Introgression of a dominant phototropin1 mutant enhances carotenoids and boosts flavour-related volatiles in genome-edited tomato RIN mutants

The tomato (Solanum lycopersicum) ripening inhibitor (rin) mutation is known to completely repress fruit ripening. The heterozygous (RIN/rin) fruits have extended shelf life, ripen normally, but have inferior taste/flavour. To address this, we used genome editing to generate newer alleles of RIN (rinCR ) by targeting the K-domain. Unlike previously reported CRISPR alleles, the rinCR alleles displayed delayed onset of ripening, suggesting that the mutated K-domain represses the onset of ripening. The rinCR fruits had extended shelf life and accumulated carotenoids at an intermediate level between rin and progenitor line. Besides, the metabolites and hormonal levels in rinCR fruits were more akin to rin. To overcome the negative attributes of rin, we crossed the rinCR alleles with Nps1, a dominant-negative phototropin1 mutant, which enhances carotenoid levels in tomato fruits. The resulting Nps1/rinCR hybrids had extended shelf life and 4.4-7.1-fold higher carotenoid levels than the wild-type parent. The metabolome of Nps1/rinCR fruits revealed higher sucrose, malate, and volatiles associated with tomato taste and flavour. Notably, the boosted volatiles in Nps1/rinCR were only observed in fruits bearing the homozygous Nps1 mutation. The Nps1 introgression into tomato provides a promising strategy for developing cultivars with extended shelf life, improved taste, and flavour.

A Comprehensive Protocol for Assembly of Multiple gRNAs into a Direct Vector for Genome Editing in Tomato

The Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-Cas 9 (CRISPR-associated protein 9) is a robust DNA-encoded, RNA-mediated sequence-specific nuclease system widely used for genome editing of various plants. Although there are many reports on the assembly of gRNAs and plant transformation, there is no single resource for the complete gene editing methodology in tomato. This chapter provides a comprehensive protocol for designing gRNAs, their assembly into the vector, plant transformation, and final mutant analysis in tomato.

The new kid on the block: a dominant-negative mutation of phototropin1 enhances carotenoid content in tomato fruits

Phototropins, the UVA-blue light photoreceptors, endow plants to detect the direction of light and optimize photosynthesis by regulating positioning of chloroplasts and stomatal gas exchange. Little is known about their functions in other developmental responses. A tomato Non-phototropic seedling1 (Nps1) mutant, bearing an Arg495His substitution in the vicinity of LOV2 domain in phototropin1, dominant-negatively blocks phototropin1 responses. The fruits of Nps1 mutant were enriched in carotenoids, particularly lycopene, compared with its parent, Ailsa Craig. On the contrary, CRISPR/CAS9-edited loss of function phototropin1 mutants displayed subdued carotenoids compared with the parent. The enrichment of carotenoids in Nps1 fruits is genetically linked with the mutation and exerted in a dominant-negative fashion. Nps1 also altered volatile profiles with high levels of lycopene-derived 6-methyl 5-hepten2-one. The transcript levels of several MEP and carotenogenesis pathway genes were upregulated in Nps1. Nps1 fruits showed altered hormonal profiles with subdued ethylene emission and reduced respiration. Proteome profiles showed a causal link between higher carotenogenesis and increased levels of protein protection machinery, which may stabilize proteins contributing to MEP and carotenogenesis pathways. The enhancement of carotenoid content by Nps1 in a dominant-negative fashion offers a potential tool for high lycopene-bearing hybrid tomatoes.

