Ground tissue specification in the plant embryo

Multicellular organisms consist of a variety of tissues and cell types that perform unique functions. A central question in biology is how these tissues and cell types are specified and maintained. Ground tissue, one of the three major plant tissues (Figure 1A), offers an excellent model to study the progression from tissue initiation to differentiation. A well-studied molecular framework for cell type specification within the root ground tissue has been established centered around the SHORT-ROOT (SHR) transcription factor (Figure 1B; 1). However, a major question - how ground tissue is initiated in the embryo - is largely unanswered due to a lack of mutants and markers specific for the first ground tissue cells.

Previous work of our lab and others has shown that the MONOPTEROS (MP) transcription factor is crucial for the specification of the root founder cell and vascular stem cells and mp is one of the few Arabidopsis mutants that does not make an embryonic root (2-5). To better understand how MP controls these processes, a number of genes have been isolated that are potentially activated by MP in the young embryo. Intriguingly, several of these TMO (TARGET OF MP) genes are specifically activated in the very first ground tissue cells (Figure 1C). A closer look at the mp mutant embryo showed that also the very first division of the ground tissue cells is disturbed (Figure 1D). Together, these findings reveal a novel role for MP in the initiation of the ground tissue. Interestingly, also SHR and several of its close homologs are potentially activated by MP in the young embryo. This implies an earlier and so far unsuspected role for SHR in ground tissue initiation.

In this project, our hypothesis that SHR is a MP target gene that acts redundantly with other family genes in ground tissue initiation will be further explored. Additionally, we will examine whether the novel putative TMO genes that are expressed in the first ground tissue cells contribute to the specification of these cells.

This work includes the construction and analysis of knockout, knockdown (RNAi or artificial microRNA), and (over-) expression lines and a detailed determination of gene expression and protein localization patterns.

Methods: (reverse) genetic analysis, molecular cloning, plant transformation, phenotype analysis, advanced microscopy, (qRT-) PCR.

Figure 1. Establishment of ground tissue in the early embryo. (A) Left: ground tissue (green) and vascular tissue (yellow) are specified at early globular stage, while epidermal identity (blue) is specified earlier and gradually restricted. Right: Schematic cross-sections in root and hypocotyl of a postembryonic seedling showing the basic tissue organization established during embryogenesis. (B) Ontogeny of ground tissue and its regulation. While regulators for initial specification and asymmetric division are lacking (?), specification of endodermis (E) and cortex (C) requires known regulators SHR, SCR, SCZ, JKD and MGP. Later steps up to differentiation have also been described in some detail. (C) Previously uncommitted cells start to express unique markers in vascular (left) and ground tissue initials (right). (D) Left: wild-type embryo showing asymmetric divisions in vascular (green) and ground tissue (yellow) stem cells. Right: monopteros (mp) mutant embryo showing altered division planes in both cells and no division of the hypophysis.
Figure 1. Establishment of ground tissue in the early embryo. (A) Left: ground tissue (green) and vascular tissue (yellow) are specified at early globular stage, while epidermal identity (blue) is specified earlier and gradually restricted. Right: Schematic cross-sections in root and hypocotyl of a postembryonic seedling showing the basic tissue organization established during embryogenesis. (B) Ontogeny of ground tissue and its regulation. While regulators for initial specification and asymmetric division are lacking (?), specification of endodermis (E) and cortex (C) requires known regulators SHR, SCR, SCZ, JKD and MGP. Later steps up to differentiation have also been described in some detail. (C) Previously uncommitted cells start to express unique markers in vascular (left) and ground tissue initials (right). (D) Left: wild-type embryo showing asymmetric divisions in vascular (green) and ground tissue (yellow) stem cells. Right: monopteros (mp) mutant embryo showing altered division planes in both cells and no division of the hypophysis.
References:
  1. Helariutta et al., Cell (2000) 101: 555-567

  2. Hamann et al., Genes Dev (2002) 16: 1610-1615
  3. Weijers D et al., Dev Cell (2006) 10: 265-270

  4. Schlereth et al., (2010) Nature 8; 464:913-916

  5. De Rybel, Möller et al., Dev Cell (2013) 24: 426-437