HISTONE DEACETYLASE 9 stimulates auxin-dependent thermomorphogenesis in Arabidopsis thaliana by mediating H2A.Z depletion

Many plant species respond to unfavorable high ambient temperatures by adjusting their vegetative body plan to facilitate cooling. This process is known as thermomorphogenesis and is induced by the phytohormone auxin. Here, we demonstrate that the chromatin-modifying enzyme HISTONE DEACETYLASE 9 (HDA9) mediates thermomorphogenesis but does not interfere with hypocotyl elongation during shade avoidance. HDA9 is stabilized in response to high temperature and mediates histone deacetylation at the YUCCA8 locus, a rate-limiting enzyme in auxin biosynthesis, at warm temperatures. We show that HDA9 permits net eviction of the H2A.Z histone variant from nucleosomes associated with YUCCA8, allowing binding and transcriptional activation by PHYTOCHROME INTERACTING FACTOR 4, followed by auxin accumulation and thermomorphogenesis

From lab to greenhouse: molecular mechanisms of physiological control of plant growth

In horticulture, especially in the production of ornamental crops, it is important to grow compact-shaped plants. Achieving this year-round can be challenging, especially during cloudy and/or warm days because elongation of stems is stimulated under such conditions, resulting in spindly-shaped plants. Chemical plant growth retardants (PGRs) are used in horticulture to control plant elongation. However, the use of chemical PGRs needs to be replaced by more sustainable methods. A commonly used alternative method for controlling plant growth without the use of PGRs is realizing a negative day-night temperature difference (-DIF: cold days/warm nights), instead of a ‘normal’ +DIF regime (warm days/cold nights). Although -DIF is effective for controlling growth in many plant species, economic realization of -DIF is not always possible. Therefore, more alternative methods for controlling plant growth in greenhouses are needed. Aim of this thesis project was to increase our understanding of the molecular regulation of the growth response to -DIF. Additionally, we aimed to find ways to manipulate the components behind this regulation (e.g. by added light treatments) in order to enhance the effect of -DIF and/or identify alternative treatments for growing compact plants in greenhouses. To study the transcriptional responses of growth-regulating components to light and temperature cues, we used firefly-luciferase (ffLUC) reporters for promoter activity of genes involved in plant growth. To this aim LUMINATOR was developed. This custom-built system was used to semi-continuously monitor ffLUC activity in four-week-old Arabidopsis reporter plants for promoter activity of PHYTOCHROME INTERACTING FACTOR 4 (PIF4), ELONGATED HYPOCOTYL 5 (HY5) and GIGANTEA (GI) under 12h light/12h dark cycles. LUMINATOR was subsequently used to study transcriptional responses of PIF4, HY5 and GI to one hour added light at the start and end of the photoperiod. Such short light pulse treatments triggered immediate and longer-lasting transcriptional responses, which were predictive for long-term plant growth responses of tomato to similar added light treatments in climate cabinets. Thus, LUMINATOR may be used to help predict long-term plant growth responses to additional light treatments. LUMINATOR was also used to monitor changes in ffLUC reporter activity in response to sudden temperature changes. However, comparing ffLUC profiles with mRNA levels of the endogenous genes showed that the effect of temperature on activity of the ffLUC enzyme itself makes results difficult to interpret. To obtain an overview of the processes that are affected by -DIF RNA sequencing (RNAseq) analysis was performed on total mRNA and miRNA of adult Arabidopsis rosettes grown under +DIF (12h L 22°C/12h D 12°C) or -DIF (12h L 12°C/12h D 22°C) at two timepoints: end of day (ED) and end of night (EN). RNA was isolated from sink material (young sink leaves and petioles of older leaves) of Col-0 WT and the phytochrome B (PHYB) mutant phyB-9, which shows reduced sensitivity to -DIF. Analysis of differentially expressed mRNAs shows that -DIF leads to differential expression of in total 2706 genes in WT. Gene Ontology (GO)-term enrichment analysis indicates that -DIF stimulates cold acclimation processes at ED and leads to a low energy status at EN. Cell-growth processes such as cell-wall modification and water transport are downregulated at either ED or EN. Overlap between genes that are differentially expressed under -DIF and PIF-target genes indicates reduced transcriptional activity of PIF proteins under -DIF, especially at EN. Differentially expressed genes under -DIF that are related to promotion of cell growth were often not differentially expressed at EN in the phyB-9 mutant. This suggests that the transcriptional responses of these genes are causal for growth suppression under -DIF and that PHYB is involved in mediating the transcriptional response to temperature at night. Analysis of differentially expressed miRNAs and their putative target mRNAs suggests that part of the transcriptional response to -DIF is through regulation of mRNA transcript stability by miRNAs. Previously it was shown that -DIF results in altered clock-controlled leaf movement in Arabidopsis, suggesting the timing of the circadian clock is altered under -DIF. We show that -DIF leads to changes in phase and amplitude of rhythmic expression of clock genes. Altered clock gene expression under -DIF is associated with altered regulation of starch metabolism and results in reduced starch levels in source leaves and reduced sucrose levels in sink leaves at EN, leading to induction of marker genes for carbohydrate (CH) starvation. Since it is known that plant growth is suppressed in response to CH starvation, these results suggest that the induction of CH starvation at EN may be causal for growth suppression under -DIF. In agreement with this, alternative methods that potentially induce CH starvation all lead to growth suppression in Arabidopsis. The growth response to -DIF is associated with reduced PIF protein activity. Activity of PIFs may be (further) reduced by changing light quality by providing additional red (R) or blue (B) light. We used LUMINATOR and ffLUC reporter plants to study the transcriptional response of PIF3/4/5, the PIF-antagonist HY5 and the PIF-induced target gene INDOLE-3-ACETIC ACID INDUCIBLE 29 (IAA29) to one-hour additional B or R light at dawn and dusk. Results show that in response to additional R light at dusk IAA29 transcription is reduced throughout the night, suggesting reduced PIF protein activity. Additional B light suppressed PIF and IAA29 transcription but stimulated HY5 transcription in the morning. These results indicate that one-hour additional R or B light at dawn and dusk may be used to suppress plant growth. Indeed, stem growth in tomato plants could be suppressed by providing additional R/B light at dawn and dusk in climate cabinets and/or greenhouses. Interestingly, timing of the additional R/B light determined whether the added light had an effect on plant growth. In agreement with the transcriptional responses, growth responses of tomato to additional R light were restricted to the end of the day, while growth responses to additional B light were restricted to the start of the day. Based on the results presented in this thesis several suggestions for novel sustainable protocols for growing compact plants in greenhouses are provided in the final chapter of this thesis

