AUTHOR=Yan Wei , Wang Runze , Zhang Yutong , Zhang Xiuxiu , Wang Qin 

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

JOURNAL=Frontiers in Plant Science

VOLUME=15

YEAR=2024

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2024.1466431

DOI=10.3389/fpls.2024.1466431

ISSN=1664-462X

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