**Xiao Han 1,**†**, Li-Jun Huang 2,**†**, Dan Feng 3,**†**, Wenhan Jiang 2, Wenzhuo Miu <sup>2</sup> and Ning Li 4,\***


Received: 27 May 2019; Accepted: 13 June 2019; Published: 17 June 2019

**Abstract:** Plant cells are separated by cellulose cell walls that impede direct cell-to-cell contact. In order to facilitate intercellular communication, plant cells develop unique cell-wall-spanning structures termed plasmodesmata (PD). PD are membranous channels that link the cytoplasm, plasma membranes, and endoplasmic reticulum of adjacent cells to provide cytoplasmic and membrane continuity for molecular trafficking. PD play important roles for the development and physiology of all plants. The structure and function of PD in the plant cell walls are highly dynamic and tightly regulated. Despite their importance, plasmodesmata are among the few plant cell organelles that remain poorly understood. The molecular properties of PD seem largely elusive or speculative. In this review, we firstly describe the general PD structure and its protein composition. We then discuss the recent progress in identification and characterization of PD-associated plant cell-wall proteins that regulate PD function, with particular emphasis on callose metabolizing and binding proteins, and protein kinases targeted to and around PD.

**Keywords:** plant cell walls; plasmodesmata; callose; callose synthase
