Phd in GENETICS AND MOLECULAR BIOLOGY

Thesis title: ASYMMETRIC CELL DIVISIONS: AN EVOLUTIONARY DEVELOPMENTAL ROADMAP TO DEFINE ROOT DIVERSITY

Asymmetric cell division is a most successful strategy in multicellular organisms to generate cell diversity. How the regulation of asymmetric cell division generate intraspecific and interspecific patterning diversity is however still unclear. In this regards, I identified the ground tissue (GT) as an optimal model to clarify the role of asymmetric cell division in controlling root patterning in plants. The GT is formed by endodermis and cortex(es), and its patterning varies in individuals and/or among different plant species; in particular the number of cortical layers can vary from one to several depending on the species and the age of plants. In my studies I utilize two close relative model systems A.thaliana, and C.hirsuta to shed light on the molecular mechanisms governing ACDs responsible of cortical layer number formation. By means of molecular and genetic analysis I demonstrate that HOMEODOMAIN LEUCINE ZIPPER III (HD-ZIPIII) transcription factor PHABULOSA (PHB) is a master regulator of multiple cortical layer formation as this transcription factor triggers ACDs. In detail, I show two different molecular mechanisms regulated by PHB: 1. In A.thaliana, during post-embryonic maturation, in a time window spanning from 7 to 14 days post germination (dpg), a second cortical layer, namely the Middle Cortex (MC), is generated by asymmetric cell divisions in the endodermis of about 80% of primary roots. Here, I show that vascular PHB activity mediates the expression of the cell cycle regulator CYCD6;1 in the endodermis. In particular PHB control the homeostasis of the plant hormone gibberellin GA catabolic promoting GIBBERELLIN 2 OXIDASE 2 (GA2ox2) in the vascular tissue. 2. In C.hirsuta a set of cells with both cortex and endodermis identity (CEMs) divides asymmetrically, generating two cortical and one endodermal layers. I demonstrate that the activity of PHB in CEMs is fundamental for regulating this ACD, prompting the formation of multiple cortical layers. In particular, I show that in C.hirsuta PHB triggers periclinal cell divisions in those cells promoting the expression of CYCD6,1 via the phytohormone cytokinin (CKs). My studies reveal for the first time a role of PHB as master regulator of GT ACDs.