Balans hormonen bepaalt ontwikkeling, groei en weerbaarheid gewas : Licht, temperatuur en stress sturen de balans

Hormonen sturen heel veel processen in de plant aan. Het gaat meestal niet om individuele hormonen, maar om de balans tussen verschillende soorten. Door teeltmaatregelen kun je die balans beïnvloeden. De inzichten groeien, maar sturing op hormonen is een complexe zaak

Sturen op stress: wanneer is het (nog) nuttig?

In this project a conceptual framework is made about different types of plant stress in a greenhouse cultivation. The aim is to get a better understanding of plant stress and to prevent the need for energy consuming actions when a plant is “out of balance”. What is plant stress? A plant strives for homeostasis: all chemical and physical processes are in balance with the environment. In a stress situation, a plant senses the change that disturbs the balance between plant and environment. The plant adapts to the new situation which leads to a new balance (acclimation). That means that stress can have a negative impact on the crop, but stress can also be beneficial. Crop and climate management both disturb the balance and steer the crop in the desired direction, which is considered as positive stress. The cultivation method “Next Generation Growing” aims for balances in crop and greenhouse. It is not always clear which actions lead to positive stress and which actions are negative for the crop. Monitoring plant balance or plant stress would be a helpful tool. But a lot is still unknown, like which plant processes should be monitored, what are critical values and what time period is needed for “a balanced crop”

LEDs: het pad naar de praktijk : een roadmap voor tomaat

In recent years, electricity consumption in greenhouse horticulture has increased rapidly due to increasing areas of lighted cultivation and the increase in installed power. However, the rise in energy prices at the beginning of 2022 has made it clear that the trade-off between costs and revenues with respect to lighting has a major influence on the choices in purchase and use of lighting. LEDs are essential in the development of sustainable, fossil-free cultivation systems, due to their high efficiency. The transition to LED lighting is a system change: replacing HPS with LEDs has consequences for the energy balance of the greenhouse, morphology and physiology of the crop, and other factors such as crop health. This report outlines which development paths there are with regard to LED lighting, where we stand on these development paths, and what challenges there are still in the development of knowledge about the use of LED lighting in a fossil-free, sustainable cultivation system for tomatoes

