Comment on “Is Plant Morphology Still Alive? Fundamental Flaws in Genetic and Transcriptomic Support for the Bipartite Interpretation of the Grass Cotyledon”

We present our morphological and transcriptomic evidence for a model of the bipartite grass cotyledon, wherein the scutellum comprises the distal, upper zone and the coleoptile is the sheathing, lower zone of the single grass cotyledon. We assert that homology is best evaluated by examining ontogeny, and not mature adult stages of development. The scutellum is the first lateral organ to arise in the grass embryo, accompanied by leaf homologous gene expression; thereafter the scutellum forms a highly-modified, non-foliar, digestive organ. Lateral organs in grasses exhibit distichous phyllotaxy, where successive organs arise on opposite sides of the shoot axis and in two ranks. Accordingly, scutellum and coleoptile arise on the same side of the shoot, as expected in a bipartite, single lateral organ that is fused medially. Likewise, transcriptomic data identifies expression of the sheath identity marker BOP1a in the coleoptile but not the scutellum, supporting the homology of the coleoptile and the grass sheathing leaf base. Taken together, these data support the bipartite model on the evolution of the grass cotyledon.