Biochemical genetics of Neurospora crassa conidial germination

Abstract

A compilation of data has been presented comparing the conidia of Neurospora with the vegetative mycelia. These two phases of the asexual life cycle were compared with respect to their content of DNA, RNA, protein, lipid, 20 different low molecular weight compounds, and cell wall composition. The specific activities of 21 enzymes from both phases are compared. In general, conidia have more similarities than differences to vegetative mycelia. Some of the important differences found between them so far are as follows: conidia have few polysomes, little oxidative phosphorylation, no cell wall galactosamine, and low cellular pool levels of arginine and ornithine. They contain higher amounts of glutamic acid, oxidized glutathione, phospholipid, and a hydrophobic surface layer. The biochemical changes that occur during conidial germination are summarized. The known biochemical events can be grouped into three classes. The first group of events occurs in the first 10 to 20 min, and only requires hydration of the conidia. Some of these early events can be grouped into a temporal sequence, similar to a multistep trigger reaction, and a model for the breaking of dormancy is proposed based on these events. The second group of events occurs after a few hours, and requires a carbon source. This group of events, such as mitochondrial changes and transport changes, appear to be related in some way to changes in the structure of the membranes. A third group of events occurs considerably later in germination and involves the activation of the synthesis of two cell wall polymers. The prominent events that occur very early in germination are the degradation of a large endogenous glutamic acid pool, a surge in the level of NADH and NADPH, the enzymatic reduction of the high content of oxidized glutathione, and the formation of polysomes. The first three of these events are, in some sense, unusual biochemical events. First, they appear to be unique to germination and were not detected at any other time. Second, they all start and finish during the initial phases of germination, as opposed to many events, such as RNA synthesis which may start in the first few minutes, but then continue throughout vegetative growth. A four point model for dormancy and conidial germination is proposed which is based primarily on these three early events. One prediction of this model is that cellular proteins in the conidia have a high content of disulfide bonds. An elaboration of this idea is put forth with respect to how internal cross linking of this type could stabilize and inactivate enzymes in a readily reversible manner. Other aspects of the model are discussed with reference to parallels found in the germination of bacterial spores and plant seeds. There is also a discussion of the previous and potential uses of genetic approaches to studies on a particular phase of development, such as germination.