Sarah E. Wyatt

Plant tropisms: From Darwin to the International Space Station

Plant tropisms play a fundamental role in shaping the growth form of plants, and these fascinating movements are the focus of this thematic issue of the American Journal of Botany. The issue includes 16 reviews of the current literature and eight original manuscripts written by a diverse group of international experts in their respective fields. This special issue emphasizes tropistic responses to three fundamental stimuli governing plant growth: water, light, and gravity. We hope this issue will inform the current generation and inspire the next generation of plant biologists.

Expression of the high capacity calcium-binding domain of calreticulin increases bioavailable calcium stores in plants

Modulation of cytosolic calcium levels in both plants and animals is achieved by a system of Ca2+-transport and storage pathways that include Ca2+ buffering proteins in the lumen of intracellular compartments. To date, most research has focused on the role of transporters in regulating cytosolic calcium. We used a reverse genetics approach to modulate calcium stores in the lumen of the endoplasmic reticulum. Our goals were two-fold: to use the low affinity, high capacity Ca2+ binding characteristics of the C-domain of calreticulin to selectively increase Ca2+ storage in the endoplasmic reticulum, and to determine if those alterations affected plant physiological responses to stress. The C-domain of calreticulin is a highly acidic region that binds 20–50 moles of Ca2+ per mole of protein and has been shown to be the major site of Ca2+ storage within the endoplasmic reticulum of plant cells. A 377-bp fragment encoding the C-domain and ER retention signal from the maize calreticulin gene was fused to a gene for the green fluorescent protein and expressed in Arabidopsis under the control of a heat shock promoter. Following induction on normal medium, the C-domain transformants showed delayed loss of chlorophyll after transfer to calcium depleted medium when compared to seedlings transformed with green fluorescent protein alone. Total calcium measurements showed a 9–35% increase for induced C-domain transformants compared to controls. The data suggest that ectopic expression of the calreticulin C-domain increases Ca2+ stores, and that this Ca2+ reserve can be used by the plant in times of stress.

Mutations in the gravity persistence signal loci in Arabidopsis disrupt the perception and/or signal transduction of gravitropic stimuli

Gravity plays a fundamental role in plant growth and development, yet little is understood about the early events of gravitropism. To identify genes affected in the signal perception and/or transduction phase of the gravity response, a mutant screen was devised using cold treatment to delay the gravity response of inflorescence stems of Arabidopsis. Inflorescence stems of Arabidopsis show no response to gravistimulation at 4°C for up to 3 h. However, when gravistimulated at 4°C and then returned to vertical at room temperature (RT), stems bend in response to the previous, horizontal gravistimulation (H. Fukaki, H. Fujisawa, M. Tasaka [1996] Plant Physiology 110: 933–943). This indicates that gravity perception, but not the gravitropic response, occurs at 4°C. Recessive mutations were identified at three loci using this cold effect on gravitropism to screen for gravity persistence signal (gps) mutants. All three mutants had an altered response after gravistimulation at 4°C, yet had phenotypically normal responses to stimulations at RT. gps1-1 did not bend in response to the 4°C gravity stimulus upon return to RT.gps2-1 responded to the 4°C stimulus but bent in the opposite direction. gps3-1 over-responded after return to RT, continuing to bend to an angle greater than wild-type plants. At 4°C, starch-containing statoliths sedimented normally in both wild-type and the gps mutants, but auxin transport was abolished at 4°C. These results are consistent with GPS loci affecting an aspect of the gravity signal perception/transduction pathway that occurs after statolith sedimentation, but before auxin transport.

