Cell-to-Cell Movement of Green Fluorescent Protein Reveals Post-Phloem Transport in the Outer Integument and Identifies Symplastic Domains in Arabidopsis Seeds and Embryos

Abstract

Developing Arabidopsis (Arabidopsis thaliana) seeds and embryos represent a complex set of cell layers and tissues that mediate the transport and partitioning of carbohydrates, amino acids, hormones, and signaling molecules from the terminal end of the funicular phloem to and between these seed tissues and eventually to the growing embryo. This article provides a detailed analysis of the symplastic domains and the cell-to-cell connectivity from the end of the funiculus to the embryo, and within the embryo during its maturation. The cell-to-cell movement of the green fluorescent protein or of mobile and nonmobile green fluorescent protein fusions was monitored in seeds and embryos of plants expressing the corresponding cDNAs under the control of various promoters (SUC2, SUC3, TT12, and GL2) shown to be active in defined seed or embryo cell layers (SUC3, TT12, and GL2) or only outside the developing Arabidopsis seed (AtSUC2). Cell-to-cell movement was also analyzed with the low-molecular-weight fluorescent dye 8-hydroxypyrene-1,3,6-trisulfonate. The analyses presented identify a phloem-unloading domain at the end of the funicular phloem, characterize the entire outer integument as a symplastic extension of the phloem, and describe the inner integument and the globular stage embryo plus the suspensor as symplastic domains. The results also show that, at the time of hypophysis specification, the symplastic connectivity between suspensor and embryo is reduced or interrupted and that the embryo develops from a single symplast (globular and heart stage) to a mature embryo with new symplastic domains.

Coordination of apical and basal embryo development revealed by tissue-specific GNOM functions

“…Being much larger than 3ϫGFP whose cell-to-cell movement was very limited, GNOM should be restricted to the cells of expression. This is also supported by the specificity of observed rescue phenotypes of different transactivation lines: although suspensor and globular embryo form a symplastic continuum for 1ϫGFP (27 kDa) (Stadler et al, 2005), GNOM transactivation from the suspensor and hypophyseal promoters showed distinct phenotypes. Detection of low-level protein expression is always problematic.…”

A Cascade of Sequentially Expressed Sucrose Transporters in the Seed Coat and Endosperm Provides Nutrition for the Arabidopsis Embryo

“…Developing Arabidopsis seeds have at least three apoplasmic borders that require membrane transporters: (1) between the outer integument (a symplasmic extension of the phloem) and the inner integument; (2) between the inner integument and the endosperm; and (3) between the endosperm and the embryo (Stadler et al, 2005). Based on expression patterns, all three SWEETs contribute in cascade-like patterns to distinct steps in seed filling.…”

“…In Arabidopsis thaliana, sugars are delivered to the maternal seed coat via the funicular phloem, which is symplasmically connected to the outer integument (Stadler et al, 2005). Sugars are released from the outer integument to the inner integument, and potentially the suspensor, followed by secretion into the seed apoplasm by yet unknown membrane transport mechanisms.…”

Endosperm and Nucellus Develop Antagonistically in Arabidopsis Seeds

“…Consistent with this hypothesis, we detected starch granules in the nucellus. Nevertheless, nutrients were shown to travel to the endosperm and embryo via the seed coat (Stadler et al, 2005;Chen et al, 2015). Further analyses are necessary to determine whether nutrients are allocated to the endosperm also through the nucellus.…”