Embryogenesis As a Model System to Dissect the Genetic and Developmental Regulation of Cell-to-Cell Transport Via Plasmodesmata

Abstract

The Arabidopsis embryo is a single symplast where all its cells allow intercellular communication via plasmodesmata (PD). However, PD apertures are dynamic and alter during embryonic development, both in terms of their general transport capacity within the entire embryo, and in mediating intercellular transport capacity between different regions of the embryo. Endogenous expression of 1X, 2X, and 3XGFP tracers illustrates that early embryos (up to the heart stage) traffic very large molecules such as 2XGFP (54 kDa). The embryo remains a single symplast for even 1XGFP (27 kDa) until the late-torpedo stage. Sub-division of the embryo into different symplastic sub-domains as a function of PD aperture is revealed at the mid-torpedo stage by more restricted movement of larger tracers; 2XGFP cannot move into the cotyledons, and 3XGFP cannot move down into the root tip. Tobacco mosaic virus (TMV) movement protein, P30, moves cell-to-cell via PD in embryos. P30-GFP targets to PD and forms puncta, just as it has been shown to do in older leaf tissues, and P30-GFP (57 kDa) exhibits more movement than similarly sized 2XGFP (54 kDa). As the embryo predominantly contains simple PD (∼90% of embryonic PD are simple) these data demonstrate that P30 also targets to simple (unbranched) PD. Two mutants allowed increased movement of exogenously added 10 kDa F-dextrans, called increased size exclusion limit 1 and ise2, and result in the increased formation of twinned and branched PD. Gene silencing of ISE1 and ISE2 leads to de novo production of secondary PD in young leaves, and concomitant increased cell-to-cell transport. ise1 and ise2 null mutants are lethal attesting to their essential function. ISE1 localizes to mitochondria and ISE2 localizes to chloroplasts. Thus, cellular homeostasis as mediated by organelle function, dramatically affects PD formation and function.