Roles for callose in plant development and intercellular communication

Abstract

Cell-to-cell communication through plasmodesmata (PD) exists between most plant cells. The small fluorescent dye Lucifer Yellow CH (LYCH) has been used to determine that in the Arabidopsis root meristem there are tissues where PD occlude the movement of small molecules. A putative callose synthesis inhibitor chlorpromazine (CPZ) was tested and found to be effective. CPZ removed restrictions on LYCH movement in root tips, and impaired root gravitropism, supporting the idea that closure of PD in the root tip can be due to formation of callose at PDs, and that this is essential to maintain the normal circulation pattern for auxin. Other studies have used Arabidopsis expressing GFP under the control of a companion cell-specific promoter AtSUC2 to conclude that GFP protein gets into all cells of the root meristem by moving through PD. We reexamined the movement of AtSUC2::GFP in an attempt to explain why GFP moves through PD where LYCH, a smaller molecule, cannot. We found that while GFP moves through the phloem and out into all cells of the Arabidopsis root meristem, movement out of the stele is very slow. More importantly we found that GFP mRNA is able to move through all the PD. The newly assigned SEL of 27 kDa for cells of the Arabidopsis root meristem may thus be a gross overestimate. We speculate that there may be two types of PD in the Arabidopsis root meristem: one which allows the quicker diffusion of small molecules such as LYCH, the other which allows slower movement and has a higher SEL. Because chemical inhibitors are not specific, we then took a genetic approach towards looking at the importance of callose in plant intercellular communication. Callose is synthesized by callose synthase. Arabidopsis has 12 putative callose synthase genes (GSL1-12 ). We used TILLING and T-DNA insertion mutants to show that GSL1 and GSL5 are essential for normal sporophytic development in Arabidopsis and are also involved in pollen grain formation during cytokinesis, tetrad dissociation, cell expansion and mitosis. While we have not yet found which GSL makes PD callose, GSL3 is a candidate.