Blossom-end rot: a century-old problem in tomato (Solanum lycopersicum L.) and other vegetables

Blossom-end rot (BER) is a devastating physiological disorder affecting vegetable production worldwide. Extensive research into the physiological aspects of the disorder has demonstrated that the underlying causes of BER are associated with perturbed calcium (Ca2+) homeostasis and irregular watering conditions in predominantly cultivated accessions. Further, Reactive Oxygen Species (ROS) are critical players in BER development which, combined with unbalanced Ca2+ concentrations, greatly affect the severity of the disorder. The availability of a high-quality reference tomato genome as well as the whole genome resequencing of many accessions has recently permitted the genetic dissection of BER in segregating populations derived from crosses between cultivated tomato accessions. This has led to the identification of five loci contributing to BER from several studies. The eventual cloning of the genes contributing to BER would result in a deeper understanding of the molecular bases of the disorder. This will undoubtedly create crop improvement strategies for tomato as well as many other vegetables that suffer from BER.

Genome-wide identification and analyses of tomato (Solanum lycopersicum L.) high-affinity nitrate transporter 2 (NRT2) family genes and their responses to drought and salinity

High-affinity nitrate transporter 2 (NRT2) proteins have vital roles in nitrate (NO3-) uptake and translocation in plants. The gene families coding NRT2 proteins have been identified and functionally characterized in many plant species. However, no systematic identification of NRT2 family members have been reported in tomato (Solanum lycopersicum). There is also little known about their expression profiles under environmental stresses. Accordingly, the present study aimed to identify NRT2 gene family in the tomato genome; then, investigate them in detail through bioinformatics, physiological and expression analyses. As a result, four novel NRT2 genes were identified in the tomato genome, all of which contain the same domain belonging to the Major Facilitator Superfamily (PF07690). The co-expression network of SlNRT genes revealed that they were co-expressed with several other genes in many different molecular pathways including transport, photosynthesis, fatty acid metabolism and amino acid catabolism. Programming many crucial physiological and metabolic pathways, various numbers of phosphorylation sites were predicted in the NRT2 proteins.

A high-continuity and annotated tomato reference genome

Background Genetic and functional genomics studies require a high-quality genome assembly. Tomato (Solanum lycopersicum), an important horticultural crop, is an ideal model species for the study of fruit development. Results Here, we assembled an updated reference genome of S. lycopersicum cv. Heinz 1706 that was 799.09 Mb in length, containing 34,384 predicted protein-coding genes and 65.66% repetitive sequences. By comparing the genomes of S. lycopersicum and S. pimpinellifolium LA2093, we found a large number of genomic fragments probably associated with human selection, which may have had crucial roles in the domestication of tomato. We also used a recombinant inbred line (RIL) population to generate a high-density genetic map with high resolution and accuracy. Using these resources, we identified a number of candidate genes that were likely to be related to important agronomic traits in tomato. Conclusion Our results offer opportunities for understanding the evolution of the tomato genome and will facilitate the study of genetic mechanisms in tomato biology.

Genome-wide survey and expression analysis of NIN-like Protein (NLP) genes reveals its potential roles in the response to nitrate signaling in tomato

Background Tomato (Solanum lycopersicum) is one of the most important horticultural crops, with a marked preference for nitrate as an inorganic nitrogen source. The molecular mechanisms of nitrate uptake and assimilation are poorly understood in tomato. NIN-like proteins (NLPs) are conserved, plant-specific transcription factors that play crucial roles in nitrate signaling. Results In this study, genome-wide analysis identified six NLP members in tomato genome. These members were clustered into three clades in a phylogenetic tree. Comparative genomic analysis showed that SlNLP genes exhibited collinear relationships to NLPs in Arabidopsis, canola, maize and rice, and that the expansion of the SlNLP family mainly resulted from segmental duplications in the tomato genome. Tissue-specific expression analysis showed that one of the close homologs of AtNLP6/7, SlNLP3, was strongly expressed in roots during both the seedling and flowering stages, that SlNLP4 and SlNLP6 exhibited preferential expression in stems and leaves and that SlNLP6 was expressed at high levels in fruits. Furthermore, the nitrate uptake in tomato roots and the expression patterns of SlNLP genes were measured under nitrogen deficiency and nitrate resupply conditions. Four SlNLPs, SlNLP1, SlNLP2, SlNLP4 and SlNLP6, were upregulated after nitrogen starvation. And SlNLP1 and SlNLP5 were induced rapidly and temporally by nitrate. Conclusions These results provide significant insights into the potential diverse functions of SlNLPs to regulate nitrate uptake.

