Acclimation of entomopathogenic nematodes to novel temperatures: trehalose accumulation and the acquisition of thermotolerance

Refugia, climatic conditions and farm management factors as drivers of adaptation in Nematodirus battus populations

Parasites are known for their ability to rapidly adapt to changing conditions. For parasitic helminths, changes in climate, along with farming and management practices associated with the intensification of livestock farming, provide novel challenges which can impact on their epidemiology and control. The sustainability of livestock production partially relies on effective control of helminth infection. Therefore, understanding changes in parasite behaviour, and what drives these, is of great importance. Nematodirus battus is an economically important helminth in the UK and temperate regions. Its infective larvae typically overwinter in eggs on pasture and hatch synchronously in spring, causing acute disease in lambs. Attempts to control disease typically rely on whole-flock benzimidazole (BZ) treatments. In recent years, the emergence of BZ-resistance, alongside the hatching of eggs without the classical over-winter 'chill stimulus', have made N. battus more difficult to control. In three previous studies, after collecting a large number of N. battus populations alongside farm management data from commercial farms, we explored the prevalence of genetic mutations associated with BZ-resistance (n = 253 farms), the ability of eggs to hatch with and without a chill stimulus (n = 90 farms) and how farm management practices varied throughout the UK (n = 187 farms). In the present study, we identify factors which may be acting as drivers, or barriers, to either the development of resistance or the variable hatching behaviour of N. battus eggs. Generalised linear mixed effect models were applied to regress experimental hatching and genotyping data on farm management and additional environmental data. Both variable hatching and resistance development appeared associated with the maintenance of parasite refugia as well as grazing management, particularly reseeding of pasture routinely grazed by young lambs each spring and the practice of set-stocked grazing. Effective quarantine measures were identified as the main protective factor for the development of BZ-resistance whereas set stocked grazing and population bottlenecks, resulting from reseeding heavily contaminated pastures, were risk factors. Spring maximum temperature and other climatic factors were associated with 'typical' hatching of eggs following a chill stimulus whilst several management factors were linked with hatching without prior chilling. For example, practices which reduce parasite numbers on pasture (e.g. re-seeding) or restrict availability of hosts (e.g. resting fields), were found to increase the odds of non-chill hatching. Retention of the timing of lambing and infection level of the host within the fitted model indicated that requirement for a chill stimulus prior to hatching may be plastic, perhaps subject to change throughout the grazing season, in response to immune development or parasite density-dependence within the host. Further investigation of the influence of the factors retained within the fitted models, particularly the theme of parasite refugia which was highlighted in relation to both the presence of BZ-resistance alleles and alternative hatching, is required to establish robust, sustainable parasite control and farm management strategies.

Stress tolerance in entomopathogenic nematodes: Engineering superior nematodes for precision agriculture

Entomopathogenic nematodes (EPNs) are soil-dwelling parasitic roundworms commonly used as biocontrol agents of insect pests in agriculture. EPN dauer juveniles locate and infect a host in which they will grow and multiply until resource depletion. During their free-living stage, EPNs face a series of internal and environmental stresses. Their ability to overcome these challenges is crucial to determine their infection success and survival. In this review, we provide a comprehensive overview of EPN response to stresses associated with starvation, low/elevated temperatures, desiccation, osmotic stress, hypoxia, and ultra-violet light. We further report EPN defense strategies to cope with biotic stressors such as viruses, bacteria, fungi, and predatory insects. By comparing the genetic and biochemical basis of these strategies to the nematode model Caenorhabditis elegans, we provide new avenues and targets to select and engineer precision nematodes adapted to specific field conditions.

Dry-freezing Steinernema carpocapsae infective juveniles for robust preservation of stocks

Cryopreservation allows strains to be stored, eliminating genetic drift and maintenance costs. Existing cryopreservation methods for the economically-important entomopathogenic nematode Steinernema carpocapsae involve multiple incubation and filtration steps to precondition the animals. The standard protocol for freezing the model organism Caenorhabditis elegans in buffer is simpler, and a recent C. elegans dry-freezing protocol allows stocks to survive multiple freeze-thaws, a possibility during a power failure. Here we report the efficacy of C. elegans cryopreservation protocols adapted for S. carpocapsae . We show that dry freezing with disaccharides, but not glycerol-based or trehalose-DMSO-based freezing buffer, allows reliable recovery of infective juveniles.

