HD-Zip II transcription factors control root meristem organization and maintenance

The Arabidopsis genome encodes for 10 HD-Zip II proteins. ATHB2, HAT1, HAT2 (HD-Zip II ? clade), HAT3 and ATHB4 (HD-Zip II ? clade) are regulated by changes in the Red/Far Red ratio of the light environment that induce shade avoidance response in most of the angiosperms. However recent work highlighted that simultaneous lack of HAT3 and ATHB4 causes developmental defects in a white light environment, and several findings indicated that these defects correlate with changes in auxin distribution and response. Here we show that these transcription factors together with ATHB2 play a role in root meristem organization and maintenance. Loss-of-function triple mutants athb2 hat3 athb4 fail to express stem cell niche-specific markers and show terminal differentiation of the root meristem. The specification of the root stem cell niche requires two parallel transcription factor pathways: the PLT and the SHR/SCR pathway. Work to get insight in possible interactions of the HD-Zip II transcription factors with the PLT and SHR/SCR regulatory networks is currently in progress.