Coordination of auxin-triggered leaf initiation by tomato <i>LEAFLESS</i>

Abstract

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 <i>leafless</i> (<i>lfs</i>) 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 <i>pPIN1:PIN1:GFP</i> and <i>DR5:YFP</i> Upon auxin microapplication, both <i>lfs</i> meristems and TIBA-pin apices activated <i>DR5:YFP</i> expression with similar kinetics; however, only <i>lfs</i> plants failed to concurrently initiate leaf primordia. We found that <i>LFS</i> encodes the single tomato ortholog of <i>Arabidopsis DORNRONSCHEN</i> (<i>DRN</i>) and <i>DRN-like</i> (<i>DRNL</i>) genes and is transiently expressed at incipient and young primordia, overlapping with auxin response maxima. <i>LFS</i> is rapidly induced by auxin application, implying feed-forward activity between <i>LFS</i> and auxin signals. However, driving <i>LFS</i> at auxin response maxima sites using the <i>DR5</i> promoter fails to fully rescue <i>lfs</i> plants, suggesting that additional, auxin-independent regulation is needed. Indeed, extended GCC-box elements upstream of <i>LFS</i> drove primordia-specific expression in a LFS-dependent but auxin-independent manner. We thus suggest that <i>LFS</i> transiently acts at the site of primordia initiation, where it provides a specific context to auxin response maxima culminating in leaf primordia initiation.