We discovered that the ZHOUPI (ZOU) gene has important roles in seed development in the flowering plant Arabidopsis thaliana. ZOU is needed for the embryo to make its cuticle, the coating on its epidermis which limits water loss when the embryo germinates and emerges from the seed. In addition, it promotes the death and liquification of a nutrient rich tissue (endosperm) which surrounds the embryo and fills the seed. This is important as it makes space inside the seed to allow the embryo to grow. Surprisingly, we found that the ZOU gene is conserved in all land plants, even in lineages like mosses or ferns whose members lack seed and endosperm. We used genetic analysis in the model liverwort Marchantia polymorpha to address what the roles of ZOU are in a plant very distantly related to Arabidopsis. We found that ZOU controls the formation of pegged rhizoids, a novel water conducting cell that is thought to have evolved independently of xylem. ZOU promotes cell death in these cells, to create hollow pipes well suited for water transport. However, it is unlikely that this is the ancestral role of ZOU genes, as pegged rhizoids are only found in the Marchantiales, an evolutionarily derived group. We are therefore exploring what roles ZOU genes may play in cuticle development and in reproductive tissues in diverse plant lineages.
Outline of the lab's research.
Confocal image of rhizoid cells in Marchantia. The rhizoids are long, tip growing cells that emerge from the epidermis on the lower side of tissues and anchor plants to soil. The nuclei of living cells are visible as green ovals near the tips of rhizoids, the cytoplasm of living cells is pink. Pegged rhizoid cells, distinguished by the internal dark spots (pegs), are differentiating and dying, in most of them the nuclei have broken down and are no longer visible. Image created by Philippe Gadient
Confocal image of cleared Marchantia gemma, cell walls stained with SCRI Renaissance. The cells in the meristem are visible in the centre of the notch at top of image. Image created by Jiayue Zheng
Pegged rhizoid cells are external and accessible, so we are exploiting this to identify the effector genes that mediate programmed cell death as well as cell wall modification. Our work is part of a long term collaboration with Gwyneth Ingram (ENS, Lyon), Moritz Nowack (VIB, Ghent) and Takayuki Kohchi (Univ of Kyoto, Japan) and more recently our colleague Sandy Hetherington.
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