Gr and hp-1 tomato mutants unveil unprecedented interactions between arbuscular mycorrhizal symbiosis and fruit ripening

Genetic and functional traits limit the success of colonisation by arbuscular mycorrhizal fungi in a tomato wild relative

To understand whether domestication had an impact on susceptibility and responsiveness to arbuscular mycorrhizal fungi (AMF) in tomato (Solanum lycopersicum), we investigated two tomato cultivars ("M82" and "Moneymaker") and a panel of wild relatives including S. neorickii, S. habrochaites and S. pennellii encompassing the whole Lycopersicon clade. Most genotypes revealed good AM colonisation levels when inoculated with the AMF Funneliformis mosseae. By contrast, both S. pennellii accessions analysed showed a very low colonisation, but with normal arbuscule morphology, and a negative response in terms of root and shoot biomass. This behaviour was independent of fungal identity and environmental conditions. Genomic and transcriptomic analyses revealed in S. pennellii the lack of genes identified within QTLs for AM colonisation, a limited transcriptional reprogramming upon mycorrhization and a differential regulation of strigolactones and AM-related genes compared to tomato. Donor plants experiments indicated that the AMF could represent a cost for S. pennellii: F. mosseae could extensively colonise the root only when it was part of a mycorrhizal network, but a higher mycorrhization led to a higher inhibition of plant growth. These results suggest that genetics and functional traits of S. pennellii are responsible for the limited extent of AMF colonisation.

Physicochemical Properties and Antioxidant Activity of CRISPR/Cas9-Edited Tomato SGR1 Knockout (KO) Line

Tomatoes contain many secondary metabolites such as β-carotene, lycopene, phenols, flavonoids, and vitamin C, which are responsible for antioxidant activity. SlSGR1 encodes a STAY-GREEN protein that plays a critical role in the regulation of chlorophyll degradation in tomato leaves and fruits. Therefore, the present study was conducted to evaluate the sgr1 null lines based on their physicochemical characteristics, the content of secondary metabolites, and the γ-Aminobutyric acid (GABA) content. The total soluble solids (TSS), titrated acidity (TA), and brix acid ratio (BAR) of the sgr1 null lines were higher than those of the wild type(WT). Additionally, the sgr1 null lines accumulated higher levels of flavor-inducing ascorbic acid and total carotenoids compared to WT. Also, the total phenolic content, total flavonoids, GABA content, and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical content of the sgr1 null lines were higher than those of the WT. Therefore, these studies suggest that the knockout of the SGR1 gene by the CRISPR/Cas9 system can improve various functional compounds in tomato fruit, thereby satisfying the antioxidant properties required by consumers.

Microbial effects on plant phenology and fitness

Plant development and the timing of developmental events (phenology) are tightly coupled with plant fitness. A variety of internal and external factors determine the timing and fitness consequences of these life-history transitions. Microbes interact with plants throughout their life history and impact host phenology. This review summarizes current mechanistic and theoretical knowledge surrounding microbe-driven changes in plant phenology. Overall, there are examples of microbes impacting every phenological transition. While most studies have focused on flowering time, microbial effects remain important for host survival and fitness across all phenological phases. Microbe-mediated changes in nutrient acquisition and phytohormone signaling can release plants from stressful conditions and alter plant stress responses inducing shifts in developmental events. The frequency and direction of phenological effects appear to be partly determined by the lifestyle and the underlying nature of a plant-microbe interaction (i.e., mutualistic or pathogenic), in addition to the taxonomic group of the microbe (fungi vs. bacteria). Finally, we highlight biases, gaps in knowledge, and future directions. This biotic source of plasticity for plant adaptation will serve an important role in sustaining plant biodiversity and managing agriculture under the pressures of climate change.

Effects of Arbuscular Mycorrhization on Fruit Quality in Industrialized Tomato Production

Industrialized tomato production faces a decrease in flavors and nutritional value due to conventional breeding. Moreover, tomato production heavily relies on nitrogen and phosphate fertilization. Phosphate uptake and improvement of fruit quality by arbuscular mycorrhizal (AM) fungi are well-studied. We addressed the question of whether commercially used tomato cultivars grown in a hydroponic system can be mycorrhizal, leading to improved fruit quality. Tomato plants inoculated with Rhizophagus irregularis were grown under different phosphate concentrations and in substrates used in industrial tomato production. Changes in fruit gene expression and metabolite levels were checked by RNAseq and metabolite determination, respectively. The tests revealed that reduction of phosphate to 80% and use of mixed substrate allow AM establishment without affecting yield. By comparing green fruits from non-mycorrhizal and mycorrhizal plants, differentially expressed genes (DEGs) were found to possibly be involved in processes regulating fruit maturation and nutrition. Red fruits from mycorrhizal plants showed a trend of higher BRIX values and increased levels of carotenoids in comparison to those from non-mycorrhizal plants. Free amino acids exhibited up to four times higher levels in red fruits due to AM, showing the potential of mycorrhization to increase the nutritional value of tomatoes in industrialized production.

