Cytoskeletal and Ca2+ regulation of hyphal tip growth and initiation

Hyphal tip growth is a complex process involving finely regulated interactions between the synthesis and expansion of cell wall and plasma membrane, diverse intracellular movements, and turgor regulation. F-actin is a major regulator and integrator of these processes. It directly contributes to (a) tip morphogenesis, most likely by participation in an apical membrane skeleton that reinforces the apical plasma membrane, (b) the transport and exocytosis of vesicles that contribute plasma membrane and cell wall material to the hyphal tips, (c) the localization of plasma membrane proteins in the tips, and (d) cytoplasmic and organelle migration and positioning. The pattern of reorganization of F-actin prior to formation of new tips during branch initiation also indicates a critical role in early stages of assembly of the tip apparatus. One of the universal characteristics of all critically examined tip-growing cells, including fungal hyphae, is the obligatory presence of a tip-high gradient of cytoplasmic Ca2+ that probably regulates both actin and nonactin components of the apparatus, and the formation of which may also initiate new tips. This review discusses the diversity of evidence behind these concepts.

Imaging of total calcium in urediospore germlings of Uromyces by ion microscopy

Calcium has been implicated in growth and appressorium formation of urediospore germlings of the bean rust fungus, Uromyces appendiculatus. Using ion microscopy, a mass spectrometry-based imaging technique, intracellular stores of calcium were analyzed by direct imaging of total calcium in frozen freeze-dried germlings. Calcium concentration was calculated by ratioing and spatially registering (40)Ca to (12)C signals. Intracellular distributions of total potassium, sodium, magnesium, and carbon were similarly imaged in the same germlings for a direct comparison of their localizations to total calcium. Calcium was remarkably heterogeneous with highest concentrations (2 to 10 mM) in the mid-region of the germling between the nuclei and the apex. A similar distribution of Ca(2+) (assessed using Fluo-3) was also noted sequestered in organelles in live germlings. Distributions of remaining elements (K, Na, Mg, and C) were mostly homogeneous throughout the cytoplasm and nuclei of the fungal cell. The K/Na ratio ranged from 17 to 31.