A High-continuity and Annotated Tomato Reference Genome

Background: Genetic and functional genomics studies require a high-quality genome assembly. Tomato (Solanum lycopersicum), an important horticultural crop, is an ideal model species for the study of fruit development. Results: Here, we assembled an updated reference genome of S. lycopersicum cv. Heinz 1706 that was 799.09 Mb in length, containing 34,384 predicted protein-coding genes and 65.66% repetitive sequences. By comparing the genomes of S. lycopersicum and S. pimpinellifolium LA2093, we found a large number of genomic fragments probably associated with human selection, which may have had crucial roles in the domestication of tomato. We also used a recombinant inbred line (RIL) population to generate a high-density genetic map with high resolution and accuracy. Using these resources, we identified a number of candidate genes that were likely to be related to important agronomic traits in tomato. Conclusion:Our results offer opportunities for understanding the evolution of the tomato genome and will facilitate the study of genetic mechanisms in tomato biology.

Title: Genome-Wide Survey and Expression Analysis of NIN-Like Protein (NLP) Genes Reveals Its Potential Roles in the Response to Nutrition Deficiency in Tomato

Background: Tomato (Solanum lycopersicum) is one of the most important horticultural crops, with a marked preference of nitrate as inorganic nitrogen source. The molecular mechanisms of nitrate uptake and assimilation are poorly understood in tomato. NIN-Like Proteins (NLPs) are conserved, plant-specific transcription factors that play crucial roles in nitrate signaling. Results: In this study, genome-wide analysis revealed six NLP members in tomato genome. They were clustered into three clades in a phylogenic tree. Comparative genomic analysis showed that SlNLP genes had collinear relationships to NLPs in Arabidopsis, canola, maize and rice, and that the expansion of the SlNLP family mainly resulted from segmental duplications in tomato genome. Tissue-specific expression analysis showed that the close homologues of AtNLP6/7, SlNLP3, was strongly expressed in roots during both seedling and flowering stages; SlNLP4 and SlNLP6 exhibited preferential expression in stems and leaves; and SlNLP6 were expressed in high levels in fruits. Further, the nitrate uptake in tomato roots and expression patterns of SlNLP genes were measured under nitrogen/phosphate/potassium deficiency and nitrate resupply conditions. The transcript abundance of SlNLP3 decreased to 70% under phosphate/potassium deficiency. Most of SlNLPs were up-regulated after nitrogen starvation. SlNLP1 and SlNLP5 were induced rapidly and temporally by nitrate. Conclusions: These results provided significant insights into the potential diverse functions of SlNLPs to regulate nitrate uptake.

Genome-Wide Identification and Molecular Characterization of the Growth-Regulating Factors-Interacting Factor Gene Family in Tomato

