WI) were adapted for use as feeding devices genizer containing 1 ml of 100 mM 3-(N-morpholino)-2- by removing the 0.45 p filter and plastic supporting hydroxypropane-sulfonic acid (MOPS)-KOH, pH 7, screen from the filtration apparatus. The open bottom was 5 mM MgC12, 1% (v/v) Triton X-100 and 5 mM 2-mer- sealed with a layer of stretched parafilm and feeding sol- captoethanol. The homogenate was centrifuged at ution was added to the cylinder. The modified micro 20 OOOxg for 10 min and the supematant used for the spin-filtration units provided a 0.2 cm2 surface for feed- assays. ing. To prevent evaporation, the top of the filtration Trehalulose synthase activity was measured by incub- device was loosely covered with the yellow cap that was ating an aliquot of the supematant at 30°C in a 50 ~1 supplied with the micro spin-filtration apparatus. To pre- assay containing 100 mM MOPS-KOH, pH 7, 5 mM vent whiteflies from leaving the feeder, the feeder units dithiothreitol and the indicated concentrations of sucrose. were inserted into 2 ml microfuge tubes that had been Reactions were terminated after 30 min by boiling for TREHALlJLOSE SYNTHESIS Ih THE SILVERLEAF WHITEFLY 459

2 min and then centrifuged for 3 min at 13 OOOxg to RESULTS remove denatured protein. The amount of trehalulose Chuucterization of the &eding system produced in the assay was determined by HPLC analysis of the supernatant (Hendrix and Wei, 1994). Trehalulose A simple, small volume system was developed for was not detected in zero time controls in which the super- feeding silverleaf whiteflies on a defined diet. The feeder natant was boiled for 2 min prior to assay. was ideal for use with radioisotopic tracers or expensive Sucrase activity was determined spectrophoto- chemicals, since the entire feeding surface could be com- metrically at 30°C by linking sucrose hydrolysis to pletely covered with as little as 50 ~1 of solution. In NADP’reduction via hexokinase and glucose-6-phos- addition, feeding solution could be adjusted or removed phate dehydrogenase. An aliquot of the whitefly super- from the cylindrical filtration device without disturbing natant was added to 0.5 ml assays containing 100 mM feeding. Addition of trace amounts of radiolabeled MOPS, pH 7.0, 5 mM 2-mercaptoethanol, 5 mM MgCl,, material to the defined diets provided a means for 2 tnM ATP, 2 mM NADP’, 2 IU hexokinase, 1 IU glu- determining the total volume of solution ingested. Pre- cose-6-phosphate dehydrogenase and the indicated con- liminary results showed that the calculated rates of diet- centrations of sucrose. Sucrose hydrolysis was determ- ary uptake were similar when based on the amount of ined continuously by monitoring the sucrose-dependent either [?S]Met/Cys or inulin-[“Cl-carboxylate taken up increase in A,,,, caused by the reduction of NADP’. Con- (data not shown). With [‘“S]Met/Cys, about 52% of the trol experiments showed that whitefly extracts contained radiolabel was retained in the whitefly body after 4 h of feeding, while the remainder was excreted in the honey- sufficient phosphoglucoisomerase activity to convert Fru- dew. In contrast, greater than 90% of the ingested inulin- 6-P to Glu-6-P as rapidly as it was produced. Thus, the [“Cl-carboxylate was recovered in the honeydew as rate of NADPH formation was twice the rate of sucrose expected for a compound that cannot be metabolized by hydrolysis. Apparent K, and C:,,,, values of both trehalu- insects (Fischer et al., 1984; Simpson et al., 1995). lose synthase and sucrase were determined by nonlinear The optimum pH for ingestion from a diet containing regression analysis of duplicate assays using the GraFit 20% sucrose was between 6.5 and 7.5 (Fig. 1). We there- program (Leatherbarrow. 1992). A unit of activity (U) fore chose a pH of 7 for all subsequent experiments. Vis- is equal to I pmol of sucrose hydrolyzed or isomerized ual observations showed that whiteflies actively fed for per min. 2448 h on a sucrose diet buffered at this pH, and had a mortality rate of less than 20% during this period.