Cold adaptive potential of pine wood nematodes overwintering in plant hosts

The pine wood nematode (PWN; Bursaphelenchus xylophilus) is the causal agent of pine wilt disease, which results in severe ecological and economic losses in coniferous forests. During overwintering, PWNs undergo morphological and physiological changes to adapt to low temperature environments. Here, the physiological changes of the PWN populations sampled in the summer and winter were compared to analyze the role of low temperatures in their response. The PWN overwinters as third-stage dispersal juveniles, which showed significantly greater survival rates than summer populations (propagative forms) at sub-zero temperatures. The major biochemical compounds in the populations were analyzed by gas chromatography. Eight dominant fatty acids, with stearic acid being the most important, were identified from PWN propagative stage and third-stage dispersal stage. Compared with the propagative stage, the dispersal stage showed significant increases in the fatty acid content and the proportion of unsaturated fatty acids. Three carbohydrates, trehalose, glycerol and glucose, were detected in the PWN. Compared with the summer population, the levels of trehalose and glycerol increased significantly, while glucose decreased, in the winter population. The modifications in fatty acid composition and cryoprotectant levels, as elements of its changing physiology, play important roles in the overwintering success of the PWN.

Summary: The modifications in fatty acid composition and cryoprotectant levels, as elements of its changing physiology, play important roles in the overwintering success of the pine wood nematode.

Infective larvae of Anisakis simplex (Nematoda) accumulate trehalose and glycogen in response to starvation and temperature stress

Anisakis simplex L3 larvae infect fish and other seafood species such as squid or octopi; therefore, humans consuming raw or undercooked fish may become accidental hosts for this parasite. These larvae are induced to enter hypometabolism by cold temperatures. It is assumed that sugars (in particular trehalose and glycogen) are instrumental for survival under environmental stress conditions. To elucidate the mechanisms of environmental stress response in A. simplex, we observed the effects of starvation and temperature on trehalose and glycogen content, the activity of enzymes metabolizing those sugars, and the relative expression of genes of trehalose and glycogen metabolic pathways. The L3 of A. simplex synthesize trehalose both in low (0°C) and high temperatures (45°C). The highest content of glycogen was observed at 45°C at 36 h of incubation. On the second day of incubation, tissue content of trehalose depended on the activity of the enzymes: TPS was more active at 45°C, and TPP was more active at 0°C. The changes in TPP activity were consistent with the transcript level changes of the TPP gene, and the trehalose level, while glycogen synthesis correlates with the expression of glycogen synthase gene at 45°C; this suggests that the synthesis of trehalose is more essential. These results show that trehalose plays a key role in providing energy during the thermotolerance and starvation processes through the molecular and biochemical regulation of trehalose and glycogen metabolism.

The cold tolerance of the northern root-knot nematode, Meloidogyne hapla

The northern root-knot nematode, Meloidogyne hapla, is one of the most important nematode pathogens occurring in cold regions. It is a sedentary, biotrophic parasites of plants and overwinter in the soil or in diseased roots. This study showed that the cold tolerance for the second-stage juveniles (J2) of M. hapla was moderate with the 50% survival temperature (S₅₀) of -2.22°C and the fatal temperature was -6°C when cooling at 0.5°C min^-1. Cryoprotective dehydration significantly enhance cold tolerance of M. hapla J2 with the lowest S₅₀ of -3.28°C after held being at -1°C for 6 h. Moreover, cold shock and cold acclimation had significant effects on the freezing survival of M. hapla J2. The lethal temperature of eggs was -18°C. Therefore, the cold tolerance of M. hapla is sufficiently favorable to withstand winters in cold temperature environments.

Infective Juveniles of the Entomopathogenic Nematode, Steinernema feltiae Produce Cryoprotectants in Response to Freezing and Cold Acclimation

Steinernema feltiae is a moderately freeze-tolerant entomopathogenic nematode which survives intracellular freezing. We have detected by gas chromatography that infective juveniles of S. feltiae produce cryoprotectants in response to cold acclimation and to freezing. Since the survival of this nematode varies with temperature, we analyzed their cryoprotectant profiles under different acclimation and freezing regimes. The principal cryoprotectants detected were trehalose and glycerol with glucose being the minor component. The amount of cryoprotectants varied with the temperature and duration of exposure. Trehalose was accumulated in higher concentrations when nematodes were acclimated at 5°C for two weeks whereas glycerol level decreased from that of the non-acclimated controls. Nematodes were seeded with a small ice crystal and held at -1°C, a regime that does not produce freezing of the nematodes but their bodies lose water to the surrounding ice (cryoprotective dehydration). This increased the levels of both trehalose and glycerol, with glycerol reaching a higher concentration than trehalose. Nematodes frozen at -3°C, a regime that produces freezing of the nematodes and results in intracellular ice formation, had elevated glycerol levels while trehalose levels did not change. Steinernema feltiae thus has two strategies of cryoprotectant accumulation: one is an acclimation response to low temperature when the body fluids are in a cooled or supercooled state and the infective juveniles produce trehalose before freezing. During this process a portion of the glycerol is converted to trehalose. The second strategy is a rapid response to freezing which induces the production of glycerol but trehalose levels do not change. These low molecular weight compounds are surmised to act as cryoprotectants for this species and to play an important role in its freezing tolerance.

