Speaker: Yunde Zhao, University of California, San Diego, professor in the Department of Cell and Developmental Biology and Tata Chancellor's Endowed Professorship in Cell & Developmental Biology  

Title: Arabidopsis knock-in technology reveals how auxin controls flower initiation

Abstract: Auxin is known to be required for flower initiation. Mutations that disrupt either auxin biosynthesis, or auxin transport, or signaling result in the development of pin-like inflorescences. Among the pin-like mutants, pin-formed 1 (pin1), which encodes an auxin transporter, has been at the center of research in plant developmental biology. PIN1 activity and localization have been reported to be positively and directly regulated by PINOID (PID), a protein kinase, through phosphorylation.  My group has developed several technologies that enable precise modifications of genes in plants. The technological advances provide unprecedented tools for us to study auxin biology. In this presentation, I will briefly describe the efficient gene targeting technology and in situ tagging genes with GFP and other tags using CRISPR-based homologous recombination. The technology avoids the pitfalls of transgenic approaches such as co-suppression and ectopic overexpression. We knocked GFP into PIN1 locus to generate in-frame PIN1-GFP fusion using our gene-editing technology. Surprisingly, the PIN1-GFP_HDR fusion suppressed pid phenotypes, in a semi-dominant fashion. We further showed that pid mutants were suppressed when one copy of PIN1 gene is inactivated, while homozygous pin1 mutations enhanced pid phenotypes.  Our observations are not compatible with the model that PIN1 is positively and directly regulated by PID. Moreover, we demonstrated that PID is not directly involved in PIN1 phosphorylation.  In addition, we have shown that NPY1, which enhances pid phenotypes when mutated and which is not a protein kinase, positively regulates PIN1 phosphorylation. Interestingly, we discovered that phosphorylation of PIN1 actually inhibits PIN activities.  Our latest results lead to a dramatic revision of previous models regarding the relationship between PIN1 and PID and overall auxin-mediated flower development.

Host: Michelle Guo, genetics, development and cell biology assistant professor