Cell-cell interactions during plant development.

The large body of information that has come from genome sequencing projects has demonstrated that plants and animals have many sequences in common, suggesting that they have either co-opted the same molecules for different purposes, or that they utilize similar mechanisms (Reuzeau and Pont-Lezica 1995; Clark 1996). One might expect many housekeeping tasks, such as regulation of the cell cycle, to be similar in plants and animals, but what of the multicellular tasks, the developmental processes that coordinate growth and differentiation of cells? Unlike animal cells, plant cells never move, thus induction by cell movement such as occurs during gastrulation in animal embryo development, is not likely to occur. Lineage-based development would be convenient in plants as they are forever attached to their parent cell; however, with few exceptions (Sachs 1978), plant cells do not appear to differentiate based on lineage. How, then do plant cells find their cues? A growing body of clonal analysis supports the statement that plant cells differentiate according to position (Steffensen 1968; Johri and Coe 1983; Jegla and Sussex 1989; Irish and Sussex 1992; Bossinger and Smyth 1996). Initial cells are as likely to contribute to somatic tissues such as leaves as they are to the eggs and sperm embedded in the flower; in other words, there are no sequestered germ cells. The cells of the epidermis, or outer layer, are clonally related, but if a cell divides such that a daughter cell is pushed into the internal tissues, that daughter cell takes on the fates of internal cells. Similarly, the internal cells that find themselves pushed into the epidermis differentiate as epidermis (Stewart 1978). Differentiation of cell types in the Arabidopsis root appears to follow specific lineages (Dolan et al. 1993, 1994; Scheres et al. 1994), however, laser ablation experiments have shown that ablated cells are replaced by cells from another lineage which differentiate according to their new position (van den Berg 1995). These experiments suggest that the apparent lineage of the root is a consequence of the simple and regular cellular arrangements. Positional information has also been shown to play a role in the differentiation of photosynthetic cell types in