Transcriptomic Analysis of Phylloclade in Ruscus aculeatus Is Consistent with Unifacial Morphology

The development of planar structures such as wings or leaves is a common feature among organisms and serves as a mechanism to increase surface to volume ratios. We wished to explore whether the recurrent and independent development of similar adaptive planar morphologies is the result of an activation of common genetic modules or toolkits. To test this, we focused on the developmental gene networks that are proposed to define leaf polarity in eudicots in phylloclades, leaf-like organs derived from branch primordia, in the monocot Ruscus aculeatus. Since branch primordia normally have a radial shape, this approach allowed us to examine the genetic changes required for the transformation from a round to a planar (flat) form. In our transcriptome analysis of phylloclade and stem tissue, we detected 76,085 annotated ORFs of which 87.2% were identified as complete out of 2026 BUSCO groups. Expression patterns clearly identify differentiation between phylloclade and stem tissues consistent with an enhanced photosynthetic function in the phylloclades. However, except for the AS1/AS2 and possibly STM module, we see little evidence that canonical leaf adaxial and abaxial modules are activated in the sampled phylloclades compared with the stems. Our results show that the unifacial nature of phylloclades is consistent with the observed lack of strong adaxial/abaxial molecular signatures. We propose that in R. aculeatus and plants with similar unifacial laminar leaves, adaxial/abaxial molecular identity may not be required for planar growth, and that lateral expansion of organ primordia and acropetal and intercalary cell division may be sufficient to generate planar versus radial organ shapes.