The key transcriptional regulators that regulate root hair initiation and tip growth are unknown. While many genes have been reported that participate in root hair development, especially root hair tip growth, none that regulate root hair initiation and tip growth has been identified to date. The aim of this thesis is to characterise the role of a family of transcription factors that control these processes and to construct a regulatory network for root hair growth and initiation. Previously, we isolated two bHLH transcription factors, RHD6 and RSL1, that positively regulate root hair initiation. These two genes belong to the bHLH VIllc subfamily, which includes four other paralogues (RSL2 to RSL5) that form a sister clade to RHD6 and RSL1. To characterize the function of the sister clade genes, I examined the phenotypes of all the available knockout mutants and determined their expression patterns. The genetic data shows that the sister clade genes function downstream of RHD6 and RSLl in root hair development, with RSL4 regulating both root hair initiation and tip growth and RSL2, RSL3 and RSL5 controlling root hair tip growth. Root hair tip growth is totally abolished in plants having mutations in both RSL2 and RSL4. Consistent with the genetic data, expression of RHD6 family genes follows a temporal pattern that reflects the timing of their roles during development. RHD6 and RSLl are expressed in root hair cells around the root meristem and elongation zones to regulate the early event of root hair initiation. RSL4 is expressed in root hair cells around root hair initiation zone to direct late event of root hair initiation; then, together with RSL2 and RSL3, which are also expressed in root hair cells around root hair initiation zone, RSL4 maintains root hair tip growth; RSL5, which is regulated by the other members, is also expressed in root hair cells. As auxin and ethylene also regulate root hair initiation and tip growth, and can partially restore the root hair development in rhd6/rsll mutant, I also investigated how auxin and ethylene interact with the RHD6 family genes to regulate root hair development. I showed that auxin and ethylene can bypass RHD6 and RSLl to regulate the down stream sister clade genes during root hair development. Furthermore, RHD6 and auxin/ethylene regulate root hair tip growth through RSL2 and RSL4. And low phosphate stress also regulates root hair by modulating RSL2 and RSL4 transcription. In summary, we found a subgroup of key regulator genes in the same bHLH subfamily form a regulatory network and temporally control root hair initiation and tip growth, and the internal and external (phosphate availability) factors regulate root hair development through these genes.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:502367 |
| Date | January 2008 |
| Creators | Yi, Keke |
| Publisher | University of East Anglia |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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