Tissue culture of oil palm : finding the balance between mass propagation and somaclonal variation

The oil palm (Elaeis guineensis Jacq.) is typically propagated in vitro by indirect somatic embryogenesis, a process in which somatic cells of an explant of choice are, via an intermediate phase of callus growth, induced to differentiate into somatic embryos. The architecture of the oil palm, lacking axillary shoots, does not allow for vegetative propagation. Therefore, somatic embryogenesis is the only alternative to seed propagation, which is hampered by long germination times and low germination rates, for the production of planting material. The current oil palm somatic embryogenesis procedure is associated with several difficulties, which are described in this review. The limited availability of explants, combined with low somatic embryo initiation and regeneration rates, necessitate the proliferation of embryogenic structures, increasing the risk for somaclonal variants such as the mantled phenotype. Several ways to improve the efficiency of the tissue culture method and to reduce the risk of somaclonal variation are described. These include the use of alternative explants and propagation techniques, the introduction of specific embryo maturation treatments and the detection of the mantled abnormality in an early stage. These methods have not yet been fully explored and provide interesting research field for the future. The development of an efficient oil palm micropropagation protocol is needed to keep up with the increasing demand for palm oil in a sustainable way. Mass production of selected, high-yielding palms by tissue culture could raise yields on existing plantations, reducing the need for further expansion of the cultivated area, which is often associated with negative environmental impacts

Measurement of plant growth in view of an integrative analysis of regulatory networks

As the regulatory networks of growth at the cellular level are elucidated at a fast pace, their complexity is not reduced; on the contrary, the tissue, organ and even whole-plant level affect cell proliferation and expansion by means of development-induced and environment-induced signaling events in growth regulatory processes. Measurement of growth across different levels aids in gaining a mechanistic understanding of growth, and in defining the spatial and temporal resolution of sampling strategies for molecular analyses in the model Arabidopsis thaliana and increasingly also in crop species. The latter claim their place at the forefront of plant research, since global issues and future needs drive the translation from laboratory model-acquired knowledge of growth processes to improvements in crop productivity in field conditions

Leaf growth in dicots and monocots : so different yet so alike

In plants, most organs grow post-embryonically through cell division and cell expansion. The coordination of these two growth processes is generally considered to be different between dicots and monocots. In dicot plants, such as the model plant Arabidopsis, leaf growth is most often described as being temporally regulated with cell division ceasing earlier at the tip and continuing longer at the base of the leaf. Conversely, in monocot leaves, the organization of the growth processes is rather viewed as spatially regulated with dividing cells at the base of the leaf, followed by expanding cells and finally mature cells at the tip. As our understanding of the leaf growth processes in the two major classes of flowering plants expands, it becomes increasingly clear that the regulation of the growth processes is to a great extent conserved between dicots and monocots. In this review, we highlight how the temporal and spatial organization of cell division and cell expansion takes place in both dicot and monocot leaves. We also show that there are similarities in the molecular wiring that coordinates these two processes during leaf development

The pivotal role of ethylene in plant growth

Being continuously exposed to variable environmental conditions, plants produce phytohormones to react quickly and specifically to these changes. The phytohormone ethylene is produced in response to multiple stresses. While the role of ethylene in defense responses to pathogens is widely recognized, recent studies in arabidopsis and crop species highlight an emerging key role for ethylene in the regulation of organ growth and yield under abiotic stress. Molecular connections between ethylene and growth-regulatory pathways have been uncovered, and altering the expression of ethylene response factors (ERFs) provides a new strategy for targeted ethylene-response engineering. Crops with optimized ethylene responses show improved growth in the field, opening new windows for future crop improvement. This review focuses on how ethylene regulates shoot growth, with an emphasis on leaves

Protein subcellular trafficking during the oxidative stress response in plants

Due to their sessile lifestyle, plants are permanently exposed to a variety of adverse environmental conditions leading to the accumulation of reactive oxygen species (ROS). Although initially considered as harmful byproducts of aerobic metabolism reacting in high concentrations with all cellular components, ROS are generally accepted as key signaling molecules that coordinate a broad range of environmental and developmental processes. At the beginning of my Ph.D., significant progress has been made in the description of oxidative stressdependent gene expression, but the regulation of the complex ROS signal transduction network remains largely unknown. The functional analysis of proteins that are encoded by genes that rapidly respond to ROS can therefore give novel insights into the early signaling steps triggered by a sudden increase in intracellular ROS. During adverse environmental conditions, the regulation of dynamic protein trafficking is an important intracellular signaling strategy to elicit a fast defense response. The aim of this project was to identify and characterize proteins that relocalize during oxidative stress. Therefore, we first made a comprehensive inventory of hydrogen peroxide (H2O2)-induced genes by comparing H2O2-related expression data sets and selected in a well-considered manner 85 candidate genes for further functional studies. We focussed our selection on genes encoding transcription factors and proteins of unknown function. To identify proteins with a potential dynamic behaviour during oxidative stress, we employed two medium-throughput localization screens of green fluorescent protein (GFP)-tagged proteins by transient expression in Nicotiana benthamiana and by stable expression in transgenic Arabidopsis thaliana lines. Transgenic Arabidopsis plants with perturbed levels of interesting candidate genes were assayed for altered tolerance to abiotic and biotic stress. The identification of proteins that dynamically relocalize during stress conditions, together with a detailed understanding of the mechanisms behind the identified oxidative stress-induced relocalizations, will provide a better understanding of stress response signaling

