Coordination of auxin-triggered leaf initiation by tomato .

Lateral plant organs, particularly leaves, initiate at the flanks of the shoot apical meristem (SAM) following auxin maxima signals; however, little is known about the underlying mechanisms. Here, we show that tomato () mutants fail to produce cotyledons and leaves and grow a naked pin while maintaining an active SAM. A similar phenotype was observed among pin-like shoots induced by polar auxin transport inhibitors such as 2,3,5-triiodobenzoic acid (TIBA). Both types of pin-like shoots showed reduced expression of primordia markers as well as abnormal auxin distribution, as evidenced by expression of the auxin reporters and Upon auxin microapplication, both meristems and TIBA-pin apices activated expression with similar kinetics; however, only plants failed to concurrently initiate leaf primordia. We found that encodes the single tomato ortholog of () and () genes and is transiently expressed at incipient and young primordia, overlapping with auxin response maxima. is rapidly induced by auxin application, implying feed-forward activity between and auxin signals. However, driving at auxin response maxima sites using the promoter fails to fully rescue plants, suggesting that additional, auxin-independent regulation is needed. Indeed, extended GCC-box elements upstream of drove primordia-specific expression in a LFS-dependent but auxin-independent manner. We thus suggest that transiently acts at the site of primordia initiation, where it provides a specific context to auxin response maxima culminating in leaf primordia initiation.