Plant-specific WUSCHEL-related homeobox (WOX) family transcription factors play critical roles in maintaining meristems and lateral organ development. The WUS clade member STF/LAM1 physically interacts with the intermediate clade member WOX9. This interaction contributes to their antagonistical functions on leaf blade outgrowth by competing for the same cis-elements in the promoter of their common target in M. truncatula and N. sylvestris. Here, we identified the main interaction domains of STF and MtWOX9 in Medicago, shedding light on the mechanism of WOX gene function. The middle domain of STF and MtWOX9 are both critical for the interaction, while the conserved motif of STF in the C-terminal domain is also required. Deletion of the middle domain of STF partially rescued the leaf blade phenotypes of the stf null mutant, indicating that the middle domain plays an essential role during leaf blade expansion. This finding provides a new insight that the versatility of WOX function is not only caused by the conserved DNA binding and repression domains but also by the middle domain that recruits different partners.
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