Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Genetics, Development and Cell Biology


Plant Biology

First Advisor



Brassinosteroids (BRs) are a class of plant steroid hormones that play important roles in plant growth and development, as well as biotic and abiotic stress responses. BR functions through membrane receptor BRI1, co-receptor BAK1 and a cascade of signaling components to regulate BES1/BZR1 family transcription factors. Comprehensive chromatin-binding and transcriptome reveal that more than 500 transcription factors are regulated by BES1/BZR1, which supports the idea that BES1/BZR1 control multiple biological processes through diverse transcription factors. Among those transcription factors, there are two classes of typical stress responsive transcription factors: TINYs in AP2/ERF family and BOS1 in MYB family. TINYs and BOS1 are shown to be regulated by BR, implying BRs may regulate plant growth and stresses responses via these two families of transcription factors. However, the detailed mechanisms by which TINY and BOS1 regulate stress response and plant growth remain to be established. Therefore, a general introduction about BR biosynthesis, BR signaling pathway, and BR mediated cross-talk on drought tolerance are discussed in Chapter 2. Given that AP2/ERF family transcription factors regulate diverse stresses and hormone pathways, it provides the potential to integrate multiple responses together through AP2/ERFs. I also discuss AP2/ERFs’ function in hormones and abiotic stresses, and generate a regulatory network to study AP2/ERFs mediated hormones and abiotic stresses cross-talk.

My research goal is to study the mechanisms of how BR-mediated the trade-off between stress and growth. In Chapter3, I characterized AP2/ERF family DREB A4 group transcription factors TINY and its homologs function in BR-regulated plants growth and drought response. I found that TINY negatively regulates plants growth, but promotes drought tolerance. TINY’s negative role in plant growth is involving disrupting BR signaling pathway to repress cell elongation related gene expression. TINY overexpression plants have a stunted growth phenotype, display higher sensitivity to BR biosynthesis inhibitors and compromised BR-responsive gene expression. In contrast, tiny tiny2 tiny4 triple mutant leads to increased plant growth and resistance to BR biosynthesis inhibitors as well as induced BR responsive gene expression. We find that TINY promotes ABA-mediated stomatal closure and confers drought tolerance by activating drought responsive genes, which is in contrast to BR regulation of drought response. We also reveal that TINY and BRs have opposite regulation of global gene expression. The mechanistic studies showed that TINY interacts with and antagonizes BES1 on either growth or stress-related genes. In addition, we demonstrate that TINY function balanced through the regulation by BIN2, the negative regulator in BR pathway. BIN2 phosphorylates and stabilizes TINY. This provides a mechanism by which BRs function to inhibit TINY function to prevent unnecessary activation of drought responses, while stresses activated BIN2 reinforced TINY’s function.

In Chapter 4, I characterize BOS1’s function in BR regulated plant growth. We find that BOS1 negatively regulates BR mediated plant growth. However, BR negatively regulates the expression of BOS1 and its homologs to promote plant growth. BES1 can directly bind to BOS1 promoter to inhibit its expression. BOS1 is a substrate of BIN2 suggesting that the post-translational modification of BOS1 protein might regulate BOS1’s function. Previous studies showed that BOS1 have positive effect on plants botrytis and abiotic stress tolerance. We aim to unravel the functional and regulatory mechanisms of BRs in biotic and abiotic stresses through BOS1.

In Chapter 5, future directions are addressed. Taken together, our findings clarify AP2/ERF and MYB family transcription factors functions in plant growth, and adds the knowledge to BR-mediated cross-talk between stresses and growth.

Copyright Owner

Zhouli Xie



File Format


File Size

149 pages