Investigating a feed-forward loop of transcription factors that acts in plant vascular development

Abstract

The plant vascular system facilitates the distribution of essential substances throughout the plant. Vascular tissue arises from highly organised cell division in the vascular meristem, known as the procambium. This displaces cells, which differentiate into phloem towards the outside of the stem or xylem towards the inside, resulting in a well-defined pattern of vascular tissue within the stem. The PHLOEM INTERCALATED WITH XYLEM (PXY) receptor and its peptide ligand, TRACHEARY DIFFERENTIATION INHIBITORY FACTOR (TDIF), regulate the development of this pattern by influencing multiple developmental processes.

Recent Y1H assays demonstrate that PXY sits within a complex transcriptional regulatory network (TRN). This thesis investigates a feed-forward loop motif of interest within this TRN that sits downstream of PXY and may act to co-ordinate vascular development. It is comprised of three transcription factors: WUSCHEL HOMEOBOX RELATED 14 (WOX14), TARGET OF MONOPTEROS 6 (TMO6) and LOB DOMAIN-CONTAINING PROTEIN 4 (LBD4).

Here, we show that lines of Arabidopsis thaliana expressing LBD4 ectopically in the xylem demonstrate difficulty in establishing xylem identities. Meanwhile, expressing LBD4 in the phloem results in increased phloem proliferation, suggesting that LBD4 plays a role in defining the phloem-procambium boundary. Furthermore, tmo6 mutants demonstrate decreased phloem proliferation, indicating that the loop as a whole may be important in phloem development.

Both the TMO6 and LBD4 transcription factors show WOX14 dependent expression. However, LBD4 does not show expression dependent on PXY signalling. As LBD4 shows high connectivity in the TRN, this suggests that LBD4 is maintained by other signalling pathways. Thus this feed-forward loop may act as a point of cross-talk between PXY signalling and other signalling pathways.