Nieuwe balansen in Het Nieuwe Telen: hormonen en ecologie

Voor het project “Nieuwe balansen in Het Nieuwe Telen (HNT): hormonen en ecologie” is een literatuurstudie gedaan naar de huidige staat van kennis met betrekking tot de hormoonbalans en de ecologische balans. Daarnaast is gekeken in hoeverre kennis van deze ‘nieuwe’ balansen nuttig is voor telers en een waardevolle toevoegingen aan het theoretisch kader van HNT waarin onder andere de waterbalans, energiebalans en assimilatenbalans in de plant en de energiebalans, vochtbalans en CO2 balans in de kas centraal staan. Uit de literatuurstudie blijkt dat de hormoonbalans en de ecologische balans verschillen van de andere plantbalansen binnen HNT. Beide maken een belangrijk onderdeel uit van de mechanismen die de plant helpen om zich aan te passen aan de omgeving en daarmee de waterbalans, energiebalans en assimilatenbalans in stand houden onder uiteenlopende omstandigheden. De hormoonbalans en ecologische balans vormen nuttige plantkundige aanvullingen binnen het kader van het HNT. Ook al weten we niet alles, door gebruik te maken van de huidige kennis van relaties met hormonen en ecologie is het beter mogelijk om goed te telen volgens de balansen van HNT met een optimaal gebruik van energie voor licht en warmte en CO2.---Within the project “Nieuwe balansen in Het Nieuwe Telen (HNT): hormonen en ecologie” a literature study was conducted on the plant hormonal and ecological balance. Besides describing the current state of knowledge on these ‘new’ balances, the study also focused on whether this knowledge is useful for growers and forma valuable addition to the theoretical framework of HNT, in which the water balance, energy balance, and assimilate balance in the plant and the energy balance, water balance and CO2 balance in the greenhouse playa central role. From the literature study it becomes clear that the hormonal and ecological balance differ from the traditional plant balances within HNT. Both form an essential part of mechanisms that help the plant to adapt to changing environments and by doing so control the water balance, energy balance and assimilate balance of the plant under different circumstances. The hormonal balance and ecological balance are valuable additions to the theoretical framework of HNT. Although there is still a lot unknown, using available knowledge on hormones and ecology can help growers to implement HNT in their cultivation strategy and make optimal use of energy for light, heating and CO2

Aardbei Fossielvrij

The increasing acreage of strawberry under protected conditions has to be cultivated fossil free and sustainably,certainly given the recent rise in energy prices. Commissioned by Kas als Energiebron, WUR and Delphy have described how such a cultivation system looks like. A general description of future strawberry cultivation has been given, current knowledge and knowledge gaps are given, and an with computation model options for further energy saving steps are quantified. Computations are performed for three situations, viz., cultivating a not-illuminated ever-bearing cultivar, an illuminated ever-bearing cultivar, and a low-chill fresh cultivar. Using thoughtful setpoints already leads to considerable energy saving. Furthermore, attention is given to lowering air temperature, increasing relative air humidity, and longer or higher lighting. The consequences for the use of gas, electricity and CO2 are estimated for all situations. The remains an energy demand for heating and lighting, however, these should be supplied from non-fossil sources

Toekomstbeeld fossielvrij houtig kleinfruit : Een deskstudie

In a desk study financed by Kas als Energiebron, WUR and Delphy investigated the possibilities of fossil-free cultivation of woody soft fruit under glass. To this end, discussions were held with growers. In addition to a description of the current situation in woody berries, a literature study was conducted in which the results of national and international research on blackberries and raspberries, which mainly related to climate factors, were listed. Partly on the basis of this, an attempt has been made to provide a vision of a fossil-free future for woody soft fruit and what measures growers of blackberries and raspberries could take to achieve this. A number of knowledge gaps with regard to woody berries have also been identified

Duurzame teeltsystemen met LEDs

To combine the development towards sustainable and climate-neutral cultivation systems for all-electric greenhouses with lighting, efficient lighting systems such as LEDs are essential. LEDs have a high energy-efficiency and make it possible to apply dynamic light recipes, in which light intensity and spectral composition can be varied throughout the day and during the cultivation depending on the developmental stage of the crop. Knowledge on the effects of light spectrum is still limited for most crops, as well as the effect of spectral composition on the control of pests and diseases in the crop. In this project, we took an integrated approach that combines research on the effects of dynamic light spectra on the growth and production of eggplant, tomatoes and cucumbers with the effects on plant resistance to pests and diseases, incorporating the underlying physiological and genetic processes. The behavior of pests and natural enemies has been further investigated in chrysanthemum and sweet pepper. The results indicate that growth and production of greenhouse grown crops can be controlled with the light spectrum, whereby resistance to diseases and pests is a precondition