Abaxial Greening Phenotype in Hybrid Aspen

Julia Nowak,Quentin Cronk,Carl Douglas

doi:10.3390/plants2020279

Julia Nowak, Quentin Cronk + Show 1 more

Open Access

https://doi.org/10.3390/plants2020279

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Abstract

The typical angiosperm leaf, as in Arabidopsis, is bifacial consisting of top (adaxial) and bottom (abaxial) surfaces readily distinguishable by the underlying cell type (palisade and spongy mesophyll, respectively). Species of the genus Populus have leaves that are either conventionally bifacial or isobilateral. Isobilateral leaves have palisade mesophyll on the top and bottom of the leaf, making the two sides virtually indistinguishable at the macroscopic level. In poplars this has been termed the “abaxial greening” phenotype. Previous work has implicated ASYMMETRIC LEAVES1 (AS1) as an essential determinant of palisade mesophyll development. This gene, as well as other genes (84 in all) putatively involved in setting the dorsiventral axis of leaves, were investigated in two Populus species: black cottonwood (Populus trichocarpa) and hybrid aspen (P. tremula x tremuloides), representative of each leaf type (bifacial and isobilateral, respectively). Poplar orthologs of AS1 have significantly higher expression in aspen leaf blade and lower in the petiole, suggestive of a potential role in the isobilateral leaf phenotype consistent with the previously observed phenotypes. Furthermore, an ABERRANT TESTA SHAPE (ATS) ortholog has significantly lower expression in aspen leaf tissue, also suggesting a possible contribution of this gene to abaxial greening.

Highlights

The genus Populus consists of approximately 29 species [1] containing two main types of leaves: bifacial and isobilateral [2]
Black cottonwood leaves are bifacial and contain palisade mesophyll tissues associated with the dark green adaxial side and spongy mesophyll associated with the lighter colored abaxial surface (Figure 2a,c)
Bifacial leaves are usually associated with rounded petioles and isobilateral with mediolaterally flattened petioles

Summary

Introduction

The genus Populus consists of approximately 29 species [1] containing two main types of leaves: bifacial and isobilateral [2]. Isobilateral leaves, on the other hand, are more commonly found within the genus [2] and are more or less uniformly green on both adaxial and abaxial leaf surfaces, in contrast to bifacial-leaved species The petiole of these isobilateral leaves is mediolaterally flattened and unifacial, allowing the leaves to flutter in the wind [3]. The development of the lower mesophyll cells is variable, from cells almost identical to upper or adaxial palisade mesophyll cells to less elongated palisade-like cells ( termed ―abaxial palisade‖) This ―abaxial greening‖ or ―abaxial greenness‖ phenotype [4] is thought to contribute to overall carbon gain due to a more even light distribution throughout the tree, improved carbon dioxide fixation following exposure to short sunflecks through the canopy, and decrease in leaf temperature [5,6,7]

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