Comparison of Nostocean hormogonium induction and its motility on solid plates between agar and gellan gum at varying gel matrix concentrations

To establish a sensitive bioassay for Nostocean hormogonium induction, we compared the effectiveness of the morpho-differentiation induction on two gelled plates, agar and gellan gum, for anacardic acid C15:1-Δ⁸ decyl ester (<b>1</b>) (100 nmol/disc). On BG-11₀ (nitrogen-free) medium-based 0.6 and 0.8% agar plates, <i>Nostoc</i> sp. strain Yaku-1 isolated from a coralloid root of <i>Cycas revoluta</i> in Yakushima Island showed clear morpho-differentiation from filamentous aggregates into hormogonia, and the induced hormogonia dispersed within 24 h; however, similar hormogonium formation was not observed at agar concentrations of 1.0% or higher. Conversely, hormogonium induction was considerably more pronounced on gellan gum plates than those on agar plates through concentrations ranging from 0.6 to 1.6% even after 12 h of incubation, particularly active on the 0.8–1.0% gellan gum plates. Thus, gellan gum plates can achieve clear results within 12 h and are thus highly useful for primary screening for hormogonium-inducing factors (HIFs). Rather than agar, effective Nostocean hormogonium induction was performed on 1% gellan plate. Differentiation into hormogonia and active gliding were observed within 12 h.