Humic matter elicits proton and calcium fluxes and signaling dependent on Ca2+-dependent protein kinase (CDPK) at early stages of lateral plant root development

Background: The humic acid (HA) fraction of soil organic matter (SOM) exerts an effective plant growth promotion through a complex mechanism involving a coordinated activation of several key ion transport and signaling systems. We investigated the effects of HA on H+ and Ca2+ cellular dynamics at the early stages of lateral plant root development. Results: Emergence of lateral root in rice seedlings were related to specific H+ and Ca2+ fluxes in the root elongation zone underlying an activation of the plasma membrane H+-ATPase and of the Ca2+-dependent protein kinase (CDPK). The latter was coupled with an increased expression of the voltage-dependent OsTPC1 Ca2+ channels and two stress responsive CDPK isoforms, such as OsCPK7 and OsCPK17. Conclusions: HA act as molecular elicitors of H+ and Ca2+ fluxes, which seem to be upstream of a complex CDPK cell-signaling cascade. These findings shed light on the first ion signaling events involved in the mechanism of action of HA on plant growth and development.