Induction in Maize

The maize ligule is an adaxial membranous structure on the leaf that develops at the boundary of the sheath and blade. The ligule and the associated auricle are dispensable structures, amenable to genetic manipulation. We present here a genetic analysis of ligulelessl ( lg l ) and liguleless2 (lg2), the two genes known to be uniquely necessary for ligule and auricle development. We show that both reference mutant alleles, lgl-R and lg2-R, are null alleles. The double mutant phenotype suggests that lgl and lg2 act in the same pathway. Indeed, the dosage of a functional allele at either gene affects the null phenotype of the other. While lgl function has previously been shown to be cell-autonomous, here we show that the lg2-R phenotype is cell-nonautonomous, suggesting lgl and lg2 play different roles in the liguleauricle induction mechanism. We present a model in which early lg2 function specifies the precise position where ligule and auricle will develop. Later lg2 function interacts with lgl function (either directly or indirectly) to transmit and receive a make-ligule-make-auricle inductive signal. T HE leaf of maize ( E a mays L.) is typical of the grasses. It is separated into blade and sheath by the ligule-auricle region (Figure 1). The ligule is a membranous fringe of epidermal tissue that grows perpendicular out of the plane of the leaf. The auricle encompasses all tissue layers and acts as a hinge that allows the leaf blade to bend out from the main axis. Our current understanding of maize leaf development involves three general phases (SYLVESTER et al. 1990; FREELING 1992; SMITH and HAKE 1992; POETHIC and SZYMKOWIAK 1995; HARPER and FREELINC 1996). First, a group of cells in the meristem is recruited to become the next phytomer, the repeating unit of leaf, node, internode and axillary bud. The subset of these cells that will become the leaf are termed the “leaf founder cells” [defined in POETHIC (1984)l. Second, the founder cells all divide about equally to establish the leaf primordium, which can be seen as a bulge on the meristem flank. Third, a basipetal polar differentiation begins accompanied by a lateral polar differentiation from the middle toward the margin. The earliest identifiable morphological features on a maize leaf primordium are three regions of different adaxial epidermal cell shapes (SYLVESTER et al. 1990). These correspond to the three regions on the mature leaf: the blade, the ligular region, and the sheath. The middle region, where the ligule and auricle eventually differentiate, represents an important marker on the leaf that we call the blade-sheath boundary. At an early point in development, this boundary occupies a disproportionately large region of the primordium (SYLWSCorresponding author: Michael Freeling, 351 Koshland Hall, Department of Plant Biology, University of California, Berkeley, CA 94720. Genetics 144: 1871-1882 (Decernher, 1996) TER et al. 1990). Because it appears so early, and because no mutants in maize remove it, we argue that establishment of the blade-sheath boundary is a fundamental defining feature of a maize leaf, and perhaps of all grass leaves (FREELING 1992). The dispensable ligule and auricle are elaborated later than the establishment of the blade-sheath boundary; however, they may utilize aspects of this boundary as cues for their development. Ligule differentiation begins when a leaf primordium is in its fourth to sixth plastochron, and differentiation is occurring at the tip of the blade. First, anticlinal divisions (new wall inserted perpendicular to the plane of the leaf) appear on the adaxial surface of the leaf to form a band of small cells termed the preligular band ( SHARMAN 1941, 1942; HAKE et al. 1985; BECRAFT et al. 1990; SYLVESTER et al. 1990; BECRAFT and FREELINC 1991). Formation of the preligular band is directional beginning at two foci, one on either side of the midrib, and proceeding outward toward the margins and inward over the midrib (SYLVESTER et al. 1990). Following preligular band formation are the periclinal divisions (new walls inserted parallel to the plane of the leaf) that allow the ligule to grow out of the plane of the leaf (SHARMAN 1941, 1942; BECRAFT et al. 1990). These periclinal divisions are also laterally directional, again following the general context of leaf differentiation from midrib to margin (HAKE et al. 1985). These divisions result in the final differentiation of the ligule. The auricle is elaborated from a subset of internal cells in the preligular band that forms between the blade and the ligule. The periclinal divisions and the elaboration of the auricle occur at a time when the leaf blade is completing differentiation, and differentiation of the leaf sheath is just beginning. 1872 L. Harper and M. Freeling