GIGANTEA regulates lateral root formation by modulating auxin signaling in <i>Arabidopsis thaliana</i>

Abstract

Lateral root (LR) formation is a vital organogenetic process that determines the root architecture in plants. The number of root branches governs the degree of anchorage, efficiency of nutrients acquisition, and water uptake. The molecular pathways involved in LR formation have been extensively studied in <i>Arabidopsis thaliana</i> (<i>At</i>). A plant hormone, Auxin, is a key regulator of root development and promotes LR formation in plants. A plethora of <i>Arabidopsis</i> genes have been identified to regulate LR initiation, patterning, and emergence processes. Recently, the involvement of flowering time control pathways and circadian clock pathways in LR development has come to light, but the connecting link between these processes is still missing. We have established that GIGANTEA (GI), a key component of photoperiodic flowering, can regulate the formation of LRs in <i>Arabidopsis</i>. GI is known to be involved in red light signaling and circadian clock signaling pathways. Here, we report that over-expression of <i>GI</i> enhances LR formation in red light in <i>At</i>. Real-time PCR analysis shows that GI positively regulates the transcription of <i>TRANSPORT INHIBITOR RESPONSE 1</i> (<i>TIR1</i>) which is an upstream component of auxin signaling. Furthermore, <i>gi-100</i> mutant downregulates the LR initiation signaling gene, <i>AUXIN RESPONSE FACTOR 7</i> (<i>ARF7</i>), and its downstream target gene, <i>LATERAL ORGAN BOUNDARIES-DOMAIN 16</i> (<i>LBD16</i>). Hence, GI acts as a positive regulator of IAA14-ARF7-LBD16 modules during LR initiation. We have also checked the effect of GI on the expression of <i>NAC1</i> and <i>AIR3</i> genes which are controlled by TIR1 during LR formation. Our results show that GI induces the <i>NAC1</i> transcription and its downstream gene, <i>AIR3</i> expression, which leads to the enhancement of LR initiation. Taken together, our results suggest that GI controls the expression of <i>TIR1</i> to govern auxin signaling during LR formation in presence of red light and GI can act as a link between circadian clock signaling, flowering time control pathways, light signaling, and lateral root development pathways.