Identification of novel auxin responses during Arabidopsis embryogenesis

Plants normally form one embryo per seed. Under special circumstances, such as death of the embryo, a second embryo can develop from a supportive structure called the suspensor. These suspensor cells therefore provide a reservoir of stem cells for the generation of secondary embryos. At the start of this project, the mechanisms that control the formation of secondary embryos were completely unclear. By conducting a systematic screen for cellular responses to the plant hormone auxin during embryogenesis we found that auxin prevents embryo development from suspensor cells. The detailed analysis of auxin response components allowed us to identify the auxin-dependent transcription factors that mediate auxin action in the suspensor. Furthermore, we found that the control of expression of these auxin response transcription factors contributes to early embryo pattern formation. This work identified the first molecular players in the control of suspensor-embryo transformation and provides a stepping stone for elucidating the genetic networks that control embryo identity in plants. <br/

Evolutionary adaptations of plant AGC kinases: from light signaling to cell polarity regulation

Moleculaire ontwikkelingsgenetic

Evolutionary adaptations of plant AGC kinases: from light signaling to cell polarity regulation

Moleculaire ontwikkelingsgenetic

Got root? Initiation of the embryonic root meristem.

Plant development relies on the activity of meristems, small groups of undifferentiated cells that produce all organs. The first meristems are formed in the embryo, and all subsequent development depends on their proper establishment, making embryonic meristem initiation a key step in plant life. The founder cells of the embryonic meristems are specified early in embryo development after the establishment of the body axis. Initiation of the root meristem in the early embryo is marked by the specification of a single cell, the hypophysis, and hence an attractive model to study meristem initiation. In this review, we will discuss the mechanisms that control embryo axis formation and root meristem initiatio

Green beginnings - Pattern formation during plant embryogenesis

Embryogenesis in plants transforms the zygote into a relatively simple structure, the seedling, which contains all tissues and organs that later form the mature plant body. Despite a profound diversity in cell division patterns among plant species, embryogenesis yields remarkably homologous seedling architectures. In this review, we describe the formative events during plant embryogenesis and discuss the molecular mechanisms that regulate these processes, focusing on Arabidopsis. Even though only a relatively small number of factors are known that regulate each patterning step, a picture emerges where locally acting transcription factors and intercellular signaling contribute to the specification and spatio-temporal coordination of the various cell types in the embryo. Notably, several patterning processes are controlled by the plant hormone auxin. Most regulators that were identified in Arabidopsis have orthologs in other sequenced plant genomes, and several of these are expressed in similar patterns. Therefore, it appears that robust conserved mechanisms may underlie pattern formation in plant embryo

A cellular expression map of the Arabidopsis AUXIN RESPONSE FACTOR gene family

The plant hormone auxin triggers a wide range of developmental and growth responses throughout a plant’s life. Most well-known auxin responses involve changes in gene expression that are mediated by a short pathway involving an auxin-receptor/ubiquitin-ligase, DNA-binding auxin response factor (ARF) transcription factors and their interacting auxin/indole-3-acetic acid (Aux/IAA) transcriptional inhibitors. Auxin promotes the degradation of Aux/IAA proteins through the auxin receptor and hence releases the inhibition of ARF transcription factors. Although this generic mechanism is now well understood, it is still unclear how developmental specificity is generated and how individual gene family members of response components contribute to local auxin responses. We have established a collection of transcriptional reporters for the ARF gene family and used these to generate a map of expression during embryogenesis and in the primary root meristem. Our results demonstrate that transcriptional regulation of ARF genes generates a complex pattern of overlapping activities. Genetic analysis shows that functions of co-expressed ARFs converge on the same biological processes, but can act either antagonistically or synergistically. Importantly, the existence of an ‘ARF pre-pattern’ could explain how cell-type-specific auxin responses are generated. Furthermore, this resource can now be used to probe the functions of ARF in other auxin-dependent processes

MONOPTEROS controls embryonic root initiation by regulating a mobile transcription factor

Acquisition of cell identity in plants relies strongly on positional information1, hence cell–cell communication and inductive signalling are instrumental for developmental patterning. During Arabidopsis embryogenesis, an extra-embryonic cell is specified to become the founder cell of the primary root meristem, hypophysis, in response to signals from adjacent embryonic cells2. The auxin-dependent transcription factor MONOPTEROS (MP) drives hypophysis specification by promoting transport of the hormone auxin from the embryo to the hypophysis precursor. However, auxin accumulation is not sufficient for hypophysis specification, indicating that additional MP-dependent signals are required3. Here we describe the microarray-based isolation of MP target genes that mediate signalling from embryo to hypophysis. Of three direct transcriptional target genes, TARGET OF MP 5 (TMO5) and TMO7 encode basic helix–loop–helix (bHLH) transcription factors that are expressed in the hypophysis-adjacent embryo cells, and are required and partially sufficient for MP-dependent root initiation. Importantly, the small TMO7 transcription factor moves from its site of synthesis in the embryo to the hypophysis precursor, thus representing a novel MP-dependent intercellular signal in embryonic root specificatio

Different Auxin Response Machineries Control Distinct Cell Fates in the Early Plant Embryo

The cell types of the plant root are first specified early during embryogenesis and are maintained throughout plant life. Auxin plays an essential role in embryonic root initiation, in part through the action of the ARF5/MP transcription factor and its auxin-labile inhibitor IAA12/BDL. MP and BDL function in embryonic cells but promote auxin transport to adjacent extraembryonic suspensor cells, including the quiescent center precursor (hypophysis). Here we show that a cell-autonomous auxin response within this cell is required for root meristem initiation. ARF9 and redundant ARFs, and their inhibitor IAA10, act in suspensor cells to mediate hypophysis specification and, surprisingly, also to prevent transformation to embryo identity. ARF misexpression, and analysis of the short suspensor mutant, demonstrates that lineage-specific expression of these ARFs is required for normal embryo development. These results imply the existence of a prepattern for a cell-type-specific auxin response that underlies the auxin-dependent specification of embryonic cell type