Emerging connections between small RNAs and phytohormones

Small RNAs (sRNAs), mainly including miRNAs and siRNAs, are ubiquitous in eukaryotes. sRNAs mostly negatively regulate gene expression via (post-)transcriptional gene silencing through DNA methylation, mRNA cleavage, or translation inhibition. The mechanisms of sRNA biogenesis and function in diverse biological processes, as well as the interactions between sRNAs and environmental factors, like (a)biotic stress, have been deeply explored. Phytohormones are central in the plant’s response to stress, and multiple recent studies highlight an emerging role for sRNAs in the direct response to, or the regulation of, plant hormonal pathways. In this review, we discuss recent progress on the unraveling of crossregulation between sRNAs and nine plant hormones

Phenotyping on microscopic scale using DIC microscopy

Image analysis of Arabidopsis (Arabidopsis thaliana) plants is an important method for studying plant growth. Most work on automated analysis focuses on full rosette analysis, often in a high-throughput monitoring system. In this talk we propose a new workflow that analysis plant growth on a microscopic scale. This approach results in more detail than the common growth measurements, i.e. analysis of the number of cells, the average cell size, etc. The proposed workflow uses differential interference contrast (DIC) microscopy to visualise cells. DIC microscopy is preferred over fluorescence techniques because it provides a very fast methodology (i.e. image analysis is already possible after 1 day) and it also results in clear contrast in the samples. Although these images are easy to interpret by a human operator, they pose several challenges for automated computer vision methods. In our proposed talk we circumvent most of these challenges by combining multiple images, acquired with different microscopy settings. This approach allows us to automatically segment and analyse cells in the images. The proposed workflow enables a new form of automated phenotyping on microscopic scale

The role of the anaphase-promoting complex/cyclosome in plant growth

The anaphase-promoting complex/cyclosome (APC/C) is a multi-subunit E3 ubiquitin ligase that plays a major role in the progression of the eukaryotic cell cycle. This unusual protein complex targets key cell cycle regulators, such as mitotic cyclins and securins, for degradation via the 26S proteasome by ubiquitination, triggering the metaphase-to-anaphase transition and exit from mitosis. Because of its essential role in cell cycle regulation, the APC/C has been extensively studied in mammals and yeasts, but relatively less in plants. Evidence shows that, besides its well-known role in cell cycle regulation, the APC/C also has functions beyond the cell cycle. In metazoans, the APC/C has been implicated in cell differentiation, disease control, basic metabolism and neuronal survival. Recent studies also have shed light on specific functions of the APC/C during plant development. Plant APC/C subunits and activators have been reported to play a role in cellular differentiation, vascular development, shoot branching, female and male gametophyte development and embryogenesis. Here, we discuss our current understanding of the APC/C controlling plant growth

3D reconstruction of maize plants in the phenoVision system

In order to efficiently study the impact of environmental changes, or the differences between various genotypes, large numbers of plants need to be measured. At the VIB, a system named \emph{PhenoVision} was built to automatically image plants during their growth. This system is used to evaluate the impact of drought on different maize genotypes. To this end, we require 3D reconstructions of the maize plants, which we obtain from voxel carving

Plant growth under suboptimal water conditions : early responses and methods to study them

Drought stress forms a major environmental constraint during the life cycle of plants, often decreasing plant yield and in extreme cases threatening survival. The molecular and physiological responses induced by drought have therefore been the topic of extensive research during the last decades. Because soil-based approaches to study drought responses are often inconvenient due to low throughput and insufficient control of the conditions, osmotic stress assays in plates were developed to mimic drought. Addition of compounds such as poly-ethylene glycol, mannitol, sorbitol, or NaCl to controlled growth media has become increasingly popular since it offers the advantage of accurate control of stress level and onset. These osmotic stress assays enabled the discovery of very early stress responses, occurring within seconds or minutes following osmotic stress exposure. In this review, we construct a detailed timeline of early responses to osmotic stress, with a focus on how they initiate plant growth arrest. We further discuss the specific responses triggered by different types and severities of osmotic stress. Finally, we compare short-term plant responses under osmotic stress vs. in-soil drought and discuss the advantages, disadvantages and future of these plate-based proxies for drought