The VIL gene CRAWLING ELEPHANT controls maturation and differentiation in tomato via polycomb silencing.

VERNALIZATION INSENSITIVE 3-LIKE (VIL) proteins are PHD-finger proteins that recruit the repressor complex Polycomb Repressive Complex 2 (PRC2) to the promoters of target genes. Most known VIL targets are flowering repressor genes. Here, we show that the tomato VIL gene CRAWLING ELEPHANT (CREL) promotes differentiation throughout plant development by facilitating the trimethylation of Histone H3 on lysine 27 (H3K27me3). We identified the crel mutant in a screen for suppressors of the simple-leaf phenotype of entire (e), a mutant in the AUX/IAA gene ENTIRE/SlIAA9, involved in compound-leaf development in tomato. crel mutants have increased leaf complexity, and suppress the ectopic blade growth of e mutants. In addition, crel mutants are late flowering, and have delayed and aberrant stem, root and flower development. Consistent with a role for CREL in recruiting PRC2, crel mutants show drastically reduced H3K27me3 enrichment at approximately half of the 14,789 sites enriched in wild-type plants, along with upregulation of many underlying genes. Interestingly, this reduction in H3K27me3 across the genome in crel is also associated with gains in H3K27me3 at a smaller number of sites that normally have modest levels of the mark in wild-type plants, suggesting that PRC2 activity is no longer limiting in the absence of CREL. Our results uncover a wide role for CREL in plant and organ differentiation in tomato and suggest that CREL is required for targeting PRC2 activity to, and thus silencing, a specific subset of polycomb targets.

Coordination of differentiation rate and local patterning in compound-leaf development.

The variability in leaf form in nature is immense. Leaf patterning occurs by differential growth, taking place during a limited window of morphogenetic activity at the leaf marginal meristem. While many regulators have been implicated in the designation of the morphogenetic window and in leaf patterning, how these effectors interact to generate a particular form is still not well understood. We investigated the interaction among different effectors of tomato (Solanum lycopersicum) compound-leaf development, using genetic and molecular analyses. Mutations in the tomato auxin response factor SlARF5/SlMP, which normally promotes leaflet formation, suppressed the increased leaf complexity of mutants with extended morphogenetic window. Impaired activity of the NAC/CUC transcription factor GOBLET (GOB), which specifies leaflet boundaries, also reduced leaf complexity in these backgrounds. Analysis of genetic interactions showed that the patterning factors SlMP, GOB and the MYB transcription factor LYRATE (LYR) coordinately regulate leaf patterning by modulating in parallel different aspects of leaflet formation and shaping. This work places an array of developmental regulators in a morphogenetic context. It reveals how organ-level differentiation rate and local growth are coordinated to sculpture an organ. These concepts are applicable to the coordination of pattering and differentiation in other species and developmental processes.

Characterization of the cytokinin sensor TCSv2 in arabidopsis and tomato.

BACKGROUND: Hormones are crucial to plant life and development. Being able to follow the plants hormonal response to various stimuli and throughout developmental processes is an important and increasingly widespread tool. The phytohormone cytokinin (CK) has crucial roles in the regulation of plant growth and development. RESULTS: Here we describe a version of the CK sensor Two Component signaling Sensor (TCS), referred to as TCSv2. TCSv2 has a different arrangement of binding motifs when compared to previous TCS versions, resulting in increased sensitivity in some examined tissues. Here, we examine the CK responsiveness and distribution pattern of TCSv2 in arabidopsis and tomato. CONCLUSIONS: The increased sensitivity and reported expression pattern of TCSv2 make it an ideal TCS version to study CK response in particular hosts, such as tomato, and particular tissues, such as leaves and flowers.

Multiple Auxin-Response Regulators Enable Stability and Variability in Leaf Development.