Genetic analysis of tomato root colonization by arbuscular mycorrhizal fungi

BACKGROUND AND AIMS

Arbuscular mycorrhizal fungi (AMF) play an important role in plant nutrition and protection against pests and diseases, as well as in soil structuration, nutrient cycling and, generally speaking, in sustainable agriculture, particularly under drought, salinity and low input or organic agriculture. However, little is known about the genetics of the AMF-plant association in tomato. The aim of this study was the genetic analysis of root AMF colonization in tomato via the detection of the quantitative trait loci (QTLs) involved.

METHODS

A population of 130 recombinant inbred lines derived from the wild species Solanum pimpinellifolium, genotyped for 1899 segregating, non-redundant single nucleotide polymorphisms (SNPs) from the SolCAP tomato panel, was characterized for intensity, frequency and arbuscular abundance of AMF colonization to detect the QTLs involved and to analyse the genes within their peaks (2-2.6 Mbp).

KEY RESULTS

The three AMF colonization parameters were highly correlated (0.78-0.97) and the best one, with the highest heritability (0.23), corresponded to colonization intensity. A total of eight QTLs in chromosomes 1, 3, 4, 5, 6, 8, 9 and 10 were detected. Seven of them simultaneously affected intensity and arbuscule abundance. The allele increasing the expression of the trait usually came from the wild parent in accordance with the parental means, and several epistatic interactions were found relevant for breeding purposes. SlCCaMK and SlLYK13 were found among the candidate genes. Carbohydrate transmembrane transporter activity, lipid metabolism and transport, metabolic processes related to nitrogen and phosphate-containing compounds, regulation of carbohydrates, and other biological processes involved in the plant defence were found to be over-represented within the QTL peaks.

CONCLUSIONS

Intensity is genetically the best morphological measure of tomato root AMF colonization. Wild alleles can improve AMF colonization, and the gene contents of AMF colonization QTLs might be important for explaining the establishment and functioning of the AMF-plant symbiosis.

Native soils with their microbiotas elicit a state of alert in tomato plants

Several studies have investigated soil microbial biodiversity, but understanding of the mechanisms underlying plant responses to soil microbiota remains in its infancy. Here, we focused on tomato (Solanum lycopersicum), testing the hypothesis that plants grown on native soils display different responses to soil microbiotas. Using transcriptomics, proteomics, and biochemistry, we describe the responses of two tomato genotypes (susceptible or resistant to Fusarium oxysporum f. sp. lycopersici) grown on an artificial growth substrate and two native soils (conducive and suppressive to Fusarium). Native soils affected tomato responses by modulating pathways involved in responses to oxidative stress, phenol biosynthesis, lignin deposition, and innate immunity, particularly in the suppressive soil. In tomato plants grown on steam-disinfected soils, total phenols and lignin decreased significantly. The inoculation of a mycorrhizal fungus partly rescued this response locally and systemically. Plants inoculated with the fungal pathogen showed reduced disease symptoms in the resistant genotype in both soils, but the susceptible genotype was partially protected from the pathogen only when grown on the suppressive soil. The 'state of alert' detected in tomatoes reveals novel mechanisms operating in plants in native soils and the soil microbiota appears to be one of the drivers of these plant responses.

Gene expression analyses in tomato near isogenic lines provide evidence for ethylene and abscisic acid biosynthesis fine-tuning during arbuscular mycorrhiza development

Plant responses to the environment and microorganisms, including arbuscular mycorrhizal fungi, involve complex hormonal interactions. It is known that abscisic acid (ABA) and ethylene may be involved in the regulation of arbuscular mycorrhiza (AM) and that part of the detrimental effects of ABA deficiency in plants is due to ethylene overproduction. In this study, we aimed to determine whether the low susceptibility to mycorrhizal colonization in ABA-deficient mutants is due to high levels of ethylene and whether AM development is associated with changes in the steady-state levels of transcripts of genes involved in the biosynthesis of ethylene and ABA. For that, tomato (Solanum lycopersicum) ethylene overproducer epinastic (epi) mutant and the ABA-deficient notabilis (not) and sitiens (sit) mutants, in the same Micro-Tom (MT) genetic background, were inoculated with Rhizophagus clarus, and treated with the ethylene biosynthesis inhibitor aminoethoxyvinylglycine (AVG). The development of AM, as well as the steady-state levels of transcripts involved in ethylene (LeACS2, LeACO1 and LeACO4) and ABA (LeNCED) biosynthesis, was determined. The intraradical colonization in epi, not and sit mutants was significantly reduced compared to MT. The epi mutant completely restored the mycorrhizal colonization to the levels of MT with the application of 10 µM of AVG, probably due to the inhibition of the ACC synthase gene expression. The steady-state levels of LeACS2 and LeACO4 transcripts were induced in mycorrhizal roots of MT, whereas the steady-state levels of LeACO1 and LeACO4 transcripts were significantly induced in sit, and the steady-state levels of LeNCED transcripts were significantly induced in all genotypes and in mycorrhizal roots of epi mutants treated with AVG. The reduced mycorrhizal colonization in sit mutants seems not to be limited by ethylene production via ACC oxidase regulation. Both ethylene overproduction and ABA deficiency impaired AM fungal colonization in tomato roots, indicating that, besides hormonal interactions, a fine-tuning of each hormone level is required for AM development.