Monocot leaves possess fundamentally different morphologies compared to those of eudicots, making the identification of homologies extremely difficult. Although leaves from both groups originate from the flanks of the shoot apical meristem, each has novel structures that, from first appearance, seem unrelated. For example, many grasses have an epidermally derived fringe of tissue at the blade sheath boundary called the ligule that functions to repel water from the stem. In contrast, the leaves of several eudicots have paired foliar appendages called stipules that have a variety of functions ranging from photosynthesis to plant defense. Recent molecular evidence indicates that the stipule and ligule do in fact share common genetic mechanisms, both requiring expression of ankyrin repeat proteins belonging to the BLADE-ON-PETIOLE (BOP) clade at the leaf base. Knockouts of BOP orthologues in the monocot barley, as well as the eudicots Medicago, Arabidopsis and pea affect development and initiation of the ligules and stipules, respectively. These results indicate that ligules and stipules may in fact be homologous structures originating from the lower leaf zone, both requiring the formation of boundaries defined by BOP genes in order to initiate.
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