Spore Germination and Pre-Symbiotic Mycelial Growth

Abstract

Germination, in arbuscular mycorrhizal (AM) fungi, is not regulated by host-derived signals, since their spores are capable of germination and growth, under adequate physical, chemical and microbiological conditions, in the absence of host plants. The molecular signals which relieve spore dormancy and activate the cell cycle still remain unknown, yet environmental conditions initiating germination processes have been detected in different genera and species of AM fungi. Environmental factors playing the most important roles in spore germination, such as pH, temperature, moisture, mineral and organic nutrients, host/nonhost plants, and microorganisms are reviewed here. Recent developments have contributed to the understanding of cellular and molecular events involved in the early stages of the life cycle of AM fungi, from relief of spore dormancy to pre-symbiotic mycelial growth and developmental arrest in the absence of colonization of roots. Biochemical and genetic studies indicated that germinating spores do possess the metabolic machinery for hyphal growth, that no vital metabolic pathway is blocked, and that spore reserves are not totally depleted when germlings cease growing within 15–20 days of germination in the absence of the host. Data on withdrawal of protoplasm from peripheral hyphae and on their senescence and decrease in metabolic activity have shown that a mechanism allowing propagule survival and long-term infectivity of mycelium operates when spores of AM fungi germinate in the absence of a carbon donor. This inconsistency, an obligate symbiont which germinates in the absence of its host, is the most fundamental puzzle in the way in which these obligately biotrophic organisms have survived the past 400 million years.