HORMONAL REGULATION AND SPECIES-SPECIFIC FUNCTION OF PLETHORA GENES IN ORCHESTRATING FIBROUS ROOT SYSTEM

Abstract

The evolution of the root architecture in plants was a prerequisite for the absorption of water and minerals from the soil, thus becoming a major determinant of terrestrial plant colonization. Cereals have a remarkably complex root system consisting of embryonic primary roots, post-embryonically formed root-borne lateral roots and shoot-borne adventitious roots that are very different from model dicot plant Arabidopsis which consists of embryonic primary roots and postembryonic lateral roots. Among grass species, rice adventitious roots (also called crown roots) are developed from compressed nodes at the stem base. The origin of post-embryonically borne lateral roots and adventitious roots is highly variable, the AR in Arabidopsis is developed from the pericycle cells present at the xylem pole of hypocotyl while in rice they arise at the stem base from innermost ground meristem cells which are peripheral to the vascular cylinder of the stem. However, lateral roots arise from the xylem pole pericycle cells of primary roots in Arabidopsis while in rice they arise from the endodermal and pericycle cells which are located opposite to the protophloem of PR and CRs. Being the backbone of the adult root system in monocots, adventitious roots are being extensively studied in rice. In recent times, molecular genetics, genomics, and proteomics-based approaches have been utilized to dissect the mechanism of post-embryonic meristem formation and tissue patterning. In rice CRs naturally arise from the crown root primordia (CRP) present at the shoot-root junction of the stem base region (i.e. crown tissue). Adventitious root development is a cumulative effect of action and interaction of crucial genetic and hormonal regulators occurring during crown root primordia formation at the stem base. Many genes regulating the initiation and emergence process of CRP including OsCRL5, OsWOX11, OsERF3, OsNAC2, and OsCKX4 are auxin cytokinin responsive genes, thus auxin and cytokinin are regulating the developmental process of CRP during initiation and emergence stage, but how this interaction is occurring at these developmental events in unknown and will be discussed in my study. Further how this interaction homeostasis is important for CRP development will be answered by genetically depleting the cytokinin in the auxin domain during different stages of CRP development in rice. Auxin is known to regulate organogenesis in plants, also exogenously applied auxin induction regulate many TFs transcriptionally, thus it is worthy to decipher how auxin regulates the metabolomic flux in rice crown root development, will also be discussed. Further, a recent study of CR development stages using laser capture microdissection coupled with RNA sequencing has identified a plethora of epigenetic modifiers and other genes including PLETHORA genes that are expressed at the onset of CRP initiation and differentiation of CRP (Lavarenne et al. 2020, Garg et al. 2022). In my thesis, I will provide a comprehensive state-of-the-art on the key member of rice PLETHORA (OsPLT) genes namely OsPLT1 and OsPLT4 involved during different stages of shoot-borne AR development in model monocot plant species rice. Further, functional analysis of key genes expressed at the onset of CRP initiation and during differentiation of CRP reveals the crucial role of PLETHORA TFs in regulating CR initiation and outgrowth will also be disclosed hereunder.