Coordination of auxin-triggered leaf initiation by tomato LEAFLESS
Friday, 2017/03/24 | 08:01:25
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SignificancePlant leaves form at the flanks of the shoot apical meristem in response to cues provided by the phytohormone auxin. Auxin signals determine the sites of leaf initiation and bulging, a process followed by gradual and ongoing differentiation of leaf tissues. We show here that the tomato ethylene response factor-type transcription factor LEAFLESS is induced by, and necessary for, auxin-triggered leaf initiation. These mechanisms provide a localized and transient developmental context for a specific morphogenetic output generated by general regulators such as auxin. AbstractLateral 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 leafless (lfs) 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 pPIN1:PIN1:GFP and DR5:YFP. Upon auxin microapplication, both lfs meristems and TIBA-pin apices activated DR5:YFP expression with similar kinetics; however, only lfs plants failed to concurrently initiate leaf primordia. We found that LFS encodes the single tomato ortholog of Arabidopsis DORNRONSCHEN (DRN) and DRN-like (DRNL) genes and is transiently expressed at incipient and young primordia, overlapping with auxin response maxima. LFS is rapidly induced by auxin application, implying feed-forward activity between LFS and auxin signals. However, driving LFS at auxin response maxima sites using the DR5 promoter fails to fully rescue lfs plants, suggesting that additional, auxin-independent regulation is needed. Indeed, extended GCC-box elements upstream of LFS drove primordia-specific expression in a LFS-dependent but auxin-independent manner. We thus suggest that LFS transiently acts at the site of primordia initiation, where it provides a specific context to auxin response maxima culminating in leaf primordia initiation.
See http://www.pnas.org/content/114/12/3246.abstract.html?etoc PNAS March 21 2017; vol.114; no.12: 3246–3251
Fig. 1. LFS and auxin response maxima direct tomato leaf initiation. (A) A 5.5-wk-old WT (Left) and lfs (Bottom Right) tomato. (Scale bar: 5 cm.) (Inset) A close-up of the apex of a 15-d-old lfs seedling. as, axillary shoot; c, collar-like structure; h, hypocotyl. (B) A 9-d-old lfs seedling in which the SAM starts to protrude from the collar-like structure surrounding it. (C and D) SEM images of vegetative WT (C) and lfs (D) apices. (E) WT apical meristem bearing one incipient and one visible primordia (P0 and P1, respectively). (F) lfs meristem. (G) TIBA-induced pin. Dashed lines in E–G mark the apex tissue, sampled for gene expression analyses. Note the proximity of trichomes to the SAM in D and F, marked by the arrowhead. (H–J) Three-dimensional reconstructions of live TCS:YFP apices captured by a light sheet microscope. (H) WT. (I) lfs. (J) TIBA pin. (Scale bars in C–G: 100 µm.) |
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