Hormonally Controlled Expression of the Arabidopsis MAX4 Shoot Branching Regulatory Gene

The Arabidopsis MORE AXILLARY BRANCHING 4 (MAX4) gene is required for the production of a long-range, graft-transmissible signal that inhibits shoot branching. Buds of max4 mutant plants are resistant to the inhibitory effects of apically applied auxin, indicating that MAX4 is required for auxin-mediated bud inhibition. The RAMOSUS 1 (RMS1) and DECREASED APICAL DOMINANCE 1 (DAD1) genes of pea and petunia, respectively, are orthologous to MAX4 and function in a similar way. Here we show that, despite the similarities between these three genes, there are significant differences in the regulation of their expression. RMS1 is known to be upregulated by auxin in the shoot, suggesting a straightforward link between the RMS1-dependent branch-inhibiting signal and auxin, whereas we find that MAX4 is only upregulated by auxin in the root and hypocotyl, and this is not required for the inhibition of shoot branching. Furthermore, both RMS1 and DAD1 are subject to feedback regulation, for which there is no evidence for MAX4. Instead, overexpression studies and reciprocal grafting experiments demonstrate that the most functionally significant point of interaction between auxin and MAX4 is post-transcriptional and indeed post-synthesis of the MAX4-dependent graft-transmissible signal.