Auxin-signal transduction is mediated by the antagonistic activity of transcriptional activators and repressors. Both activators and repressors belong to gene families, but the biological importance of this complexity is not clear. Here, we addressed this question using tomato leaf development as a model by generating and analyzing mutants in multiple auxin-response components. In developing compound tomato leaves, auxin promotes leaflet formation and blade growth, and in the intercalary regions between leaflets, auxin response is inhibited by the Aux/IAA protein ENTIRE (E). e mutants form simple leaves due to ectopic blade growth in the intercalary domain. Using this unique loss-of-function phenotype and genome editing of auxin-response factor (ARF) genes, encoding auxin-response activators, we identified the contribution of specific ARFs to the e phenotype. Mutations in the related ARFs SlMP, SlARF19A, and SlARF19B, but not SlARF7, reduced the leaf blade and suppressed the e phenotype in a dosage-dependent manner that correlated with their relative expression, leading to a continuum of shapes. While single e and slmp mutants affected blade growth in an opposite manner, leaves of e slmp double mutants were similar to those of the wild type. However, the leaf shape of e slmp was more variable than that of the wild type, and it showed increased sensitivity to auxin. Our findings demonstrate that the existence of multiple auxin-response repressors and activators stabilizes the developmental output of auxin and that tuning their activity enables shape variability. The increased complexity of the auxin response therefore balances stability and flexibility in leaf patterning.

Hormones in tomato leaf development.

Leaf development serves as a model for plant developmental flexibility. Flexible balancing of morphogenesis and differentiation during leaf development results in a large diversity of leaf forms, both between different species and within the same species. This diversity is particularly evident in compound leaves. Hormones are prominent regulators of leaf development. Here we discuss some of the roles of plant hormones and the cross-talk between different hormones in tomato compound-leaf development.

A role for APETALA1/fruitfull transcription factors in tomato leaf development.

Flexible maturation rates underlie part of the diversity of leaf shape, and tomato (Solanum lycopersicum) leaves are compound due to prolonged organogenic activity of the leaf margin. The CINCINNATA-teosinte branched1, cycloidea, PCF (CIN-TCP) transcription factor lanceolate (LA) restricts this organogenic activity and promotes maturation. Here, we show that tomato APETALA1/fruitfull (AP1/FUL) MADS box genes are involved in tomato leaf development and are repressed by LA. AP1/FUL expression is correlated negatively with LA activity and positively with the organogenic activity of the leaf margin. LA binds to the promoters of the AP1/FUL genes MBP20 and TM4. Overexpression of MBP20 suppressed the simple-leaf phenotype resulting from upregulation of LA activity or from downregulation of class I knotted like homeobox (KNOXI) activity. Overexpression of a dominant-negative form of MBP20 led to leaf simplification and partly suppressed the increased leaf complexity of plants with reduced LA activity or increased KNOXI activity. Tomato plants overexpressing miR319, a negative regulator of several CIN-TCP genes including LA, flower with fewer leaves via an SFT-dependent pathway, suggesting that miR319-sensitive CIN-TCPs delay flowering in tomato. These results identify a role for AP1/FUL genes in vegetative development and show that leaf and plant maturation are regulated via partially independent mechanisms.

ENTIRE and GOBLET promote leaflet development in tomato by modulating auxin response.

Compound leaves produce leaflets in a highly controlled yet flexible pattern. Here, we investigate the interaction between auxin, the putative auxin response inhibitor ENTIRE (E, SlIAA9) and the CUC transcription factor GOBLET (GOB) in compound-leaf development in tomato (Solanum lycopersicum). Auxin maxima, monitored by the auxin response sensor DR5, marked and preceded leaflet and lobe initiation. The DR5 signal increased, but maxima were partially retained in response to the external or internal elevation of auxin levels. E directly interacted with the auxin receptors SlTIR1 and SlAFB6. Furthermore, E was stabilized by a mutation in domain II of the protein and by the inhibition of auxin or proteasome activity, implying that E is subjected to auxin-mediated degradation. In e mutants the DR5 signal expanded to include the complete leaf margin, and leaf-specific overexpression of a stabilized form of E inhibited the DR5 signal and lamina expansion. Genetic manipulation of GOB activity altered the distribution of the DR5 signal, and the inhibition of auxin transport or activity suppressed the GOB overexpression phenotype, suggesting that auxin mediates GOB-regulated leaf patterning. Whereas leaves of single e or gob mutants developed only primary leaflets, the downregulation of both E and GOB resulted in the complete abolishment of leaflet initiation, and in a strong DR5 signal throughout the leaf margin. These results suggest that E and GOB modulate auxin response and leaflet morphogenesis via partly redundant pathways, and that proper leaflet initiation and separation requires distinct boundaries between regions of lamina growth and adjacent regions in which growth is inhibited.