From Goethe’s plant archetype via Haeckel’s biogenetic law to plant evo-devo 2016

The "plant neurobiology" revolution

The 21st-century "plant neurobiology" movement is an amalgam of scholars interested in how "neural processes", broadly defined, lead to changes in plant behavior. Integral to the movement (now called plant behavioral biology) is a triad of historically marginalized subdisciplines, namely plant ethology, whole plant electrophysiology and plant comparative psychology, that set plant neurobiology apart from the mainstream. A central tenet held by these "triad disciplines" is that plants are exquisitely sensitive to environmental perturbations and that destructive experimental manipulations rapidly and profoundly affect plant function. Since destructive measurements have been the norm in plant physiology, much of our "textbook knowledge" concerning plant physiology is unrelated to normal plant function. As such, scientists in the triad disciplines favor a more natural and holistic approach toward understanding plant function. By examining the history, philosophy, sociology and psychology of the triad disciplines, this paper refutes in eight ways the criticism that plant neurobiology presents nothing new, and that the topics of plant neurobiology fall squarely under the purview of mainstream plant physiology. It is argued that although the triad disciplines and mainstream plant physiology share the common goal of understanding plant function, they are distinct in having their own intellectual histories and epistemologies.

The role of fossils for reconstructing the evolution of plant development

Many of the developmental innovations that underpin the diversity of plant form alive today, such as those facilitating apical growth, branching, leaves, roots, wood and seeds, all evolved over 360 million years ago. Fossils, as our only direct record of plant form in the past, are thus essential for interpreting the origin and evolution of these innovations. The focus of this Spotlight is to showcase the rich plant fossil record open for developmental interpretation and to cement the role that fossils play at a time when increases in genome sequencing and new model species make tackling major questions in the area of plant evolution and development tractable for the first time.

On the sequence heterochrony of cranial ossification of bats in light of Haeckel's recapitulation theory

Haeckel's recapitulation theory has been a controversial topic in evolutionary biology. However, we have seen some recent cases applying Haeckel's view to interpret the interspecific variation of prenatal ontogeny. To revisit the validity of Haeckel's recapitulation theory, we take bats that have undergone drastic morphological changes and possess a characteristic ecology as a case study. All members of Rhinolophoidea and Yangochiroptera can generate an ultrasonic pulse from the larynx to interpret surrounding objects (laryngeal echolocation) whereas Pteropodidae lacks such ability. It is known that the petrosal bone is particularly derived in shape and expanded in laryngeal echolocators. If Haeckel's recapitulation theory holds, the formation of this derived trait should occur later than those of other bones. Therefore, we compared the prenatal ossification timing of the petrosal in 15 bat species and five outgroup species. We found that the ossification of the petrosal is accelerated in laryngeal echolocators while it is the last bone to ossify in non-laryngeal echolocating bats and non-volant mammals, which runs counter to the prediction generated by Haeckel's recapitulation theory. We point out the evolutionarily labile nature of trait developmental timing and emphasize that Haeckel's recapitulation theory does not hold in many cases. We caution that generating predictions on ancestral conditions and evolutionary history leading from Haeckel's recapitulation theory is not well supported.

Regulation of root development in Arabidopsis thaliana by phytohormone-secreting epiphytic methylobacteria

In numerous experimental studies, seedlings of the model dicot Arabidopsis thaliana have been raised on sterile mineral salt agar. However, under natural conditions, no plant has ever grown in an environment without bacteria. Here, we document that germ-free (gnotobiotic) seedlings, raised on mineral salt agar without sucrose, develop very short root hairs. In the presence of a soil extract that contains naturally occurring microbes, root hair elongation is promoted; this effect can be mimicked by the addition of methylobacteria to germ-free seedlings. Using five different bacterial species (Methylobacterium mesophilicum, Methylobacterium extorquens, Methylobacterium oryzae, Methylobacterium podarium, and Methylobacterium radiotolerans), we show that, over 9 days of seedling development in a light-dark cycle, root development (hair elongation, length of the primary root, branching patterns) is regulated by these epiphytic microbes that occur in the rhizosphere of field-grown plants. In a sterile liquid culture test system, auxin (IAA) inhibited root growth with little effect on hair elongation and significantly stimulated hypocotyl enlargement. Cytokinins (trans-zeatin, kinetin) and ethylene (application of the precursor ACC) likewise exerted an inhibitory effect on root growth but, in contrast to IAA, drastically stimulated root hair elongation. Methylobacteria are phytosymbionts that produce/secrete cytokinins. We conclude that, under real-world conditions (soil), the provision of these phytohormones by methylobacteria (and other epiphytic microbes) regulates root development during seedling establishment.

The "Biogenetic Law" in zoology: from Ernst Haeckel's formulation to current approaches

150 years ago, in 1866, Ernst Haeckel published a book in two volumes called "Generelle Morphologie der Organismen" (General Morphology of Organisms) in which he formulated his biogenetic law, famously stating that ontogeny recapitulates phylogeny. Here we describe Haeckel's original idea and follow its development in the thinking of two scientists inspired by Haeckel, Alexei Sewertzoff and Adolf Naef. Sewertzoff and Naef initially approached the problem of reformulating Haeckel's law in similar ways, and formulated comparable hypotheses at a purely descriptive level. But their theoretical viewpoints were crucially different. While Sewertzoff laid the foundations for a Darwinian evolutionary morphology and is regarded as a forerunner of the Modern Synthesis, Naef was one of the most important figures in 'idealistic morphology', usually seen as a type of anti-Darwinism. Both Naef and Sewertzoff aimed to revise Haeckel's biogenetic law and came to comparable conclusions at the empirical level. We end our review with a brief look at the present situation in which molecular data are used to test the "hour-glass model", which can be seen as a modern version of the biogenetic law.