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Megaphyll leaf evolution was a critical event in Earth history that had major consequences for the biotic regulation of the global environment. Zimmermann''s telome theory has been widely accepted for over seventy years as the leading explanation for this evolutionary innovation. According to the telome theory, megaphylls evolved from the three-dimensional lateral branches of early vascular land plants in a hypothetical series of three transformations; first, the formation of determinate lateral branches (overtopping); second, the development of ‘flattened’ branch systems (planation); and third, the fusion of planated branches with lateral outgrowths of photosynthetic mesophyll tissue to form the leaf blade (webbing). A critical review of the molecular and cellular evidence identifies plausible genetic, cellular and physiological mechanisms in extant higher plants for overtopping and planation but more limited evidence for the process of webbing (lateral outgrowth fusion). We highlight key outstanding questions concerning the telome theory that are likely to be resolved when gene identification and functional analysis techniques are applied to photosynthetic organisms that have different evolutionary histories. [Copyright &y& Elsevier]

Summary: Stomatal pores evolved more than 410 million years ago [] and allowed vascular plants to regulate transpirational water loss during the uptake of CO2 for photosynthesis []. Here, we show that stomata on the sporophytes of the moss Physcomitrella patens [] respond to environmental signals in a similar way to those of flowering plants [] and that a homolog of a key signaling component in the vascular plant drought hormone abscisic acid (ABA) response [] is involved in stomatal control in mosses. Cross-species complementation experiments reveal that the stomatal ABA response of a flowering plant (Arabidopsis thaliana) mutant, lacking the ABA-regulatory protein kinase OPEN STOMATA 1 (OST1) [], is rescued by substitution with the moss P. patens homolog, PpOST1-1, which evolved more than 400 million years earlier. We further demonstrate through the targeted knockout of the PpOST1-1 gene in P. patens that its role in guard cell closure is conserved, with stomata of mutant mosses exhibiting a significantly attenuated ABA response. Our analyses indicate that core regulatory components involved in guard cell ABA signaling of flowering plants are operational in mosses and likely originated in the last common ancestor of these lineages more than 400 million years ago [], prior to the evolution of ferns []. [ABSTRACT FROM AUTHOR]