Next, we investigate the interplay between adaxial–abaxial and mediolateral patterning in monocot and eudicot leaves, and compare and contrast the shared and disparate programs of leaf dissection during the development of compound leaf morphologies. We first describe the comparative ontogeny of leaf development in monocots and eudicots and present testable models for the differential elaboration of upper and lower leaf zones, the formation of sheathing leaf bases and the patterning of monocot and eudicot vasculature. The two main differences defining most monocot and eudicot leaves are the patterning of the vasculature, which is typically parallel in monocots and reticulate in eudicots, and the presence of a sheathing leaf base in monocots that encircles the stem (Kaplan, 1973). This review focuses on the comparative ontogeny of monocot and eudicot foliar leaves, which develop from the shoot apical meristem (SAM) and are dorsiventrally asymmetric from their inception (Roth, 1949 Hagemann, 1970 Caggiano et al., 2017). The monophyletic monocots diverged from their eudicot relatives quite early in angiosperm evolution (Hertweck et al., 2015), although details of their evolutionary relationships are blurred by evolutionary convergences among the nearly 300 000 species within the two orders (Christenhusz & Byng, 2016). Monocotyledonous and eudicotyledonous plants are ultimately classified according to the morphological differences in the number and arrangement of their embryonic leaves. We anticipate that the incorporation of an ever-expanding number of sequenced genomes will enrich our understanding of the developmental mechanisms generating eudicot and monocot leaves. Although many fundamental mechanisms of leaf development are shared in eudicots and monocots, variations in the timing, degree and duration of these ontogenetic events may contribute to key differences in morphology. Future studies may investigate correlations in the ontogenies of parallel venation and linear stomatal files in monocots, and the reticulate patterning of veins and dispersed stoma in eudicots. Likewise, comparisons of stomatal development are limited to Arabidopsis and a few grasses.
Although models for the homologies of enigmatic tissues, such as ligules and stipules, are proposed, tests of these hypotheses are rare. Furthermore, lineage-specific mechanisms in compound leaf development are discussed. We propose that monocot and eudicot leaves undergo expansion of mediolateral domains at different times in ontogeny, directly impacting features such as venation and leaf bases. Monocot and eudicot leaves are distinguished by the differential elaborations of upper and lower leaf zones, the formation of sheathing/nonsheathing leaf bases and vasculature patterning. Comparisons of concepts in monocot and eudicot leaf development are presented, with attention to the morphologies and mechanisms separating these angiosperm lineages.
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