SlTIR1 is involved in crosstalk of phytohormones, regulates auxin-induced root growth and stimulates stenospermocarpic fruit formation in tomato

A GARP transcription factor SlGCC positively regulates lateral root development in tomato via auxin-ethylene interplay

MAIN CONCLUSION

SlGCC, a GARP transcription factor, functions as a root-related transcriptional repressor. SlGCC synchronizes auxin and ethylene signaling involving SlPIN3 and SlIAA3 as intermediate targets sketching a molecular map for lateral root development in tomato. The root system is crucial for growth and development of plants as it performs basic functions such as providing mechanical support, nutrients and water uptake, pathogen resistance and responds to various stresses. SlGCC, a GARP family transcription factor (TF), exhibited predominant expression in age-dependent (initial to mature stages) tomato root. SlGCC is a transcriptional repressor and is regulated at a transcriptional and translational level by auxin and ethylene. Auxin and ethylene mediated SlGCC protein stability is governed via proteasome degradation pathway during lateral root (LR) growth development. SlGCC over-expressor (OE) and under-expressed (UE) tomato transgenic lines demonstrate its role in LR development. This study is an attempt to unravel the vital role of SlGCC in regulating tomato LR architecture.

SlEAD1, an EAR motif-containing ABA down-regulated novel transcription repressor regulates ABA response in tomato

EAR motif-containing proteins are able to repress gene expression, therefore play important roles in regulating plants growth and development, plant response to environmental stimuli, as well as plant hormone signal transduction. ABA is a plant hormone that regulates abiotic stress tolerance in plants via signal transduction. ABA signaling via the PYR1/PYLs/RCARs receptors, the PP2Cs phosphatases, and SnRK2s protein kinases activates the ABF/AREB/ABI5-type bZIP transcription factors, resulting in the activation/repression of ABA response genes. However, functions of many ABA response genes remained largely unknown. We report here the identification of the ABA-responsive gene SlEAD1 (Solanum lycopersicum EAR motif-containing ABA down-regulated 1) as a novel EAR motif-containing transcription repressor gene in tomato. We found that the expression of SlEAD1 was down-regulated by ABA treatment, and SlEAD1 repressed reporter gene expression in transfected protoplasts. By using CRISPR gene editing, we generated transgene-free slead1 mutants and found that the mutants produced short roots. By using seed germination and root elongation assays, we examined ABA response of the slead1 mutants and found that ABA sensitivity in the mutants was increased. By using qRT-PCR, we further show that the expression of some of the ABA biosynthesis and signaling component genes were increased in the slead1 mutants. Taken together, our results suggest that SlEAD1 is an ABA response gene, that SlEAD1 is a novel EAR motif-containing transcription repressor, and that SlEAD1 negatively regulates ABA responses in tomato possibly by repressing the expression of some ABA biosynthesis and signaling genes.

Tomato Fruit Development and Metabolism

Tomato (Solanum lycopersicum L.) belongs to the Solanaceae family and is the second most important fruit or vegetable crop next to potato (Solanum tuberosum L.). It is cultivated for fresh fruit and processed products. Tomatoes contain many health-promoting compounds including vitamins, carotenoids, and phenolic compounds. In addition to its economic and nutritional importance, tomatoes have become the model for the study of fleshy fruit development. Tomato is a climacteric fruit and dramatic metabolic changes occur during its fruit development. In this review, we provide an overview of our current understanding of tomato fruit metabolism. We begin by detailing the genetic and hormonal control of fruit development and ripening, after which we document the primary metabolism of tomato fruits, with a special focus on sugar, organic acid, and amino acid metabolism. Links between primary and secondary metabolic pathways are further highlighted by the importance of pigments, flavonoids, and volatiles for tomato fruit quality. Finally, as tomato plants are sensitive to several abiotic stresses, we briefly summarize the effects of adverse environmental conditions on tomato fruit metabolism and quality.

