YABBY Genes in the Development and Evolution of Land Plants

Mounting evidence from genomic and transcriptomic studies suggests that most genetic networks regulating the morphogenesis of land plant sporophytes were co-opted and modified from those already present in streptophyte algae and gametophytes of bryophytes sensu lato. However, thus far, no candidate genes have been identified that could be responsible for “planation”, a conversion from a three-dimensional to a two-dimensional growth pattern. According to the telome theory, “planation” was required for the genesis of the leaf blade in the course of leaf evolution. The key transcription factors responsible for leaf blade development in angiosperms are YABBY proteins, which until recently were thought to be unique for seed plants. Yet, identification of a YABBY homologue in a green alga and the recent findings of YABBY homologues in lycophytes and hornworts suggest that YABBY proteins were already present in the last common ancestor of land plants. Thus, these transcriptional factors could have been involved in “planation”, which fosters our understanding of the origin of leaves. Here, we summarise the current data on functions of YABBY proteins in the vegetative and reproductive development of diverse angiosperms and gymnosperms as well as in the development of lycophytes. Furthermore, we discuss a putative role of YABBY proteins in the genesis of multicellular shoot apical meristems and in the evolution of leaves in early divergent terrestrial plants.

Linking Leaf Spectra to the Plant Tree of Life

Evolutionary trees recount the history of how biological diversity came to be and how evolution gave rise to the incredible variation in plant form and function that can be captured by spectral reflectance. Understanding plant spectra in light of evolution is thus important for assessing biodiversity and critical for explaining how spectral diversity is generated. Here, we focus on leaf spectra and how they are linked to the plant tree of life. We review what evolutionary trees (phylogenies) are and how to interpret them. We then describe how to model the evolution of quantitative traits, discuss which evolutionary processes are involved, and explain specific concepts and metrics, such as phylogenetic signal and evolutionary rates, and how they can be applied to reflectance spectra. Next, we describe a framework that links phylogenies and leaf spectra by coupling models of evolution and radiative transfer models. In doing so, we review some of the challenges of subjecting spectra to evolutionary analyses. We then discuss how spectra can help us to understand leaf evolution and to differentiate plant taxa at different phylogenetic scales from populations to lineages, advancing our potential to remotely detect biodiversity.

Leaf structure in Amorimia and closely related Neotropical genera and implications for their systematics and leaf evolution in Malpighiaceae

Amorimia (Malpighiaceae) was recently segregated from the polyphyletic Mascagnia and placed in the malpighioid clade; identifying new characters based on leaf structure is among the first steps towards a proper generic delimitation of these segregates of Mascagnia. A comprehensive study describing and testing the relevance of leaf-structure characters in the evolution of Amorimia and related Neotropical genera is presented. We sampled all 15 Amorimia spp. and, as outgroups, eight species from the closely related Neotropical genera (Diplopterys, Ectopopterys, Mascagnia, Peixotoa and Stigmaphyllon). We scored 85 structural characters and mapped them on the most recent phylogenetic tree recovered for the genera. The presence of druses in the palisade parenchyma, the position of fibres alongside the vascular bundle and the occurrence of fibre blocks near the margin of the leaf blade were recovered as anatomical synapomorphies for Amorimia. Our results are a first step towards recovering anatomical and macromorphological synapomorphies for newly identified lineages of Malpighiaceae, such as Amorimia.

A microtubule-mediated mechanical feedback controls leaf blade development in three dimensions

Many plant species have thin leaf blades, which is an important adaptation that optimizes the exchanges with the environment. Here, we provide evidence that their three-dimensional geometry is governed by microtubule alignment along mechanical stress patterns in internal walls. Depending on the primary shape of the primordium, this process has the potential to amplify an initial degree of flatness, or promote the formation of nearly axisymmetric, mostly elongating organs, such as stems and roots. This mechanism may explain leaf evolution from branches, which is alternative to Zimmermann’s influential, but widely questioned,telometheory.One Sentence SummaryMechanical feedback controls leaf development in three dimensions

Proportional Relationship between Leaf Area and the Product of Leaf Length and Width of Four Types of Special Leaf Shapes

The leaf area, as an important leaf functional trait, is thought to be related to leaf length and width. Our recent study showed that the Montgomery equation, which assumes that leaf area is proportional to the product of leaf length and width, applied to different leaf shapes, and the coefficient of proportionality (namely the Montgomery parameter) range from 1/2 to π/4. However, no relevant geometrical evidence has previously been provided to support the above findings. Here, four types of representative leaf shapes (the elliptical, sectorial, linear, and triangular shapes) were studied. We derived the range of the estimate of the Montgomery parameter for every type. For the elliptical and triangular leaf shapes, the estimates are π/4 and 1/2, respectively; for the linear leaf shape, especially for the plants of Poaceae that can be described by the simplified Gielis equation, the estimate ranges from 0.6795 to π/4; for the sectorial leaf shape, the estimate ranges from 1/2 to π/4. The estimates based on the observations of actual leaves support the above theoretical results. The results obtained here show that the coefficient of proportionality of leaf area versus the product of leaf length and width only varies in a small range, maintaining the allometric relationship for leaf area and thereby suggesting that the proportional relationship between leaf area and the product of leaf length and width broadly remains stable during leaf evolution.

The origin and early evolution of vascular plant shoots and leaves

The morphology of plant fossils from the Rhynie chert has generated longstanding questions about vascular plant shoot and leaf evolution, for instance, which morphologies were ancestral within land plants, when did vascular plants first arise and did leaves have multiple evolutionary origins? Recent advances combining insights from molecular phylogeny, palaeobotany and evo–devo research address these questions and suggest the sequence of morphological innovation during vascular plant shoot and leaf evolution. The evidence pinpoints testable developmental and genetic hypotheses relating to the origin of branching and indeterminate shoot architectures prior to the evolution of leaves, and demonstrates underestimation of polyphyly in the evolution of leaves from branching forms in ‘telome theory’ hypotheses of leaf evolution. This review discusses fossil, developmental and genetic evidence relating to the evolution of vascular plant shoots and leaves in a phylogenetic framework. This article is part of a discussion meeting issue ‘The Rhynie cherts: our earliest terrestrial ecosystem revisited’.