Commentary Unlocking the mysteries of leaf primordia formation Robert E. Cleland* Botany Department, Box 355325, University of Washington, Seattle, WA 98195-5325 he shoot apical meristem (SAM), at the (Ahtet) induced the expression of expansin sulted in the induction of LPs that Ttip of the plant stem, has two roles; it is in those cells to which Ahtet was applied. subsequently grew into fairly normal leaves. the source of the new cells that are needed When Ahtet was applied to a small region of If IAA was applied to the I2 position on for stem growth, and it is the site of small the SAM expansin was induced in L1–L3 normal apices, a primordium would appear cellular outgrowths, called leaf primordia layers. Induction of expansion at the I2 there, instead of the normal I1 position. (LP), that develop into the leaves (1). These location, where a LP would normally arise It is not too surprising that auxin and LPs occur in predictable positions, with the only after one appeared at the I1 location, expansin might have similar effects on LP site of the next primordium (I1) being spec- resultedinaLPatthatspot.Anup- initiation. Enlargement of plant cells is con- ified by the location of the most recently regulation of expansin, then, is sufficient to strained by a resistant cell wall composed of appearing primordia (e.g., P1,P2). The or- cause a leaf to be formed at the SAM, even cellulose microfibrils, crosslinked by other dered arrangement of LPs around the cir- at a spot where its development would nor- polysaccharides such as xyloglucans (16). cumference of the SAM is known as phyl- mally be strongly inhibited. When LPs form The wall prevents the osmotic uptake of lotaxy (2). A major question in plant in vivo, there can also be an up-regulation of water into the cells until the wall is loosened; development is what events occur at the I1 expansin. In tomato apices (10) one expan- i.e., until crosslinks in the wall are cleaved position, so that the primordium develops at sin gene, LeExp18, is up-regulated in LP (5). In many cases where auxin promotes this spot rather than elsewhere on the SAM. coincident with the origin of the primor- cell enlargement, it does so by inducing cells Two recent papers now provide some of the dium. Likewise, in deep-water rice, the Os- to excrete protons into the cell walls, and the answers. In this issue of PNAS, Pien et al. (3) EXP1 expansin gene is expressed primarily lowered apoplastic pH activates expansins have demonstrated that a localized induc- in the youngest LP (11). The LP, once that break the crosslinks between the cellu- tion in the SAM of the wall-loosening pro- induced by Ahtet, continued to develop into lose microfibrils (17). The hydrostatic pres- tein expansin is sufficient to induce a pri- a complete, normal leaf, even though the sure of the cell contents, the turgor pressure, mordium and set into motion all of the expansin was induced for only the first day then expands the loosened walls. The rate of events needed to produce a mature leaf. (3). This finding indicates that initiating a cell enlargement can depend on both the Reinhardt et al. (4) obtained similar results, LP is sufficient to set into motion the com- amount of expansin present and the apo- COMMENTARY but used instead a localized application of plete developmental pathway leading to a plastic pH. This is shown by results obtained the plant hormone auxin. Together, these mature leaf. Some targeted signal that acti- with tobacco BY2 cells (18). Addition of two papers are starting to answer some of vates these specific expansin genes in a expansin caused these cells to expand, indi- the main questions about LP formation. particular set of SAM cells could be con- cating that the endogenous expansin was Expansins are a family of small (25–27 trolling the location of leaf formation. How- nonoptimal. But expansion also was induced kDa) proteins that are localized in the cell ever, the subsequent growth of the leaf still when the walls were acidified by the fungal wall (5). They have no known enzymatic is influenced by expansin. When Ahtet was toxin fusicoccin., indicating that the apo- activity (6), but have the ability to break applied to one side of a P2 primordium, plastic pH was also suboptimal. Most cells hydrogen bonds between cell wall polysac- there was a subsequent increase in the size may have suboptimal levels of both expansin charides when activated by an acidic envi- of the leaf blade on that side. and wall pH and therefore limited potential ronment (7). The expansins are primarily But expansin is not the only exogenous for enlargement; such