Divergent thermal specialisation of two South African entomopathogenic nematodes

Thermal physiology of entomopathogenic nematodes (EPN) is a critical aspect of field performance and fitness. Thermal limits for survival and activity, and the ability of these limits to adjust (i.e., show phenotypic flexibility) depending on recent thermal history, are generally poorly established, especially for non-model nematode species. Here we report the acute thermal limits for survival, and the thermal acclimation-related plasticity thereof for two key endemic South African EPN species, Steinernema yirgalemense and Heterorhabditis zealandica. Results including LT50 indicate S. yirgalemense (LT50 = 40.8 ± 0.3 °C) has greater high temperature tolerance than H. zealandica (LT50 = 36.7 ± 0.2 °C), but S. yirgalemense (LT50 = -2.4 ± 0 °C) has poorer low temperature tolerance in comparison to H. zealandica (LT50 = -9.7 ± 0.3 °C), suggesting these two EPN species occupy divergent thermal niches to one another. Acclimation had both negative and positive effects on temperature stress survival of both species, although the overall variation meant that many of these effects were non-significant. There was no indication of a consistent loss of plasticity with improved basal thermal tolerance for either species at upper lethal temperatures. At lower temperatures measured for H. zealandica, the 5 °C acclimation lowered survival until below -12.5 °C, where after it increased survival. Such results indicate that the thermal niche breadth of EPN species can differ significantly depending on recent thermal conditions, and should be characterized across a broad range of species to understand the evolution of thermal limits to performance and survival in this group.

Involvement of trehalose in hydrogen sulfide donor sodium hydrosulfide-induced the acquisition of heat tolerance in maize (Zea mays L.) seedlings

BACKGROUND

Trehalose, a non-reducing disaccharide, which involves in the acquisition of various stress tolerance, while hydrogen sulfide (H₂S) is considered as a cell signal molecule, but H₂S-induced heat tolerance and involvement of trehalose in plants is still unclear.

RESULTS

In present study, pretreatment with hydrogen sulfide donor sodium hydrosulfide (NaHS) markedly increased the accumulation of endogenous H₂S in maize seedlings under normal culture conditions, which in turn improved survival percentage of maize seedlings and mitigated increase in electrolyte leakage and malonaldehyde (MDA) accumulation under heat stress. In addition, treatment with NaHS activated increase in the activity of trehalose-6-phosphate phosphatase (TPP) under normal culture conditions, followed by induced the accumulation of endogenous trehalose, but this accumulation was eliminated by addition of sodium citrate, an inhibitor of TPP. During the process of heat stress, maize seedlings treated with NaHS maintained higher TPP activity and trehalose content than those of control. On the other hand, exogenous application of trehalose also increased the content of endogenous trehalose in maize seedlings under normal culture conditions, alleviated increase in electrolyte leakage and MDA accumulation under heat stress, which in turn improved survival percentage of maize seedlings, and the heat tolerance induced by trehalose was enhanced by exogenous supplement of NaHS, but exogenous trehalose treatment had not significant effect on the accumulation of endogenous hydrogen sulfide in maize seedlings.

CONCLUSION

These data suggest that sodium hydrosulfide pretreatment could improve heat tolerance of maize seedlings and this improvement may be involved in trehalose accumulation by activating TPP activity.