effect qf sucrose concentration on its uptake und metu- To determine the relative rates of hydrolysis of trehal- holism ulose, sucrose and stachyose, sucrase was partially pur- The total volume of solution ingested by the whiteflies ified from whitefly extracts by precipitation with decreased progressively as the sucrose concentration ammonium sulfate (35-60%) and polyethylene glycol increased from 3 to 30% (Fig. 2). However, the higher (25%). Trehalulose was produced by incubating crude sucrose concentration offset the decrease in volume so whitefly extracts overnight at 30°C with 0.7 M sucrose. that the amount of sucrose ingested increased with diet- Trehalulose and sucrose in the reaction mixtures were ary sucrose concentration. separated by HPLC on a 4.6x250 mm Absorbosil-NH, column (Alltech Inc, Deerfield, IL) using an isocratic mobile phase of acetonitrile: H,O (77.5:22.5). Fractions containing sucrose and trehalulose were lyophilized and the dried material was resuspended in a small amount of H,O. The isolated sucrose and trehalulose, as well as commercially prepared stachyose, were incubated with partially purified sucrase at 30°C for 15 min in an assay containing 0.1 M MOPS, pH 7, 5 mM 2-mercaptoethanol. Reactions were terminated by boiling. The rates of hydrolysis of the three substrates were determined from the amount of product formed and substrate used after separation and quantitation by HPLC (Hendrix and Wei, 1994). 4.5 5.5 6.5 8.5 PH

FIGURE I. Effect of pH on uptake of diet solution by silverleaf white- Protein concentration in whitefly extracts was determ- flies. WhIteflies were fed on solutions containing 20% sucrose, ined by the method of Bradford (1976) using bovine 0.14 /.LM [‘?S]Met/Cys (14 &i) and 100 mM potassium phosphate at serum albumin as the standard. the indicated pH. 460 MICHAEL E. SALVUCCI et al.

the honeydew increased five-fold. At sucrose concen- trations of 3 and 7.5%, there was slightly more radioac- tivity in the whitefly body compared with the honeydew. In contrast, at 30% sucrose more than twice as much carbon was excreted in the honeydew than was retained in the whitefly body. The composition of the excreted carbohydrates showed that the level of dietary sucrose had an effect on the metabolic fate of ingested sucrose. Representative chro- matograms at a high (30%) and low (5%) sucrose con- centration are presented in Fig. 4. In general, whiteflies feeding on solutions containing less than 10% sucrose produced honeydew that consisted of primarily glucose [Sucrose], (% w/v) and fructose with little trehalulose. In fact, in some experiments trehalulose was virtually undetectable in the FIGURE 2. Effect of sucrose concentration on uptake of solution by honeydew when dietary sucrose concentration was at or silverleaf whiteflies. Whiteflies were fed on solutions containing below 5% (data not shown). As dietary sucrose concen- 100 mM potassium phosphate, pH 7, 1 &i inulin-[Y]-carboxylate and sucrose at the indicated concentrations. The volume of solution tration increased, trehalulose became the predominant (open bar) and the amount of sucrose (closed bar) ingested were carbohydrate of honeydew. determined from the amount of radioactivity ingested.

To determine the effect of dietary sucrose concen- tration on carbon partitioning and metabolism, silverleaf whiteflies were fed on diets containing various concen- trations of radiolabeled sucrose. The amount of radiolab- eled carbon excreted in the honeydew increased mark- t 20 edly with increasing concentrations of sucrose in the diet, t exhibiting a progressively greater increase with concen- tration over the entire range of dietary sucrose concen- t 10 trations (Fig. 3). The amount of radiolabeled carbon t retained in the whitefly body also increased, but the increase was much more gradual above a dietary sucrose concentration of 7.5%. For example, over the range of dietary sucrose concentrations from 7.5 to 30%, the 3 amount of carbon that partitioned into the body increased only two-fold, whereas the amount that was excreted in

0 5 10 15

0 10 20 30 Time, (min)

[Sucrose], (% w/v) FIGURE4. Comparison of the components of whitefly honeydew FIGURE 3. Effect of sucrose concentration on the excretion and incor- after feeding on solutions containing 5 and 30% sucrose. Whiteflies poration of carbon by silverleaf whiteflies. Whiteflies were fed on sol- were fed on solutions containing 100 mM potassium phosphate, pH 7, utions containing 100 mM potassium phosphate, pH 7, 14 PCi [U- 0.14 PM [35S]Met/Cys (14 yCi) and either 30% (A) or 5% (B) sucrose, “C]sucrose and unlabeled sucrose at the indicated concentrations. The after which the individual components of the honeydew were separated amount of carbon excreted (0) was determined by measuring the by HPLC and detected using a pulsed amperometric detector. The fig- amount of radioactivity in the honeydew. The amount of carbon incor- ure shows the chromatograms of the HPLC separation. Detector sensi- porated (0) was determined by measuring the amount of radioactivity tivity toward trehalulose is 57% of that of glucose and fructose. Vol- retained in the whitefly. The dashed line indicates the percentage of umes ingested on 5 and 30% sucrose were 473+35 and 274+42 nl, carbon excreted (w). respectively TREHALULOSESYNTHESIS IN THE SILVERLEAFWHITEFLY 461