a situation may exist localized in the expanding regions of plants factor that can induce the formation of LPs. in the SAM. But if there is an increase in and are believed to be responsible for the The auxin hormone indoleacetic acid (IAA) either expansin or a lowering of the apo- cell wall loosening that is required for plant has a similar effect. The site of IAA synthe- plastic pH in response to auxin the set of cell expansion (5). In 1997 Fleming et al. (8) sis in plants is uncertain, but is believed to be cells may grow and form a LP. applied a cucumber expansin to the surface in young leaves and perhaps the apical mer- In meristematic regions in plants growth of tomato SAMs, and a primordia-looking istem (12). IAA then moves by polar auxin consists of an increase in volume coupled outgrowth developed at that spot. This out- transport (PAT), a process that involves with division, so as to result in additional growth did not result in a recognizable leaf, symmetrical uptake of IAA into cells cou- cells of approximately the size of the original however. They believed that the problem pled with asymmetrical efflux of auxin from cells. There has long been a controversy as was that the expansin did not penetrate only one end of the cell (13). This results in to whether growth starts by an increase in beyond the outer (L1) layer of the SAM (9); a one-directional movement of auxin. Inhib- cell size, which triggers division, or whether it was well known (1) that the LPs are itors of PAT such as naphthylphthalamic division occurs first, followed by restoration formed from cells from least three cell lay- acid or 2,4,5-triiodobenzoic acid are known of the original cell size (19). The fact that the ers in the SAM (the L2 and L3 layers in to alter the phyllotaxy (14) or to completely first visible sign of a new LP is a periclinal addition to the L1). prevent the formation of LPs (4). Mutants division in the L1 or L2 layer has suggested Pien et al. (3) have used an imaginative of Arabidopsis that are blocked in PAT, such that division comes first (1). However, ␥- approach to overcome the penetration as pin1, likewise fail to produce LPs (15). irradiated wheat seeds, which could not problem. They introduced a cucumber ex- Reinhardt et al. (4) found that application of pansin gene into tobacco plants combined IAA to a localized position on naphth- with the tetracycline-inducible promoter ylphthalamic acid-treated tomato stem api- See companion article on page 11812. system. Application of anhydrotetracycline ces that normally would form no LPs, re- *E-mail: [email protected]. www.pnas.org͞cgi͞doi͞10.1073͞pnas.211443498 PNAS ͉ September 25, 2001 ͉ vol. 98 ͉ no. 20 ͉ 10981–10982 Downloaded by guest on September 24, 2021 pin1 mutant (15) can completely block LP formation. Polarly transported auxin is not needed for the maintenance of the CZ, or the progression of cells from the CZ to the peripheral zone, because both processes occur normally in the pin1 mutants (23). PIN1, itself, is up-regu- lated in developing LPs (23). The initi- ation of a LP may simply require that sufficient auxin accumulates in a set of cells so as to set off the necessary cell enlargement. This accumulation of auxin requires active PAT, which will occur only when the level of the PAT inhibitor from the existing primordia drops below some threshold. The site of the minimum of the inhibitor will have a maximum of PAT and will accumulate auxin. In a feed-forward loop, as auxin accumulates it induces more PIN1 and thus more Fig. 1. An hypothesis to explain the location where a new LP arises. This occurs by the growth in volume auxin accumulation. As yet unknown is of a set of cells that have sufficient expansin and auxin, and where the physical stresses exerted by whether in LP formation auxin acts neighboring cells are permissive. Existing primordia influence the location by generating incompatible simply by causing cell wall acidification, physical stresses close to themselves, and by release of inhibitors of PAT. At a sufficient distance from or whether it is also capable of up- existing primordia the PAT inhibitors would be low enough to permit auxin to move, by PAT, to the cells that will form the primordium. regulating the endogenous expansin gene. An alternative to the concept of a chem- ical inhibitor of LP formation is the idea that undergo any cell division, upon germination zone (PZ), a ring of cells below the CZ (21). the location of the next LP is determined by formed one new primordium at the SAM by Cells in the CZ are unable to form primor- the physical interactions between the cells.