A KNOTTED1-LIKE HOMEOBOX PROTEIN1-interacting transcription factor SlGATA6 maintains the auxin-response gradient to inhibit abscission.

The KNOTTED1-LIKE HOMEOBOX PROTEIN1 (SlKD1) is a master abscission regulator in tomato (Solanum lycopersicum). Here, we identified an SlKD1-interacting transcription factor GATA transcription factor 6 (SlGATA6), which is required for maintaining the auxin-response gradient and preventing abscission. SlGATA6 up-regulates the expression of SlLAX2 and SlIAA3. The AUXIN RESISTANT/LIKE AUXIN RESISTANT (AUX/LAX) proteins SlLAX2-dependent asymmetric auxin distribution causes differential accumulation of Auxin/Indole-3-Acetic Acid 3 (SlIAA3) and its homolog SlIAA32 across different abscission zone cells. It is also required for SUMOylation of AUXIN RESPONSE FACTOR 2a (SlARF2a), a key suppressor of auxin signaling and abscission initiator. Moreover, SlIAA3 and SlIAA32 depress SUMOylated SlARF2a, thus suppressing SlARF2a function. The interaction between SlKD1 and SlGATA6 suppresses SlGATA6 binding to the promoters of SlLAX2 and SlIAA3, thereby disrupting the auxin-response gradient and triggering abscission. This regulatory mechanism is conserved under low light-induced abscission in diverse Solanaceae plants. Our findings reveal a critical role of SlKD1 in modulating the auxin-response gradient and abscission initiation.