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 <i>Medicago truncatula</i> HD-Zip II genes, <i>MtHB2</i> has been previously linked to abiotic stress responses. In this study, we conducted a functional characterization of <i>MtHB2</i> in the regulation of root growth and development. Upon auxin stimulation, expression of <i>MtHB2</i> was promptly up-regulated. Overexpression of <i>MtHB2</i> in <i>Arabidopsis thaliana</i> led to reduced primary root growth and inhibited lateral root formation. Interestingly, the transgenic plants expressing <i>MtHB2</i> 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 <i>MtHB2</i> 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 <i>PIN</i>-<i>FORMED</i> (<i>PIN1</i>)::PIN1-GFP, <i>PIN3</i>::PIN3-GFP, <i>PIN7</i>::PIN7-GFP, and <i>DR5</i>::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 <i>MtHB2</i> plays a critical regulatory role in root growth and development by modulating auxin accumulation and response in the roots.