Tomato AUXIN RESPONSE FACTOR 5 regulates fruit set and development via the mediation of auxin and gibberellin signaling

Auxin response factors (ARFs) encode transcriptional factors that function in the regulation of plant development processes. A tomato ARF gene, SlARF5, was observed to be expressed at high levels in emasculated ovaries but maintained low expression levels in pollinated ovaries. The amiRNA SlARF5 lines exhibited ovary growth and formed seedless fruits following emasculation. These parthenocarpic fruits developed fewer locular tissues, and the fruit size and weight were decreased in transgenic lines compared to those of wild-type fruits. Gene expression analysis demonstrated that several genes involved in the auxin-signaling pathway were downregulated, whereas some genes involved in the gibberellin-signaling pathway were enhanced by the decreased SlARF5 mRNA levels in transgenic plants, indicating that SlARF5 may play an important role in regulating both the auxin- and gibberellin-signaling pathways during fruit set and development.

Exogenous auxin represses soybean seed germination through decreasing the gibberellin/abscisic acid (GA/ABA) ratio

Auxin is an important phytohormone which mediates diverse development processes in plants. Published research has demonstrated that auxin induces seed dormancy. However, the precise mechanisms underlying the effect of auxin on seed germination need further investigation, especially the relationship between auxins and both abscisic acid (ABA) and gibberellins (GAs), the latter two phytohormones being the key regulators of seed germination. Here we report that exogenous auxin treatment represses soybean seed germination by enhancing ABA biosynthesis, while impairing GA biogenesis, and finally decreasing GA1/ABA and GA4/ABA ratios. Microscope observation showed that auxin treatment delayed rupture of the soybean seed coat and radicle protrusion. qPCR assay revealed that transcription of the genes involved in ABA biosynthetic pathway was up-regulated by application of auxin, while expression of genes involved in GA biosynthetic pathway was down-regulated. Accordingly, further phytohormone quantification shows that auxin significantly increased ABA content, whereas the active GA1 and GA4 levels were decreased, resulting insignificant decreases in the ratiosGA1/ABA and GA4/ABA.Consistent with this, ABA biosynthesis inhibitor fluridone reversed the delayed-germination phenotype associated with auxin treatment, while paclobutrazol, a GA biosynthesis inhibitor, inhibited soybean seed germination. Altogether, exogenous auxin represses soybean seed germination by mediating ABA and GA biosynthesis.

New insights into the roles of cucumber TIR1 homologs and miR393 in regulating fruit/seed set development and leaf morphogenesis

BACKGROUND

TIR1-like proteins act as auxin receptors and play essential roles in auxin-mediated plant development processes. The number of auxin receptor family members varies among species. While the functions of auxin receptor genes have been widely studied in Arabidopsis, the distinct functions of cucumber (Cucumis sativus L.) auxin receptors remains poorly understood. To further our understanding of their potential role in cucumber development, two TIR1-like genes were identified and designated CsTIR1 and CsAFB2. In the present study, tomato (Sonanum lycopersicum) was used as a model to investigate the phenotypic and molecular changes associated with the overexpression of CsTIR1 and CsAFB2.

RESULTS

Differences in the subcellular localizations of CsTIR1 and CsAFB2 were identified and both genes were actively expressed in leaf, female flower and young fruit tissues of cucumber. Moreover, CsTIR1- and CsAFB2-overexpressing lines exhibited pleotropic phenotypes ranging from leaf abnormalities to seed germination and parthenocarpic fruit compared with the wild-type plants. To further elucidate the regulation of CsTIR1 and CsAFB2, the role of the miR393/TIR1 module in regulating cucumber fruit set were investigated. Activation of miR393-mediated mRNA cleavage of CsTIR1 and CsAFB2 was revealed by qPCR and semi-qPCR, which highlighted the critical role of the miR393/TIR1 module in mediating fruit set development in cucumber.

CONCLUSION

Our results provide new insights into the involvement of CsTIR1 and CsAFB2 in regulating various phenotype alterations, and suggest that post-transcriptional regulation of CsTIR1 and CsAFB2 mediated by miR393 is essential for cucumber fruit set initiation. Collectively, these results further clarify the roles of cucumber TIR1 homologs and miR393 in regulating fruit/seed set development and leaf morphogenesis.