Intracellular freezing in the infective juveniles of Steinernema feltiae: an entomopathogenic nematode

Taking advantage of their optical transparency, we clearly observed the third stage infective juveniles (IJs) of Steinernema feltiae freezing under a cryo-stage microscope. The IJs froze when the water surrounding them froze at -2°C and below. However, they avoid inoculative freezing at -1°C, suggesting cryoprotective dehydration. Freezing was evident as a sudden darkening and cessation of IJs' movement. Freeze substitution and transmission electron microscopy confirmed that the IJs of S. feltiae freeze intracellularly. Ice crystals were found in every compartment of the body. IJs frozen at high sub-zero temperatures (-1 and -3°C) survived and had small ice crystals. Those frozen at -10°C had large ice crystals and did not survive. However, the pattern of ice formation was not well-controlled and individual nematodes frozen at -3°C had both small and large ice crystals. IJs frozen by plunging directly into liquid nitrogen had small ice crystals, but did not survive. This study thus presents the evidence that S. feltiae is only the second freeze tolerant animal, after the Antarctic nematode Panagrolaimus davidi, shown to withstand extensive intracellular freezing.

Acclimation responses to temperature vary with vertical stratification: implications for vulnerability of soil-dwelling species to extreme temperature events

The occurrence of summer heat waves is predicted to increase in amplitude and frequency in the near future, but the consequences of such extreme events are largely unknown, especially for belowground organisms. Soil organisms usually exhibit strong vertical stratification, resulting in more frequent exposure to extreme temperatures for surface-dwelling species than for soil-dwelling species. Therefore soil-dwelling species are expected to have poor acclimation responses to cope with temperature changes. We used five species of surface-dwelling and four species of soil-dwelling Collembola that habituate different depths in the soil. We tested for differences in tolerance to extreme temperatures after acclimation to warm and cold conditions. We also tested for differences in acclimation of the underlying physiology by looking at changes in membrane lipid composition. Chill coma recovery time, heat knockdown time and fatty acid profiles were determined after 1 week of acclimation to either 5 or 20 °C. Our results showed that surface-dwelling Collembola better maintained increased heat tolerance across acclimation temperatures, but no such response was found for cold tolerance. Concordantly, four of the five surface-dwelling Collembola showed up to fourfold changes in relative abundance of fatty acids after 1 week of acclimation, whereas none of the soil-dwelling species showed a significant adjustment in fatty acid composition. Strong physiological responses to temperature fluctuations may have become redundant in soil-dwelling species due to the relative thermal stability of their subterranean habitat. Based on the results of the four species studied, we expect that unless soil-dwelling species can temporarily retreat to avoid extreme temperatures, the predicted increase in heat waves under climatic change renders these soil-dwelling species more vulnerable to extinction than species with better physiological capabilities. Being able to act under a larger thermal range is probably costly and could reduce maximum performance at the optimal temperature.

A conidial protein (CP15) of Beauveria bassiana contributes to the conidial tolerance of the entomopathogenic fungus to thermal and oxidative stresses

Aerial conidia are central dispersing structures for most fungi and represent the infectious propagule for entomopathogenic fungus Beauveria bassiana, thus the active ingredients of commercial mycoinsecticides. Although a number of formic-acid-extractable (FAE) cell wall proteins from conidia have been characterized, the functions of many such proteins remain obscure. We report that a conidial FAE protein, termed CP15, isolated from B. bassiana is related to fungal tolerance to thermal and oxidative stresses. The full-length genomic sequence of CP15 was shown to lack introns, encoding for a 131 amino acid protein (15.0 kDa) with no sequence identity to any known proteins in the NCBI database. The function of this new gene with two genomic copies was examined using the antisense-RNA method. Five transgenic strains displayed various degrees of silenced CP15 expression, resulting in significantly reduced conidial FAE protein profiles. The FAE protein contents of the strains were linearly correlated to the survival indices of their conidia when exposed to 30-min wet stress at 48°C (r (2) = 0.93). Under prolonged 75-min heat stress, the median lethal times (LT(50)s) of their conidia were significantly reduced by 13.6-29.5%. The CP15 silenced strains were also 20-50% less resistant to oxidative stress but were not affected with respect to UV-B or hyperosmotic stress. Our data indicate that discrete conidial proteins may mediate resistance to some abiotic stresses, and that manipulation of such proteins may be a viable approach to enhancing the environmental fitness of B. bassiana for more persisting control of insect pests in warmer climates.

