UC Davis

The precise spatial control of the division plane is crucial for tissue and organpatterning in plants, and the preprophase band (PPB) is a cytoskeletal structure that regulates division plane orientation in somatic cells. The PPB is primarily composed of bundled cortical microtubules (MTs) with actin and protein components and functions to organize a cortex region that becomes the future division site. The PPB-dependent cue is a positional landmark that guides centrifugal phragmoplast expansion during cytokinesis. Chapter 1 of this thesis offers a comprehensive literature review on the PPB, delving into our current understanding of the molecular mechanisms controlling its formation and function and tracing its evolutionary origins. In Chapter 2, we identify and characterize the IQ Domain (IQD) containing proteins 6/7/8 as microtubule-binding proteins associated with PPB MTs. Triple mutant plants of these IQD genes exhibit oblique cell division planes caused by disorganized PPBs that led to the misspecification of the division plane. Our studies show that robust and solid PPBs are crucial for precise division plane control and that IQD6/7/8 are necessary for properly organizing PPB MTs. Another discovery highlights the cooperation between the MTpolymerase CLIP Associated Protein (CLASP) and IQD6/7/8 in maintaining PPB MT organization and stability. Considering the conserved IQ67 domain in IQD proteins is known to bind calmodulin (CaM) in the presence of calcium (Ca2+), it raises intriguing questions about the role of Ca2+/CaM in division plane regulation. Chapter 3 focuses on the regulation of PPB formation by cyclin and cyclin-dependent kinase (CDK). We confirm that the cyclin A1;1 (CYCA1;1) and CDKA;1 localize at the PPB when expressed under native promoters, revealing an interdependence between CDKA;1 and CYCA1;1 for their PPB localization. However, the specific functions of CDKA;1 and CYCA1;1 at the PPB still need to be clarified. Our findings introduce a connection between cell cycle regulators and the mitotic cytoskeleton machinery potentially influencing PPB emergence or disassembly. In summary, these studies contribute to our understanding of molecular modules that control plant development by studying how plants control their division plane placement. Such knowledge is crucial for understanding how plants build their tissues and organs, which constitute the basis of human food systems.