Key Morphological Alterations in the Evolution of Leaves

Evolution of plant form proceeds through sequential alterations in the development of plant organs. Leaves (or fronds) are organs that have diagnostic characteristics, including definite arrangement on a stem, bilateral symmetry (abaxial/adaxial identity), and determinate growth. Evolution of those diagnostic characteristics represents a series of critical steps in plant evolution that resulted from specific developmental alterations. The fossil record reveals a transformational series in leaf evolution that highlights steps that have occurred in parallel but independently in both leptosporangiate ferns and seed plants, resulting in superficially similar frond morphologies. In this study, the fronds of the most ancient fossil fern, Psalixochlaena antiqua, and the most ancient reconstructed seed plant, Elkinsia polymorpha, are characterized and compared with leaves of modern plants in order to identify the sequence in which features of leaves in two distinct clades of euphyllophytes arose. While both fronds show a combination of characters attributable to ancestral vegetative axes and characters attributable to leaves, each plant displays different combinations of those characters. These data document dissimilar sequences of character originations and, therefore, the independent evolution of developmental mechanisms in seed plants and ferns.

A Fossil Record for Growth Regulation: The Role of Auxin in Wood Evolution1

Abstract Appreciation for the role of ontogeny in plant evolution has been heightened by advances in studying development using molecular techniques, with a growing number of specific structural features now understood in terms of the genetic, regulatory, and biochemical mechanisms by which they are produced. Paleontological approaches to plant development provide a vehicle for extending that understanding to the ontogeny and evolution of whole organisms through time. Recent studies have shown that developmentally diagnostic features can be identified in the fossil record, where they represent fingerprints for gene-mediated regulatory pathways. The first paleontological evidence for the regulation of cambial activity via the polar axial flow of auxin consists of circular patterns of tracheary elements above buds and branch junctions in the wood of the 375-million-year-old fossil progymnosperm Archaeopteris Dawson. That evidence strongly supports homology of secondary vascular tissues in progymnosperms and seed plants, and monophylesis of the lignophytes (progymnosperms and seed plants). Similar anatomical patterns at the same position have now been identified in the wood of tree-sized fossil equisetophytes and arborescent lycophytes that belong to independent lineages, demonstrating that a similar mechanism involving the regulation of secondary vascular tissue production by the polar axial flow of auxin characterizes those clades as well. These data imply that the wood in lignophytes, lycophytes, and equisetophytes originated in conjunction with the parallel evolution of regulation of secondary tissue production by auxin in each clade. The independent origins of secondary vascular tissue in three major clades of Paleozoic plants sensu Kenrick and Crane (1997) reveal evolutionary patterns that are not represented in the living flora and illuminate promising avenues for combining future paleontological studies with molecular and genetic studies to substantially impact our understanding of the role of developmental regulation in vascular plant evolution.

Early evolution of the vascular plant body plan - the missing mechanisms.

The complex body plan of modern vascular plants evolved by modification of simple systems of branching axes which originated from the determinate vegetative axis of a bryophyte-grade ancestor. Understanding body plan evolution and homologies has implications for land plant phylogeny and requires resolution of the specific developmental changes and their evolutionary sequence. The branched sporophyte may have evolved from a sterilized bryophyte sporangium, but prolongation of embryonic vegetative growth is a more parsimonious explanation. Research in the bryophyte model system Physcomitrella points to mechanisms regulating sporophyte meristem maintenance, indeterminacy, branching and the transition to reproductive development. These results can form the basis for hypotheses to identify and refine the nature and sequence of changes in development that occurred during the evolution of the indeterminate branched sporophyte from an unbranched bryophyte-grade sporophyte.

Paleontological Context for the Developmental Mechanisms of Evolution

The emerging field of evolutionary developmental biology (evo‐devo) seeks to identify the specific genetic basis for plant evolution. Whereas molecular biological studies are rapidly exposing the developmental mechanisms of evolution, paleontological studies reveal the sequence in which evolutionary changes have occurred. Aspects of both fields are reviewed in this article. Plant structure develops along genetically patterned pathways, some of which produce distinctive morphological features. Such features serve as “fingerprints” for regulatory mechanisms, thus providing a record of growth regulation for both living species and plant fossils. Transformational series of structure through geological time form the contextual framework for inferring evolution from the fossil record, but the most widely practiced application of transformational series for plants, Telome Theory, has no developmental basis. By contrast, the fossil record of leaf evolution in seed plants documents morphological changes to which a growing understanding of genetic pathways can be correlated. Leaves are defined by a number of features, including determinacy and adaxial/abaxial identity. Fossils of early seed plants show stepwise acquisition of these characters, from which we can infer developmental modifications leading to the evolution of the characters. Previous work has revealed that an interregulated network of genes is involved in the specification of these features. The genes identified as being involved in various aspects of leaf development provide clues to the evolution of each character. In particular, expression of the AS1 and AS2 genes appears to facilitate both determinacy and bilateral patterning. The novel expression pattern of these genes may have been the key innovation that resulted in the evolution of leaves in seed plants. Combining molecular biological studies with knowledge of evolutionary patterns gleaned from the fossil record brings a new depth of understanding to the evolution of fundamental plant form.