To quantitate the above results, honeydew from white- contained a separate enzymatic activity that hydrolyzed flies feeding on various concentrations of radiolabeled sucrose to glucose and fructose. Partially purified prep- sucrose was separated and analyzed. The absolute arations containing only the sucrolytic activity were also amounts of all the honeydew carbohydrates increased capable of hydrolyzing p-nitrophenol-glucose, but not with increasing sucrose concentration [Fig. 5(A)], but the stachyose [a-n-galactose (1,6) a-D-galactose (1,6) (Y-D- increase was much more marked for trehalulose and glucose (1,2) p-n-fructose]. Since invertases (i.e. p- carbohydrates with a degree of polymerization (dp) 23 fructofuranosidases) readily hydrolyze stachyose, these (i.e. dp3) than for sucrose and the monosaccharides. results indicate that most if not all of the sucrose hydro- Consequently, the composition of the carbohydrates in lytic activity was associated with a sucrase, an cw-glucop- the honeydew changed from predominantly glucose and yranosidase. Partially purified whitefly sucrase was able fructose and 2dp3 oligosaccharides at low dietary to hydrolyze trehalulose; however, at equivalent concen- sucrose concentrations to predominately trehalulose and trations the rates of trehalulose hydrolysis were only rdp3 oligosaccharides at high concentrations [Fig. about 10% of the rate of sucrose hydrolysis (data not 5(B)]. Interestingly, the proportion of sucrose converted shown). to rdp3 oligosaccharides was similar at the various The effect of sucrose concentration on the activities of sucrose concentrations, and there was no change in the trehalulose synthase and sucrase are presented in Fig. 6. relative distribution among the various rdp3 oligo- Sucrase activity was saturated by sucrose concentrations saccharides (data not shown). greater than 100 mu, whereas trehalulose synthase activity required much higher levels of sucrose for satu- Enzymatic hydroZysis and isomerization of sucrose ration. The apparent KM values for the sucrase and trehal- The potential for conversion of sucrose to trehalulose ulose synthase were 15*2 and 181*8 mu, respectively, or hydrolysis to glucose and fructose was investigated by equivalent to 0.5 and 6.2% (w/v). The maximum velocity conducting enzyme assays with whole body extracts of of trehalulose synthase, 1.3 U mg protein’, was 2.5-fold adult whiteflies. Preliminary experiments showed that higher than for sucrase. However, because of the large whitefly extracts catalyzed trehalulose synthesis and also difference in sucrose affinity between the two enzymes, the rate of sucrose hydrolysis by sucrase at low concen- trations of sucrose was significantly higher than the rate of sucrose isomerization via trehalulose synthase (Fig. 6, inset). 400 G DISCUSSION

= % 200 Uptake and metabolism of sucrose by the silverleaf 8 whitejy 5 The inclusion of radiolabeled tracer enabled accurate B 0 measurement of the volume of feeding solution ingested I I I I I and excreted by silverleaf whiteflies on defined diets. The g B’- 'Z maximum volume ingested over a 4 h feeding experiment 40 - 3 F 3 "g? I 1 20 - P 5 0.6 F 9 " a ‘c 0 $n I I I I I 5 g O.6 0 I ae 10 0 20 30 8 2 o.4 3 [Sucrose],(%w/v) i6 = 0.2 2 c FIGURE 5. Effect of sucrose concentration on the distribution of 0 WO newly acquired carbon among the various components of whitefly 0 200 400 600 honeydew. Whiteflies were fed on solutions containing 100 mM potass- ium phosphate, pH 7, 14 &i [U-‘4C]sucrose and unlabeled sucrose at [Sucrose], (mM) the indicated concentrations. The distribution of carbon among glucose and fructose (O), trehalulose (O), sucrose (0)and >dp3 sugars (A) FIGURE 6. Effect of sucrose concentration on the activities of trehalu- was determined by measuring the radioactivity of the individual lose synthase (0) and sucrase (0) in extracts of the silverleaf whitefly. components after separation by HPLC. (A) The amount of carbon in The inset shows the activities at low substrate concentrations. Error the various components; (B) The per cent distribution. bars represent standard errors. 462 MICHAEL E. SALVUCCI et cd.

