Microspore embryogenesis: reprogramming cell fate from pollen to embryo development

Microspore embryogenesis is an expression of plant cell totipotency that leads to the production of haploid embryos. Besides being a widely exploited plant breeding tool, microspore embryogenesis is also a fascinating system that can be used to obtain a deeper mechanistic understanding of plant totipotency. This thesis aims to provide more insight into the process of microspore embryogenesis, from the formation of embryogenic cells to the outgrowth of differentiated embryos. In Chapter 1 background information is provided on the various aspects of Brassica napus microspore culture and plant development that intersect with the topics that are studied in this thesis. Emphasis is placed on the basic requirements and limitations for successful microspore embryo culture, as well as on the roles of the plant hormone auxin and epigenetic regulation in the development of plant embryos, during both zygotic and in vitro embryo development. Chapter 2 reviews the recent advances that have been made in understanding the developmental and molecular changes that take place during microspore embryogenesis in model systems. The commonly reported cellular changes associated with the establishment of embryo cell fate are summarized and evaluated. The subsequent differentiation of the embryo is also discussed, specifically, what is known about the establishment of polarity, with emphasis on the importance of exine rupture as a positional clue, and the processes that influence meristem maintenance during culture. Finally, the studies on the molecular changes during microspore embryo induction are put into context of male gametophytic development. Overall, the current perspective on microspore embryo initiation presents a landscape in which several routes can lead to the same final destination. Stress treatments are widely applied to induce embryogenic growth in microspore culture. Chapter 3 explores the role of histone acetylation status in stress-induced microspore embryogenesis in Brassica napus. Inhibition of histone deactylases (HDACs) using the HDAC inhibitor trichostatin A (TSA), phenocopies the heat stress treatment that is normally used to induce embryogenic cell proliferation in B. napus microspore culture. Arabidopsis is recalcitrant for haploid embryogenesis, yet treatment with TSA also induced embryogenic cell divisions in this model species. Our observations suggest that the totipotency of the male gametophyte is kept in check by an HDAC-dependent mechanism and that the stress treatments used to induce haploid embryo development in culture impinge on this HDAC-dependent pathway. The repression of HDACs or HDAC-mediated pathways by stress and the accompanying changes in histone acetylation status could provide a single, common regulation point for the induction of haploid embryogenesis. Chapter 4 builds on the knowledge developed in Chapter 3 on the role of HDAC proteins in plant totipotency. A wide variety of chemically distinct HDAC inhibitors was evaluated and additional inhibitors that enhance embryogenic cell induction and/or embryo yield were identified. One surprising observation was made during the course of this study: the initial donor microspore/pollen stage affects the quality of the embryo that is formed. In control cultures, embryos from progressively older stages of donor microspores/pollen became progressively compromised in their basal (axis region) region, characterized by a shift from normal embryos with apical (cotyledons) and basal (root) polarity to abnormal embryos with a reduced basal pole and ball-shaped embryos. These abnormal phenotypes could be partially complemented by treatment with HDAC inhibitors, which promoted growth of the basal region of the embryo. Progressive enhancement of embryo basal identity was accompanied by enhanced and broadened expression of the DR5 auxin response reporter. The embryo phenotypes observed in control and HDAC inhibitor treated microspore cultures are similar to the phenotypes induced by altered expression of the Arabidopsis TOPLESS (TPL)/HDAC19/BODENLOS (BDL) repressor complex, which acts to restrict expression of the AUXIN RESPONSE FACTOR ARF5/MONOPTEROS (MP) to the basal region of the embryo during zygotic embryo development. To understand why most embryogenic callus failed to develop further, we examined the transcriptome of globular-shaped embryos that have started to histodifferentiate and compared it with embryogenic callus. The transcriptome analysis showed that the expression of many genes that regulate (auxin-related) embryo patterning were downregulated in embryogenic callus compared to globular stage embryos. This result may simply reflect the lack of patterning in these embryos or might indicate a role of auxin-signalling in embryogenic callus formation. Chapter 5 examines how embryo identity and patterning is established in two B. napus microspore embryo pathways, a zygotic-like pathway, characterized by suspensor and then embryo proper formation, and a pathway characterized by initially unorganized structures that lack a suspensor. We specifically asked the question: how can embryo patterning be established in the absence of an initial asymmetric division and in the absence of a suspensor, two key events in zygotic embryo development. Analysis of embryo fate (GRP) and auxin (PIN1, PIN7 and DR5) markers showed that embryo fate was established prior to cell division, and independent of subsequent division pattern. The suspensorless embryo program was marked by a transient auxin maximum, followed by establishment of the apical and basal poles at the globular stage, coincident with release of the embryo from the pollen exine. Unlike zygotic embryo development, polar auxin transport (PAT) was not required for embryo initiation or polarity establishment in this system. Suspensor embryos developed in a similar fashion as zygotic embryos, PAT was required for specification of the embryo proper from the suspensor. Haploid embryogenesis therefore follows at least two programs, a PAT-dependent program that requires embryo proper specification from the suspensor, and an alternative PAT-independent program marked by an initial auxin maximum. In the final chapter, Chapter 6, the work presented in this thesis is put in context of the broader plant development field. The epigenetic regulation of developmental transitions that respond to stress and during pollen development are highlighted. A model is provided that histone acetylation levels mediated by HAT and HDAC regulate pollen fate.