Suppressing a Putative Sterol Carrier Gene Reduces Plasmodesmal Permeability and Activates Sucrose Transporter Genes during Cotton Fiber Elongation.

Plasmodesmata (PDs) play vital roles in cell-to-cell communication and plant development. Emerging evidence suggests that sterols are involved in PD activity during cytokinesis. However, whether sterols contribute to PD gating between established cells remains unknown. Here, we isolated , a putative sterol carrier protein gene from elongating cotton () fibers. In contrast to wild-type fiber PDs, which opened at 5 to 10 d postanthesis (DPA) and closed only at 15 to 25 DPA, plants with suppressed expression had reduced sterol contents and closed PDs at 5 through 25 DPA The suppressed fibers exhibited callose deposition at the PDs, likely due to reduced expression of , which encodes a PD-targeting β-1,3-glucanase. Both expression and callose deposition were sensitive to a sterol biosynthesis inhibitor. Moreover, suppressing upregulated a cohort of and sucrose transporter genes in fiber cells. Collectively, our results indicate that (1) GhSCP2D is required for expression to degrade callose at the PD, thereby contributing to the establishment of the symplasmic pathway; and (2) blocking the symplasmic pathway by downregulating activates or increases the expression of and , leading to the switch from symplasmic to apoplasmic pathways.