Calcium signaling contributes to xylem vessel cell differentiation via post-transcriptional regulation of VND7 downstream events.

Secondary cell walls (SCWs) accumulate in specific cell types of vascular plants, notably xylem vessel cells. Previous work has shown that calcium ions (Ca) participate in xylem vessel cell differentiation, but whether they function in SCW deposition remains unclear. In this study, we examined the role of Ca in SCW deposition during xylem vessel cell differentiation using suspension-cultured cells carrying the VND7-inducible system, in which VND7 activity can be post-translationally upregulated to induce transdifferentiation into protoxylem-type vessel cells. We observed that extracellular Ca concentration was a crucial determinant of differentiation, although it did not have consistent effects on the transcription of VND7-downstream genes as a whole. Increasing the Ca concentration reduced differentiation but the cells could generate the spiral patterning of SCWs. Exposure to a calcium-channel inhibitor partly restored differentiation but resulted in abnormal branched and net-like SCW patterning. These data suggest that Ca signaling participates in xylem vessel cell differentiation via post-transcriptional regulation of VND7-downstream events, such as patterning of SCW deposition.