Stem cell specification and regeneration

Zygotic embryogenesis in many plants, including Arabidopsis thaliana, usually kicks off with the asymmetric division of the zygote, forming a smaller apical cell that generates the embryo proper and large basal cell initiating the suspensor. Natural zygotic poly-embryony originating from this normally quiescent suspensor is observed in several species but can also be experimentally induced, thereby presenting an intriguing example of the reprogramming potential of cells during embryogenesis.

Following germination, the anchoring and foraging root system architecture (RSA) develops to support the growing plant. RSA includes two aspects of the root system: its shape and its structure. We study the genes that control RSA in lettuce for optimal growth, either on soil or on hydroponics. To identify key RSA genes, we explore natural variation in a large collection of Lactuca species. The identified genes as well as known root architecture genes are functionally characterized in lettuce by genetic modification (Anneke Horstman).

Mixed cell fate divisions, giving rise to daughter cells with characteristics of two tissue layers, are common in plants but near-absent in other organisms. We investigate how the SCHIZORIZA protein can influence several such occurrences during embryogenesis and root development. Specifically, the schizoriza mutant phenotype is characterized by root tissue fates moving inwards. Initial tissues are generated during embryogenesis indicating that SCHIZORIZA does not appear to represent a typical patterning gene. Instead, our results point to a novel control mechanism for plant cell fate stabilization. As SCHIZORIZA encodes a repressor heat-shock transcription factor, a related goal is to investigate whether and how its role in stress response is related to fate stability.  

Fate decisions are also instrumental during plant and tissue regeneration. However, phytohormone induced regenerative capacity varies widely between species and tissue types. This regenerative recalcitrance can be particularly distressing for the application of modern plant propagation and breeding techniques. Experiments in Arabidopsis demonstrated that during de novo shoot regeneration, cells are first persuaded to change fate towards root stem cell-like identity and subsequently are reprogrammed to acquire shoot fate. We apply our knowledge on root stem cell niche biology to induce stemness in relation to regeneration in phytohormone free conditions. Central in our rationale to induce regeneration are transcription factors such as PLETHORAs, SHORT ROOT/ SCARECROW and WOX5, that are involved in root stem cell niche specification. We showed that using such root stem cell factors we are able to induce regeneration via somatic embryogenesis and that we can break recalcitrance in natural Arabidopsis accessions, all in the absence of exogeneous phytohormones. We also showed that this system is able to induce somatic embryogenesis mediated regeneration in crops species such as tomato, lettuce and bell pepper. Using the obtained knowledge, we continue to investigate what determines competence for regeneration on a tissue and cellular level, and how we can translate this knowledge towards application.

Related to regeneration is the induction of callus friability for the purpose of obtaining stable cell cultures. Friability indicates loosely aggregated cells with high cel division activity that are typically induced through phytohormone treatment. However, friability can be an unstable and unpredictable process, also depending on environmental conditions. Similar to the approach used in regeneration, we aim to identify the causal factors that determine friability, whereby we will make use of Arabidopsis and cocoa.