Plan B for Stimulating Stem Cell Division

Plant development relies on two kinds of coordinated regulatory inputs to generate an optimal plant body. First are inputs regulating the spatial organization of cells in the plant. These ‘‘hardwired’’ inputs are invariant between individuals and their actions are buffered from the environment. Second are variable inputs that modify the development of tissues to optimize growth for given conditions of water, gravity, nutrients, and light. Defining these pathways and understanding how they work together is a major challenge for plant biologists. Work by Turner and colleagues in this issue of PLOS Genetics [1] moves us a step closer by elucidating a link between two pathways that control proliferation of a stem cell population that produces vascular cells. These two pathways are a receptor–ligand pathway, which represents the first type of hardwired machinery, and the ethylene signaling pathway, which traditionally has been considered an environmentally dependent pathway. In growing plants, stem cells at the tips of roots and shoots add new cells to the plant body. In shoots these cells generate new organs, leaves, and stem sections, each segment of leaf and stem adding additional length to the plant body. This can lead to very long branches. However, in order for plants to reach a significant size, they must add cells to their girth as well. Adding girth allows the plant to support branches and lets these expand the canopy where newly made leaves can compete for sunlight. Girth is added through the action of a second source of stem cells. These stem cells are located in a ring within the stem or trunk of a tree, where they generate new vascular cells (Figure 1A). In the stem of Arabidopsis, these are called procambial cells. The procambial stem cells are sandwiched between the two vascular cell types they give rise to: phloem cells (toward the outside of the plant), the carriers of sugars from the leaves to roots, fruits, and other ‘‘sink’’ organs; and xylem cells (toward the inside of the plant), the carriers of water and minerals. Procambium is used to denote the stem cells in vascular bundles of newly formed organs. In plants such as trees that show persistent lateral growth, the procambium gives rise to a more substantial, continuous ring of stem cells called a cambium. Arabidopsis, while not a perennial, and certainly not a tree, nevertheless exhibits secondary growth and a well-developed cambial stem cell population in the hypocotyl, the short stem below the rosette. The position of the procambium between the two types of descendant cells is critical to its production of organized phloem and xylem strands. Regulated orientation of cell divisions within the procambium maintains this organization as newly generated cells are fed into the differentiation pathways. Cell divisions in the long, narrow progenitor cells are oriented along the long axis of the stem (Figure 1B). Since most plant cell division planes cut across the narrowest dimension of the cell, orienting new walls such that they span the longest dimension likely requires specialized machinery controlled by specialized regulators. Environmental cues affect the activity of cambial stem cells. In trees, the vascular cambium goes through cycles of activity and inactivity with seasons. In winter the cambium is dormant, but it becomes active again during summer, resulting in the characteristic annular rings of wood. Gravity also regulates cambial growth: when trees lean, the cambium on the upper side of the trunk grows at a different rate from the lower side to generate structural support (i.e., ‘‘tension wood’’) [2]. In Arabidopsis, the tracheary element differentiation inhibition factor/CLE41 (TDIF/CLE41) peptide ligand is secreted from the phloem and interacts with the TDIF RECEPTOR/PHLOEM INTERCALATED WITH XYLEM (TDR/PXY) membrane receptor kinase expressed in adjacent cambial stem cells (Figure 1B) [3]. This signal accomplishes three things. First, it stimulates cell division within the cambium. To do this, it requires the downstream transcription factor WOX4. Second, it prevents stem cells in the cambium from becoming xylem cells. Third, it regulates the orientation of cell divisions. The latter two steps do not require WOX4 action [4,5]. Regulation of cell division orientation in the procambium requires the polar production of TDIF peptide [4]. If TDIF peptide is produced on both sides of the cambium, or only on the xylem side, cell division planes in the procambial cells become highly irregular. Thus, the tissuespecific synthesis of TDIF (in the phloem) and the detection of its asymmetric distribution (in the procambium) are part of an invariant developmental pathway that produces spatially organized vascular strands. The phenotypes caused by overexpressed TDIF peptide were eliminated in pxy mutants, indicating that the TDIF signal requires the PXY receptor to act. However, surprisingly, loss-of-function pxy mutants exhibited only a mild decrease in procambial cell numbers. This suggests that the plant possesses a ‘‘plan B’’ to stimulate procambial stem cell division. In their new work, Turner and colleagues [1] identify the bypass mechanism as signaling through the gaseous hormone ethylene. The first clue that ethylene mediates the bypass pathway came when Etchells et al. found increases in mRNA abundance for some members of the APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ ERF) family of transcription factors in pxy mutants. Since AP2/ERF gene family expression is elevated in response to