Effect of slow desiccation and freezing on gene transcription and stress survival of an Antarctic nematode

Nematodes are the dominant soil animals of the Antarctic Dry Valleys and are capable of surviving desiccation and freezing in an anhydrobiotic state. Genes induced by desiccation stress have been successfully enumerated in nematodes; however, little is known about gene regulation by Antarctic nematodes that can survive multiple types and incidences of environmental stress. In order to reveal the molecular response of anhydrobiotic survival, we investigated the genetic response of an Antarctic nematode, Plectus murrayi, which can survive desiccation and freezing. Using molecular approaches, we provide insight into the regulation of desiccation-induced transcripts during different stages of stress survival under conditions characteristic of the Antarctic Dry Valley environment. Our study revealed that exposure to slow desiccation and freezing plays an important role in the transcription of stress, metabolism and signal transduction-related genes and improves desiccation and freezing survival compared with nematodes exposed to fast desiccation and freezing. Temporal analyses of gene expression showed that pre-exposure to mild stress promotes survival of harsher stress. Our results further show that exposure to slow dehydration not only improves extreme desiccation survival but also promotes enhanced cold tolerance. We also provide evidence that slow dehydration can enhance freeze tolerance in an Antarctic nematode. Expression profiling of P. murrayi transcripts is an important step in understanding the genome-level response of this nematode to different environmental stressors.

Cold tolerance in sealworm (Pseudoterranova decipiens) due to heat-shock adaptations

Third-stage larvae ofPseudoterranova decipienscommonly infect whitefish such as cod, and the parasite can be transferred to humans through lightly prepared (sushi) meals. Because little is known about the nematode's cold tolerance capacity, we examined the nematode's ability to supercool, and whether or not cold acclimation could induce physiological changes that might increase its ability to tolerate freezing conditions. Even if third-stagePseudoterranova decipienslarvae have some supercooling ability, they show no potential for freezing avoidance because they are not able to withstand inoculative freezing. Still, they have the ability to survive freezing at high subzero temperatures, something which suggests that these nematodes have a moderate freeze tolerance. We also show that acclimation to high temperatures triggers trehalose accumulation to an even greater extent than cold acclimation. Trehalose is a potential cryoprotectant which has been shown to play a vital role in the freeze tolerance of nematodes. We suggest that the trehalose accumulation observed for the cold acclimation is a general response to thermal stress, and that the nematode's moderate freeze tolerance may be acquired through adaptation to heat rather than coldness.

Beneficial effect of intracellular free high-mannose oligosaccharides on cryopreservation of mammalian cells and proteins

The cryoprotective effect of intracellular free high-mannose oligosaccharides (HMOS) on mammalian cells and proteins was examined by monitoring PC-12 cell viability and assaying protein kinase C (PKC)-epsilon activity. 1-Deoxymannojirimycin, an inhibitor of alpha-mannosidase, to cause an increase in intracellular free HMOS, significantly rescued PC-12 cells with 2-h freezing insult at -15 degrees C in a concentration (1-50mM)- and pretreatment time (48-72h)-dependent manner, as compared with unpretreated cells; full rescue from freezing injury was obtained with 1-deoxymannojirimycin at more than 25mM for 48-h pretreatment and more than 3mM for 72- and 96-h pretreatment. For PC-12 cells pretreated with 1-deoxymannojirimycin at 1mM for 72h, thawed cell viability after more than 8-w cryopreservation at -80 degrees C in 10% (v/v) dimethyl sulfoxide was much higher than that for cells without pretreatment. PKC-epsilon activity was well preserved after 16-h cryopreservation at -20 degrees C in the presence of mannose 9-N-acetylglucosamine 2 (Man9-GlcNAc2) (1 mM), an HMOS, while the activity was reduced to 15% without Man9-GlcNAc2. Collectively, the results of the present study suggest that intracellular free HMOS is a key molecule to protect mammalian cells and proteins from freezing injury; in other words, HMOS could be a new target for cryopreservation of mammalian cells and proteins.

Both heat-shock and cold-shock influence trehalose metabolism in an entomopathogenic nematode

Heat-shock response is highly conserved in animals and microorganisms, and it results in the synthesis of heat-shock proteins. In yeast, heat-shock response has also been reported to induce trehalose accumulation. We explored the relationship between heat- (35 C) or cold-shock (1 and 10 C) and trehalose metabolism in the entomopathogenic nematode, Heterorhabditis bacteriophora. Because both heat- and cold-shocks may precede desiccation stress in natural soil environments, we hypothesized that nematodes may accumulate a general desiccation protectant, trehalose, under both situations. Indeed, both heat- and cold-shocks influenced trehalose accumulation and activities of enzymes of trehalose metabolism in H. bacteriophora. Trehalose increased by 5- and 6-fold in heat- and cold-shocked infective juveniles, respectively, within 3 hr of exposure, compared with the nematodes maintained at 25 C (culture temperature). The activity of trehalose-6-phosphate synthase (T6PS), an enzyme involved in the synthesis of trehalose, also significantly increased in both heat- and cold-shocked nematodes during the first 3 hr of exposure. Generally, the trehalose levels and activities of T6PS declined to their original levels within 3 hr when nematodes were transferred back to 25 C. In both heat- and cold-shocked nematodes, trehalase activity decreased significantly within the first 3 hr and generally returned to the original levels within 3 hr when these nematodes were transferred back to 25 C. The results demonstrate that the trehalose concentrations in H. bacteriophora are influenced by both heat- and cold-shocks and are regulated by the action of 2 trehalose-metabolizing enzymes, T6PS and trehalase. The accumulated trehalose may enhance survival of nematodes under both cold and warm conditions, but it may also provide simultaneous protection against desiccation that may result from subsequent evaporation or freezing. This is the first report of the relationship between trehalose metabolism and heat-shock for the Nematoda.

