Involvement of calcium in auxin-induced cell differentiation in the protonema of the wild strain and auxin mutants of the moss Funaria hygrometrica

Protonemata of the wild strain of the moss Funaria hygrometrica and its auxin-sensitive and -insensitive mutants have been used to demonstrate the modulation of auxin-induced protonema differentiation by calcium. All the strains under investigation (except wild type) require a certain threshold level of Ca2+ in the medium in order to exhibit caulonema differentiation by auxin treatment. Verapamil (an extracellular calcium channel blocker) suppresses caulonema induction by IAA in the auxin-sensitive mutant 87.13 and nullifies auxin action in the wild type. The involvement of intracellular Ca2+ and the mediation of the response through calmodulin have been demonstrated by the use of TMB-8 (an intracellular Ca2+ antagonist) and TFP (calmodulin inhibitor), respectively. Calcium ionophore A-23187 substitutes IAA by triggering caulonema differentiation in both auxin-sensitive and -insensitive mutants. Using the metallochromic dye arsenate III, measurement of calcium fluxes shows that chloronema cultures do not exhibit any rapid calcium uptake due to auxin treatment. Induction of caulonema in the auxin-sensitive mutant (87.13) and its further stimulation by IAA in the wild strain, are accompanied by a rapid increase in calcium uptake within 5 min of IAA treatment. The auxin-insensitive mutant (86.1) does not show any marked change in calcium uptake due to IAA treatment as also its inability to exhibit caulonema differentiation. Thus, IAA-induced protonema differentiation appears to be accompanied by a rapid influx of Ca2+.