Arabidopsis thaliana as a model for gelatinous fiber formation

Trees and herbaceous plants continuously monitor their position to maintain vertical stem growth and regulate branch orientation. When orientation is altered from the vertical, they form a special type of wood called reaction wood that differs chemically and structurally from normal wood and forces reorientation of the organ or whole plant. The reaction wood of dicotyledons is called tension wood and is characterized by nonlignified gelatinous fibers. The altered chemical and mechanical properties of tension wood reduce wood quality and represent a major problem for the timber and pulping industries. Repeated clipping of the emerging inflorescence stems of Arabidopsis thaliana augments wood formation in organs, including those inflorescence stems that are allowed to develop later. Gravistimulation of such inflorescence stems induces tension wood formation, allowing the use of A. thaliana for a molecular and genetic analysis of the mechanisms of tension wood formation.

A proteomics approach identifies novel proteins involved in gravitropic signal transduction.

PREMISE Plant organs use gravity as a guide to direct their growth. And although gravitropism has been studied since the time of Darwin, the mechanisms of signal transduction, those that connect the biophysical stimulus perception and the biochemical events of the response, are still not understood. METHODS A quantitative proteomics approach was used to identify key proteins during the early events of gravitropism. Plants were subjected to a gravity persistent signal (GPS) treatment, and proteins were extracted from the inflorescence stem at early time points after stimulation. Proteins were labeled with isobaric tags for relative and absolute quantification (iTRAQ) reagents. Proteins were identified and quantified as a single step using tandem mass-spectrometry (MS/MS). For two of the proteins identified, mutants with T-DNA inserts in the corresponding genes were evaluated for gravitropic phenotypes. KEY RESULTS A total of 82 proteins showed significant differential quantification between treatment and controls. Proteins were categorized into functional groups based on gene ontology terms and filtered using groups thought to be involved in the signaling events of gravitropism. For two of the proteins selected, GSTF9 and HSP81-2, knockout mutations resulted in defects in root skewing, waving, and curvature as well as in the GPS response of inflorescence stems. CONCLUSION Combining a proteomics approach with the GPS response, 82 novel proteins were identified to be involved in the early events of gravitropic signal transduction. As early as 2 and 4 min after a gravistimulation, significant changes occur in protein abundance. The approach was validated through the analysis of mutants exhibiting altered gravitropic responses.

The Inflorescence Stem Fibers of Arabidopsis thaliana Revoluta (ifl1) Mutant

Arabidopsis thaliana is gradually gaining significance as a model for wood and fiber formation. revolute/ifl1 is an important mutant in this respect. To better characterize the fiber system of the revolute/ifl1 mutant, we grew plants of two alleles (rev-9 in Israel and rev-1 in the USA) and examined the fiber system of the inflorescence stems using both brightfield and polarized light. Microscopic examination of sections of plants belonging to the two different alleles clearly revealed that, contrary to previous views, in 18 (13 in Israel and 5 in Ohio) out of 30 stems (20 in Israel and 10 in Ohio) the mutant produced the primary wavy fiber system of the inflorescence stems. Our findings are further supported by the fact that fibers are seen in the figures published in other studies of the mutant even when it was stated that there were no fibers. The impression of a total lack of the wavy band of fibers is in many cases just a result of poorly lignified secondary walls. This specific gene that reduces lignification in fibers is of great significance for biotechnological developments for the paper industry and thus for the global economy and ecology. We propose that revoluta, the first name given to this mutant (Talbert and others 1995), is more appropriate than ifl1.