Xanthomonas campestris cell-cell signalling molecule DSF (diffusible signal factor) elicits innate immunity in plants and is suppressed by the exopolysaccharide xanthan

Several secreted and surface-associated conserved microbial molecules are recognized by the host to mount the defence response. One such evolutionarily well-conserved bacterial process is the production of cell-cell signalling molecules which regulate production of multiple virulence functions by a process known as quorum sensing. Here it is shown that a bacterial fatty acid cell-cell signalling molecule, DSF (diffusible signal factor), elicits innate immunity in plants. The DSF family of signalling molecules are highly conserved among many phytopathogenic bacteria belonging to the genus Xanthomonas as well as in opportunistic animal pathogens. Using Arabidopsis, Nicotiana benthamiana, and rice as model systems, it is shown that DSF induces a hypersensitivity reaction (HR)-like response, programmed cell death, the accumulation of autofluorescent compounds, hydrogen peroxide production, and the expression of the PATHOGENESIS-RELATED1 (PR-1) gene. Furthermore, production of the DSF signalling molecule in Pseudomonas syringae, a non-DSF-producing plant pathogen, induces the innate immune response in the N. benthamiana host plant and also affects pathogen growth. By pre- and co-inoculation of DSF, it was demonstrated that the DSF-induced plant defence reduces disease severity and pathogen growth in the host plant. In this study, it was further demonstrated that wild-type Xanthomonas campestris suppresses the DSF-induced innate immunity by secreting xanthan, the main component of extracellular polysaccharide. The results indicate that plants have evolved to recognize a widely conserved bacterial communication system and may have played a role in the co-evolution of host recognition of the pathogen and the communication machinery.

microRNA160 dictates stage-specific auxin and cytokinin sensitivities and directs soybean nodule development

Legume nodules result from coordinated interactions between the plant and nitrogen-fixing rhizobia. The phytohormone cytokinin promotes nodule formation, and recent findings suggest that the phytohormone auxin inhibits nodule formation. Here we show that microRNA160 (miR160) is a key signaling element that determines the auxin/cytokinin balance during nodule development in soybean (Glycine max). miR160 appears to promote auxin activity by suppressing the levels of the ARF10/16/17 family of repressor ARF transcription factors. Using quantitative PCR assays and a fluorescence miRNA sensor, we show that miR160 levels are relatively low early during nodule formation and high in mature nodules. We had previously shown that ectopic expression of miR160 in soybean roots led to a severe reduction in nodule formation, coupled with enhanced sensitivity to auxin and reduced sensitivity to cytokinin. Here we show that exogenous cytokinin restores nodule formation in miR160 over-expressing roots. Therefore, low miR160 levels early during nodule development favor cytokinin activity required for nodule formation. Suppression of miR160 levels using a short tandem target mimic (STTM160) resulted in reduced sensitivity to auxin and enhanced sensitivity to cytokinin. In contrast to miR160 over-expressing roots, STTM160 roots had increased nodule formation, but nodule maturation was significantly delayed. Exogenous auxin partially restored proper nodule formation and maturation in STTM160 roots, suggesting that high miR160 activity later during nodule development favors auxin activity and promotes nodule maturation. Therefore, miR160 dictates developmental stage-specific sensitivities to auxin and cytokinin to direct proper nodule formation and maturation in soybean.

Ectopic expression of miR160 results in auxin hypersensitivity, cytokinin hyposensitivity, and inhibition of symbiotic nodule development in soybean

Symbiotic root nodules in leguminous plants result from interaction between the plant and nitrogen-fixing rhizobia bacteria. There are two major types of legume nodules, determinate and indeterminate. Determinate nodules do not have a persistent meristem, while indeterminate nodules have a persistent meristem. Auxin is thought to play a role in the development of both these types of nodules. However, inhibition of rootward auxin transport at the site of nodule initiation is crucial for the development of indeterminate nodules but not determinate nodules. Using the synthetic auxin-responsive DR5 promoter in soybean (Glycine max), we show that there is relatively low auxin activity during determinate nodule initiation and that it is restricted to the nodule periphery subsequently during development. To examine if and what role auxin plays in determinate nodule development, we generated soybean composite plants with altered sensitivity to auxin. We overexpressed microRNA393 to silence the auxin receptor gene family, and these roots were hyposensitive to auxin. These roots nodulated normally, suggesting that only minimal/reduced auxin signaling is required for determinate nodule development. We overexpressed microRNA160 to silence a set of repressor auxin response factor transcription factors, and these roots were hypersensitive to auxin. These roots were not impaired in epidermal responses to rhizobia but had significantly reduced nodule primordium formation, suggesting that auxin hypersensitivity inhibits nodule development. These roots were also hyposensitive to cytokinin and had attenuated expression of key nodulation-associated transcription factors known to be regulated by cytokinin. We propose a regulatory feedback loop involving auxin and cytokinin during nodulation.