was 600 nl per feeder, which is equivalent to an average tration on trehalulose synthesis may account for some feeding rate per whitefly of about 6 nl hh’. The pH of of the differences in whitefly honeydew composition on the diet markedly affected the rate of solution ingestion. different plant hosts (Byrne and Miller, 1990; Hendrix et Rates were highest at pH values of 6.5 and 7.5, and con- al., 1992). siderably lower above and below these pH values. A Some of the ingested sucrose was undoubtedly used similar effect of pH was reported by Berlinger et al. by the whiteflies for respiration, but the amount was not (1983) who showed that the number of resting whiteflies measured. In locusts, respiration serves as a removal and the per cent survival on buffered sucrose solutions mechanism for excess carbohydrates (Zanotto et al., were highest at neutral pH values. 1993), and the same may apply for whiteflies. In an inter- The concentration of sucrose in the diet also affected esting study of the pea aphid, Rhodes et al. (1996) the volume of solution ingested by the whiteflies. The showed that respiration rates were similar at dietary volume ingested was greatest at the lowest sucrose con- sucrose concentrations of 11 and 22%, but decreased centration and decreased with increasing concentrations when the sucrose concentration in the artificial diet was of dietary sucrose. Abisgold et al. (1994) and Simpson changed from 22 to 0%. Measurements of respiration et al. (1995) obtained a similar result with the pea aphid, rates at various dietary sucrose concentrations are needed Acyrthosiphon pisum, and concluded that aphids regulate to determine if whiteflies consume excess carbohydrates carbohydrate uptake primarily by altering consumption. by increasing respiration. The similarity of our results to their findings suggests that this conclusion is also applicable to whiteflies. The Biochemical mechanism for trehalulose synthesis at high presence of amino acids in the diet is also known to dieta y sucrose concentrations affect the consumption of carbohydrates by aphids The changes in honeydew sugar composition that (Mitler, 1967; Abisgold et al., 1994; Simpson et al., occurred in response to a change in dietary sucrose con- 1995). The effect of amino acids on carbohydrate uptake centration were entirely consistent with the kinetics and by whiteflies is unknown since physiologically signifi- activities of the major whitefly enzymes responsible for cant levels of amino acids were not included in the white- isomerizing and hydrolyzing sucrose. Enzyme assays fly diets. showed that trehalulose synthase from silverleaf white- In a study of two aphid species, Mittler and Meikle flies had a much higher maximum velocity than sucrase, ( 199 1) showed that the volume of honeydew excreted by but its affinity for sucrose was more than an order of Acyrthosiphon pisum and Myzus persicae progressively magnitude lower. Thus, when these two enzymes act decreased as the dietary sucrose concentration increased together on sucrose the enzymatic potential shifts with from 10 to 30%. We measured a similar response in sucrose concentration from favoring sucrose hydrolysis whiteflies using the undigestable compound inulin-[‘4C]- and the resultant production of glucose and fructose at carboxylate. In the same study, the proportion of sucrose low sucrose concentrations, to sucrose isomerization and retained by A. pisum and M. persicae decreased as the the production of trehalulose at high sucrose concen- dietary sucrose concentration increased from 10 to 40%. trations. Our measurements of the honeydew composition We observed a similar trend with whiteflies (Fig. 3). of whiteflies on various sucrose concentrations followed However, in comparison to aphids (see also Rhodes et this pattern, indicating that sucrose metabolism in the al., 1996) whiteflies excreted a much larger proportion whitefly reflects the kinetic parameters of trehalulose of the ingested carbon. synthase and sucrase. Above a dietary sucrose concentration of about IO%, silverleaf whiteflies excreted a greater amount of carbon Possible physiological role fbr trehalulose synthesis than they retained and the proportion of carbon that was The effect of dietary sucrose concentration on carbo- excreted increased with increasing concentrations of hydrate metabolism in B. argent$dii suggests a possible sucrose in the diet. As in aphids, the increased partition- physiological role for trehalulose. At low sucrose con- ing of carbon to excretion was accompanied by a major centrations, a relatively large percentage of the newly change in the composition of the honeydew, including ingested carbon was retained in the whitefly body pre- an