Cross-stress tolerance and expression of stress-related proteins in osmotically desiccated entomopathogenicSteinernema feltiaeIS-6

Infective juveniles (IJs) of the entomopathogenic nematode (EPN)Steinernema feltiaeIS-6 can survive exposure to 24% glycerol solution by entering an osmotically desiccated state. Exposure of osmotically desiccated nematodes to extreme temperature assays (40 °C for 10 h and −20 °C for 360 h) resulted in gradual reduction in survival, whereas non-desiccated IJs died within a short exposure to the assay conditions. Through SDS-PAGE, a stress-related protein UNC-87 was found in osmotically desiccated IJs exposed to 40 °C for 3, 6, and 8 h, whose survival rates were 98·9±1·43, 78·5±5·87 and 20·9±4·93%, respectively. The protein was not found in IJs following exposure of osmotically desiccated individuals to 40 °C for 10 h, in which none of the IJs survived. After exposure to −20 °C for 360 h, the survival of osmotically desiccated EPNs with a weak band of UNC-87 was 13·0±3·32%. To identify other responsive proteins that are required for osmotic stress, we used 2-dimensional electrophoresis to analyse the proteins in osmotically desiccated EPNs. The results revealed that 10 novel protein spots and 10 up-regulated protein spots in osmotically desiccated IJs were detected by digital image analysis. Mass spectrometry analysis of 7 significant spots indicated that osmotic stress in desiccated IJs was associated with the induction of actin, Proteasome regulatory particle (ATPase-like), GroEL chaperonin, GroES co-chaperonin and transposase family member. It seems to show actin, UNC-87 and Proteasome regulatory particle may play distinct roles in specific aspects of organization of macromolecular structures under desiccation stress. GroEL and GroES are members of the Hsp60 family of chaperons.

Relationship between thermotolerance and hydrophobin-like proteins in aerial conidia of Beauveria bassiana and Paecilomyces fumosoroseus as fungal biocontrol agents

AIMS

This study was to illustrate the relationship between the thermotolerance and the contents of hydrophobin-like or formic-acid-extractable (FAE) proteins in aerial conidia of Beauveria bassiana and Paecilomyces fumosoroseus produced on rice-based substrate.

METHODS AND RESULTS

Survival indices of 11 isolates were separately assessed as a ratio of the viability of conidia after 3-150 min thermal stress at 48 degrees C over that of unstressed conidia and fitted well to a survival model (r(2) >/= 0.97). For a given isolate, the fitted model generated an LT(50), the time for 50% viability loss under the stress. The LT(50)s of six B. bassiana isolates (10.1-61.9 min) and five P. fumosoroseus isolates (2.8-6.2 min) were correlated (r(2) = 0.81) with FAE protein contents (6.9-23.4 microg mg(-1)). The survival indices of a fixed B. bassiana isolate after 45-min thermal stress at 48 degrees C were also correlated to the FAE protein contents from conidia produced on glucose-, sucrose-, or starch-based substrate (0.79 </= r(2) </= 0.86). Three bands of 12.0, 15.0 and 17.5 kDa were recognized from SDS-PAGE profiles of protein extracts from different isolates or from the fixed isolate grown on the three carbon sources, but the 12 kDa was less likely related to the thermotolerance.

CONCLUSIONS

Approximately 80% of variability in conidial thermotolerance was attributed to either 15.0- or 17.5-kDa FAE protein or both.

SIGNIFICANCE AND IMPACT OF THE STUDY

The relationship between conidial thermotolerance and FAE protein contents implicates a new way to search for fungal biocontrol agents with more thermotolerance and environmental persistence.