Intron hairpin and transitive RNAi mediated silencing of orfH522 transcripts restores male fertility in transgenic male sterile tobacco plants expressing orfH522

The present work was aimed at developing vector construct(s) suitable for restoring fertility in transgenic male sterile tobacco plants expressing male-sterility-inducing ORFH522 in tapetal cell layer (Nizampatnam et al. Planta 229:987-1001, 2009). PTGS vectors that could produce either intron spliced hairpin RNA against the orfH522 or induce silencing of orfH522 by heterologous 3'UTR region were developed using the selected 316 bp (orf316) fragment of orfH522. The constructs were independently mobilized into Agrobacterium and used for transforming tobacco. The T(1) generation plants carrying the restorer gene cassettes in homozygous condition were identified and crossed with the male sterile transgenic tobacco plants to obtain the hybrid seeds. PCR analysis of hybrid plants indicated segregation for the sterility inducing cassette while all the plants carried the restorer cassette. Hybrid plants produced fertile pollen grains and formed normal capsules upon selfing. Further molecular analyses of these hybrid plants with RT-PCR, Northern blotting and siRNA detection, revealed that intron interrupted hairpin RNA (ihp-RNA) mediated gene silencing was more effective compared to silencing by heterologous 3'UTR (SHUTR) as indicated by the complete degradation of orfH522 transcripts and formation of higher levels of orf316 specific siRNA molecules in plants carrying ihp-RNA restorer construct. Segregation analyses of F(2) (selfed hybrid) plants confirmed the co-segregation of gene cassettes and the traits in Mendelian di-hybrid ratio (9:3:3:1). Taken together, the results established that intron hairpin and transitive RNAi mediated silencing of orfH522 transcripts restored fertility in transgenic male sterile tobacco plants expressing orfH522 and ihp-RNA was more efficient in silencing orfH522 transcripts.

Expression of sunflower cytoplasmic male sterility-associated open reading frame, orfH522 induces male sterility in transgenic tobacco plants

Sterility in the universally exploited PET1-CMS system of sunflower is associated with the expression of orfH522, a novel mitochondrial gene. Definitive evidence that ORFH522 is directly responsible for male sterility is lacking. To test the hypothesis that ORFH522 is sufficient to induce male sterility, a set of chimeric constructs were developed. The cDNA of orfH522 was cloned in-frame with yeast coxIV pre-sequence, and was expressed under tapetum-specific promoter TA29 (construct designated as TCON). For developing control vectors, orfH522 was cloned without the transit peptide under TA29 promoter (TON) or orfH522 was cloned with or without transit peptide under the constitutive CaMV35S promoter (SCOP and SOP). Among several independent transformants obtained with each of the gene cassettes, one third of the transgenics (6/17) with TCON were completely male sterile while more than 10 independent transformants obtained with each of the control vectors were fertile. The male sterile plants were morphologically similar to fertile plants, but had anthers that remained below the stigmatic surface at anthesis. RT-PCR analysis of the sterile plants confirmed the anther-specific expression of orfH522 and bright-field microscopy demonstrated ablation of the tapetal cell layer. Premature DNA fragmentation and programmed cell death was observed at meiosis stage in the anthers of sterile plants. Stable transmission of induced male sterility trait was confirmed in test cross progeny. This constitutes the first report at demonstrating the induction of male sterility by introducing orfH522 gene that could be useful for genetic engineering of male sterility.