A Medicago truncatula HD-ZIP gene MtHB2 is involved in modulation of root development by regulating auxin response

HD-Zip proteins are plant-specific transcription factors known for their diverse functions in regulating plant growth, development, and responses to environmental stresses. Among the HD-Zip II genes, has been previously linked to abiotic stress responses. In this study, we conducted a functional characterization of in the regulation of root growth and development. Upon auxin stimulation, expression of was promptly up-regulated. Overexpression of in led to reduced primary root growth and inhibited lateral root formation. Interestingly, the transgenic plants expressing exhibited differential responses to three types of auxins (IAA, NAA, and 2,4-D) in terms of root growth and development compared to the wild-type plants. Specifically, primary root growth was less affected, and lateral root formation was enhanced in the transgenic plants when exposed to auxins. This differential response suggests a potential role for in modulating auxin transport and accumulation, as evidenced by the reduced sensitivity of the transgenic plants to the auxin transport inhibitor NPA and lower expression levels of auxin-related reporters such as - ( )::PIN1-GFP, ::PIN3-GFP, ::PIN7-GFP, and ::GFP compared to wild-type plants. Additionally, microarray analysis of the root tissues revealed down-regulation of several auxin-responsive genes in transgenic seedlings compared to wild-type plants. These findings collectively indicate that plays a critical regulatory role in root growth and development by modulating auxin accumulation and response in the roots.