Studies on Isaria cretacea. Nutritional and morphological characteristics of two strains and morphogenesis of the synnema.

Abstract

A strain of Isaria cretacea van Beyma was isolated from soil and from powdery leaves of a tomato plant. The white phototropic synnema is referred to as an indeterminate synnema because it bears spores on the lower portion only and the tip is capable of indeterminate growth.This isolate (strain (A)) can utilize ammonia, amino and nitrate nitrogen, and a number of carbohydrates and related compounds as carbon sources for growth. Germination requires an energy source but little or no exogenous vitamins. Vegetative growth requires exogenous biotin and the pyrimidine moiety of thiamin. Biotin can be replaced with biotin-L-sulphoxide and biocytin and partially with desthiobiotin, but not with homobiotin, norbiotin, aspartic acid, pimelic acid, oleic acid, Tween 80, or various combinations of them.Strain (A) continuously gave rise de novo to a variant (variant strain (B)) which did not develop synnemata and which differed from the parent in possessing a higher growth rate. The two could enter into a heterokaryotic state which suppressed sporulation and synnemata formation.The synnema consists of three structures: a terminal growing point, a central column of closely packed, sterile hyphae, and a mantle of hyphae. Spores are borne on the portion of mantle enveloping the lower half of the column.Morphogenesis of the synnema can be divided into four phases: differentiation of certain hyphae of the assimilative mat into those capable of developing into synnema, orientation of the hyphae away from their origin, continued growth, and branching and sporulation of the hyphae. The entire synnema remains metabolically active and may continue growing, since 2 hours after injecting C14-labelled glucose into the agar beneath a mature synnema, all hyphae of the structure become labelled.Neither negative geotropism nor phototropism is essential for the initiation of synnemata formation. Synnemata formation is influenced by the carbon source but not by the nitrogen source. The requirement for biotin and thiamin appears related to the requirement for general hyphal growth.