Growth-regulating factors-interacting factor (GIF) proteins play crucial roles in the regulation of plant growth and development. However, the molecular mechanism of GIF proteins in tomato is poorly understood. Here, four SlGIF genes (named SlGRF1a, SlGIF1b, SlGIF2, and SlGIF3) were identified from the tomato genome and clustered into two major clades by phylogenetic analysis. The gene structure and motif pattern analyses showed similar exon/intron patterns and motif organizations in all the SlGIFs. We identified 33 cis-acting regulatory elements (CAREs) in the promoter regions of the SlGIFs. The expression profiling revealed the four GIFs are expressed in various tissues and stages of fruit development and induced by phytohormones (IAA and GA). The subcellular localization assays showed all four GIFs were located in nucleus. The yeast two-hybrid assay indicated various growth-regulating factors (SlGRFs) proteins interacted with the four SlGIF proteins. However, SlGRF4 was a common interactor with the SlGIF proteins. Moreover, a higher co-expression relationship was shown between three SlGIF genes and five SlGRF genes. The protein association network analysis found a chromodomain helicase DNA-binding protein (CHD) and an actin-like protein to be associated with the four SlGIF proteins. Overall, these results will improve our understanding of the potential functions of GIF genes and act as a base for further functional studies on GIFs in tomato growth and development.

Light and ripening-regulated BBX protein-encoding genes in Solanum lycopersicum

Light controls several aspects of plant development through a complex signalling cascade. Several B-box domain containing proteins (BBX) were identified as regulators of Arabidopsis thaliana seedling photomorphogenesis. However, the knowledge about the role of this protein family in other physiological processes and species remains scarce. To fill this gap, here BBX protein encoding genes in tomato genome were characterised. The robust phylogeny obtained revealed how the domain diversity in this protein family evolved in Viridiplantae and allowed the precise identification of 31 tomato SlBBX proteins. The mRNA profiling in different organs revealed that SlBBX genes are regulated by light and their transcripts accumulation is directly affected by the chloroplast maturation status in both vegetative and fruit tissues. As tomato fruits develops, three SlBBXs were found to be upregulated in the early stages, controlled by the proper chloroplast differentiation and by the PHYTOCHROME (PHY)-dependent light perception. Upon ripening, other three SlBBXs were transcriptionally induced by RIPENING INHIBITOR master transcriptional factor, as well as by PHY-mediated signalling and proper plastid biogenesis. Altogether, the results obtained revealed a conserved role of SlBBX gene family in the light signalling cascade and identified putative members affecting tomato fruit development and ripening.

Transcriptome Analysis Combined With Glutathione S-transferase 1 Gene Family Whole-genome Identification Reveals the Mechanism Underlying the Exogenous Application of 5-aminolevulinic Acid to Alleviate Low Temperature Stress in Solanum Lycopersicum Seedling

Background: Transcriptome sequencing was conducted to screen out genes that actively respond to exogenous 5-aminolevulinic acid (ALA) induction under low temperature stress. The study used two versions of the tomato genome database to strictly screen and identify tomato glutathione S-transferase (GST) gene families and carried out the related bioinformatics analysis of tomato GST gene family. The expression pattern of SlGST genes induced by exogenous application ALA under low temperature stress was also analysed. Related physiological indicators were determined, and related chemical stains were performed.Results: RNA sequencing (RNA-seq) results showed that the expression of SlGST gene was different under various treatments, and a large number of SlGST genes widely responded to ALA induction under low temperature stress. Sixty-nine full-length GST genes were identified by screening the two versions of tomato genome databases combined with protein domain analysis. Analysis of gene family phylogenetic tree divided the tomato GST gene family into eight subfamilies. Tandem replication of genes is one of the driving forces for the evolution of tomato GST gene family, and a large number of cis-acting elements are related to stress resistance on the promoter of the GST gene family. Exogenous ALA application under low temperature stress induces a broad response of tomato leaf SlGST gene (qRT-PCR verification), increases GST activity and decreases reactive oxygen species (ROS) accumulation.Conclusions: RNA sequencing results revealed that a large number of tomato GST genes are differentially expressed, and Sixty-nine GSTs are identified in the tomato genome. Tandem replication of genes is the driving force for 68 the evolution of tomato GST family, and the promoter contains a large number of cis-acting elements related to stress resistance. Test results show that exogenous ALA induces the expression of SlGST genes under low temperature stress, thereby increasing GST activity to eliminate the ROS produced under low temperature stress and increase the tomato tolerance.