The first morphological sign of ligule and auricle development is the formation of the preligule band ( PLB), a uniquely linear band of smaller cells that runs perpendicular to the proximal-distal axis of the developing leaf ( Figures 1C to 1H). Together, the ligule and auricle act as a hinge, extending the leaf blade at an angle at the blade-sheath boundary. Maize leaves have a proximal sheath and distal blade, separated by a specialized fringe-like ligule adjacent to an auricle ( Figure 1A). The maize ( Zea mays) leaf is particularly useful to study positional information because it is subdivided into unique regions along its relatively linear axis. The positional cues required to initiate and maintain these axial domains and their boundaries during plant development are largely unknown. Cell and tissue differentiation proceeds along these proximal-distal and adaxial-abaxial axes to generate anatomically and functionally discrete regions of the leaf. Once initiated from the SAM, leaf primordia acquire distinct polarity relative to the meristem: Cells in the leaf initial are either proximal or distal to the SAM and are either adjacent to (adaxial) or away from (abaxial) the meristem. Shoot apical meristems ( SAMs), which contain populations of plant stem cells that produce lateral organs such as leaves, are the primary starting points for establishing positional information in the plant shoot. Plant morphogenesis relies on positional information in part because plant cells do not move, but instead respond in place to diverse morphogenetic signals. Morphogenesis proceeds from positional determinants that establish the body axis. We propose that genetic modules utilized in leaf and/or branch initiation are redeployed to regulate ligule outgrowth from leaf primordia. A surprising number of these “ligule genes” have also been shown to function during leaf initiation or lateral branching and intersect multiple hormonal signaling pathways. We identified transcripts that are specifically upregulated at the blade-sheath boundary. We used laser microdissection and RNA sequencing to identify genes that are differentially expressed in discrete cell/tissue-specific domains along the proximal-distal axis of wild-type leaf primordia undergoing ligule initiation and compared transcript accumulation in wild-type and liguleless1-R mutant leaf primordia. Recessive liguleless1 mutants lack ligules and auricles and have upright leaves. The auricles act as a hinge, allowing the leaf blade to project at an angle from the stem, while the ligule comprises an epidermally derived fringe. Specialized ligule and auricle structures form at the blade-sheath boundary. The maize ( Zea mays) leaf is an ideal system to study plant morphogenesis as it is subdivided into a proximal sheath and a distal blade, each with distinct developmental patterning. Development of multicellular organisms proceeds via the correct interpretation of positional information to establish boundaries that separate developmental fields with distinct identities.
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