Stem and Leaf Anatomy of Aragoa (Plantaginaceae): In Search of Lost Rays.

Alexei Oskolski,Nathi Vuza,Alexey Shipunov

doi:10.3390/plants10091773

Abstract

Aragoa is a shrubby genus endemic to páramo in the northern Andes representing the sister group to Plantago and Limosella. Stem and leaf structure of Aragoa corrugatifolia were studied to clarify the evolutionary pathways and ecological significance of their anatomical traits. Aragoa and Plantago share a non-fascicular primary vascular system, rayless wood and secondary phloem, and anomocytic stomata. Aragoa is distinctive from most Plantaginaceae in the presence of cortical aerenchyma and of helical thickenings in vessels. Its procambium emerges in the primary meristem ring as a continuous cylinder. The view on the ring meristem and procambial strands as developmental stages in the formation of a primary vascular system is not relevant for Aragoa, and probably for other Plantaginaceae. The raylessness is synapomorphic for the crown clade of Plantaginaceae comprising Aragoa, Littorella, Plantago, Veronica, Picrorhiza, Wulfenia, and Veronicastrum. The loss of rays is thought to be predetermined by procambium rather than by the vascular cambium. The extremely narrow vessels with helical thickenings are presumably adaptive to hydric and thermic conditions of páramo. Cortical aerenchyma is thought to be a response to the local hypoxia caused by the water retained by ericoid leaves. Trichomes on juvenile leaves are expected to be the traits of considerable taxonomic importance.

Highlights

Aragoa Kunth is the genus with approximately 19 species endemic to the alpine páramo biome in the northern Andes of Colombia and Venezuela
The procambium is initiated in the peripheral zone of Shoot apical meristem (SAM) near the shoot tip
Aragoa is distinctive from most Plantaginaceae in the presence of aerenchyma in the cortex and of helical thickenings on the vessel walls

Summary

Introduction

Aragoa Kunth is the genus with approximately 19 species endemic to the alpine páramo biome in the northern Andes of Colombia and Venezuela. Aragoa was been considered as a member of Scrophulariaceae until the polyphyly of this family was strongly confirmed by molecular phylogenetic data. A molecular analysis based on rbcL and ITS sequences showed that Aragoa is a sister group to a large cosmopolitic genus Plantago of about 200 species [7]. Close relationships between these two genera were confirmed by essential similarities in morphology of their inflorescences and flowers [4]. Recent phylogenomic analyses based on plastome sequences data [8] established, that Aragoa is sister to a broader lineage comprising Plantago and Littorella (L.) Asch., a disjunct genus with three amphibious species [9]

Methods

Results

Conclusion