GERMINATION TRIGGERS OF METARHIZIUM-ANISOPLIAE CONIDIA ARE RELATED TOHOST SPECIES

The role of selectable strain variations in the development of pathoge n strategies was examined using lines of Metarhizium anisopliae isolat ed from homopteran (isolate 549) or coleopteran (isolate 808) hosts. C onidia of strain 549 germinated in either alanine, glucose, cyclic AMP or the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) . The non-metabolizable glucose analogues, 3-O-methylglucose and 6-deo xyglucose, did not allow germination by themselves but stimulated germ ination when added to IBMX. By contrast, 2-deoxyglucose (dGlc) blocked germination on glucose or IBMX and inhibited hyphal growth on other c arbon sources including alanine and glycerol. Conidia of strain 808 ge rminated rapidly in alanine but responded slowly to glucose or IBMX in the medium and were resistant to the growth inhibitory effects of dGl c. Radioactive dGlc was taken up by conidia of strains 549 and 808 at similar rates and was recovered mainly as 2-deoxyglucose 6-phosphate. Competition experiments utilizing both strains demonstrated that gluco se, dGlc and 3-O-methylglucose were transported by the same system. Fr uctose was much less able than glucose to inhibit uptake of dGlc indic ating that fructose is taken up by a different transport system than t hat for glucose. It is unlikely, therefore, that the resistance of str ain 808 to dGlc is explained by reduced sugar transport compared with strain 549 but that strains 549 and 808 differ in the regulation of ca rbon metabolism with some systems in strain 808 showing resistance to the catabolite-repressing effects of glucose. Apparently, catabolite r epression is subdivided into different segments as glucose inhibited t he derepression of a number of catabolite repressible enzymes in strai n 808, including the pathogenicity determinant protease Pr1. The same effect was produced by dGlc but not by 3-O-methylglucose, indicating t hat the trigger for catabolite repression occurs at the revel of trans port-associated glucose phosphorylation. A comparative study of 26 iso lates indicated that most lines from coleopteran hosts were dGlc resis tant and germinated poorly on glucose. Conversely, isolates germinatin g well on glucose (mostly from hemipteran and lepidopteran hosts) were dGlc susceptible.