Calcium homeostasis is required for contact-dependent helical and sinusoidal tip growth in Candida albicans hyphae

Hyphae of the dimorphic fungus, Candida albicans, exhibit directional tip responses when grown in contact with surfaces. On hard surfaces or in liquid media, the trajectory of hyphal growth is typically linear, with tip re-orientation events limited to encounters with topographical features (thigmotropism). In contrast, when grown on semisolid surfaces, the tips of C. albicans hyphae grow in an oscillatory manner to form regular two-dimensional sinusoidal curves and three-dimensional helices. We show that, like thigmotropism, initiation of directional tip oscillation in C. albicans hyphae is severely attenuated when Ca(2+) homeostasis is perturbed. Chelation of extracellular Ca(2+) or deletion of the Ca(2+) transporters that modulate cytosolic [Ca(2+)] (Mid1, Cch1 or Pmr1) did not affect hyphal length but curve formation was severely reduced in mid1Δ and cch1Δ and abolished in pmr1Δ. Sinusoidal hypha morphology was altered in the mid1Δ, chs3Δ and heterozygous pmr1Δ/PMR1 strains. Treatments that affect cell wall integrity, changes in surface mannosylation or the provision of additional carbon sources had significant but less pronounced effects on oscillatory growth. The induction of two- and three-dimensional sinusoidal growth in wild-type C. albicans hyphae is therefore the consequence of mechanisms that involve Ca(2+) influx and signalling rather than gross changes in the cell wall architecture.