increase in the absolute amount of rdp3 oligosaccha- sumably to satisfy nutritional requirements. At the same rides. However, unlike in aphids (Fischer et al., 1984; time, trehalulose constituted a much lower percentage of Walters and Mullin, 1988) the change in whitefly honey- the total carbon in the honeydew than glucose and fiuc- dew composition did not involve a change in the pro- tose. As the dietary sucrose concentration increased, a portion of carbon partitioned into rdp3 oligosaccharides greater proportion of the ingested carbon was excreted or in the composition of the >dp3 oligosaccharides. and trehalulose became the most abundant component in Instead, the major change in whitefly honeydew involved the honeydew. That trehalulose was most abundant in an increase in the proportion of trehalulose to hexose silverleaf whitefly honeydew when the ingested sucrose monosaccharides (i.e. glucose and fructose). Since plant was in excess of the insect’s metabolic requirements indi- species differ in their phloem sucrose concentrations cates that trehalulose is primarily synthesized for (compare the work of Haritatos et al., 1996 with Fischer excretion of excess carbon. and Gifford, 1986), the marked effect of sucrose concen- Synthesis of trehalulose from sucrose involves TREHALULOSE SYNTHESIS IN THE SILVERLEAF WHITEFLY 463 rearranging the glycosidic bond of sucrose from the two Brown J. K. (1094) A global position paper. The status of 5rmi.G to the one position of fructose. This rearrangement tuhuci as a plant pest and virus vector in world agroecosystems. FAO Plunt Prot. Bull. 42, 3 -32. affords no advantage in terms of osmotic strength, but Bymc D. N. and Bellows T. S. (1991) Whitefly biology. Ann. Rrlx. does significantly change the susceptability of the disac- Entomol. 36, 43 I 457. charide to hydrolysis by sucrase. Our data show Byrne D. N. and Miller W. B. (1990) Carbohydrate and amino acid that at equivalent concentrations silverleaf whitefly suc- composition of phloem sap and honeydew produced by Bemisiu rase hydrolyzed trehalulose at only about 10% of the rate tuhuci. J. Insect Phy.vio/. 36, 433439. of sucrose hydrolysis. Because of the slower rate of Chcctham P. S. J. (lYR4) The extraction and mechanism of a novel -synthesizing enzyme from Erwiniu rhupontici. hydrolysis, conversion of sucrose to trehalulose would Biochcm J. 220, 2 13-220. tend to maintain ingested carbohydrate in the form of a Clccro J. M., lllebert E. and Webb S. E. (1995) The alimentary canal disaccharide. Preserving carbon in the form of a dissach- of Bcmi.siu tahwi and Triuleurodes utnrrilonru (Homopterd, aride rather than two monosaccharides has obvious Stcmorrhynchi): histology, ultrastructurc and correlations to func- advantages in terms of osmotic potential. It is also poss- tlon. Zoomorpholog~~ I IS, 3 l-3’). Cock M. J. (1993) Bemisiu tuhucr. an update, l986-19Y2. C,4B Inter.- ible that the reduced rate of disaccharide hydrolysis is nutionul lnstimtc qf Rwlog~~l Conrrol. Ascot. p. 7X. required for metabolic homeostasis, ensuring that the lcv- Davidson E. W., Segura B. J., Steele T. and Hendrix D. L. (1994) els of monosaccharides do not greatly exceed the meta- Microorganisms influence the compositlon of honcydcw produced bolic needs of the insect. Both possibilities are consistent by the silverleaf whltcfly, Bemisiu urgentifdii. J. Insect IViysio/. with an essential role for trehalulose synthesis in main- 40, 1069% 1076. taining proper metabolic activity on a high sucrose diet. De Barre P.J. (19Y5) kwisirr tuhuci biotype B: a review of Its biology, distribution and control. CS’IRlRODir. Entond. Tech. Puprr 33, 57. Thus, inhibiting trehalulose synthase may provide a strat- Fischer D. B. and Gifford R. M. (1986) Accumulation and conversion egy for controlling silverleaf whitefly on plant species of sugars by developing wheat grains. /‘/ant Ph~xiol. 82, 1024 such as cotton that translocate sucrose. 1030. Unlike whiteflies, aphids respond to increasing dietary Fischer II. B., Wright J. P. and Mittler T. E. (lY84) Osmorcgulation sucrose concentration by increasing the average size of by the aphid MJX~ persic~ur: a physiological role for honeydew their honeydew oligosaccharides (Fischer et ul., 1984; oligosaccharides. J. fnsrc~t Pl~~.vd. 30, 387-393. Gerling D.. Motro U. and Horowitz R. (1980) Dynamics of Brrnisiu Walters and Mullin, 1988). The reason silverleaf white- tuhwi (Gennadius) (HOtnOpterd. Aleyrodidac) attackmg cotton in flies do not respond similarly is unknown, but may be the coastal plain of Israel. Rul[. Enton~ol. Res. 70, 213-219. related to the presence of a filter chamber. It is likely that Goodchild J. P. 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