Abstract
Leaves constitute the main photosynthetic plant organ and even though their importance is not debated, the origin and development of leaves still is. The leaf developmental network has been elucidated for angiosperms, from genes controlling leaf initiation, to leaf polarity and shape. There are four KANADI (KAN) paralogs in Arabidopsis thaliana needed for organ polarity with KAN1 and KAN2 specifying abaxial leaf identity. Yet, studies of this gene lineage outside angiosperms are required to better understand the evolutionary patterns of leaf development and the role of KAN homologs. We studied the evolution of KAN genes across vascular plants and their expression by in situ hybridization in the fern, Equisetum hyemale and the lycophyte Selaginella moellendorffii. Our results show that the expression of KAN genes in leaves is similar between ferns and angiosperms. However, the expression patterns observed in the lycophyte S. moellendorffii are significantly different compared to all other vascular plants, suggesting that the KAN function in leaf polarity is likely only conserved across ferns, gymnosperms, and angiosperms. This study indicates that mechanisms for leaf development are different in lycophytes compared to other vascular plants.
Highlights
Leaves are the most recognizable plant organ, yet surprisingly they may have evolved independently more than once [1]
We studied the evolution of KAN genes across vascular plants and their expression by in situ hybridization in the fern, Equisetum hyemale and the lycophyte Selaginella moellendorffii
Angiosperm sequences are for the most part complete coding sequences as they were downloaded from genome databases, whereas some gymnosperm and fern sequences are missing 30–60 amino acids (AA) from the start codon, and others lack the stop codon
Summary
Leaves are the most recognizable plant organ, yet surprisingly they may have evolved independently more than once [1]. Leaves are thought to have evolved between 2 and 11 times in vascular plants (lycophytes, ferns, and seed plants). The fossil record indicates that leaves evolved independently in lycophytes and euphyllophytes (ferns and seed plants), and molecular genetics generally supports this idea [2,3,5,6,7,8]. The discrepancy in the number of times leaves have evolved is due to varying results mainly within ferns due to an incomplete fossil record and the paucity of developmental genetic studies in this group [1]. The most widely accepted theory for leaf evolution is the telome theory where leaves are proposed to have evolved through independent processes of branching, overtopping, planation, and webbing [9]. The evolution of webbing would result in the development of a laminar structure
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