Histopathology and quantification of green fluorescent protein-tagged Fusarium oxysporum f. sp. luffae isolate in resistant and susceptible Luffa germplasm

Fusarium oxysporum f. sp. luffae (Folu) is a severe plant pathogen that causes vascular wilt and root rot in Luffa plants worldwide. A green fluorescent protein (GFP)-tagged isolate of Folu (Fomh16-GFP) was utilized to investigate the infection progress and colonization of Fomh16-GFP in resistant (LA140) and susceptible (LA100) Luffa genotypes. Seven days post-inoculation (dpi), it was observed that Fomh16-GFP had successfully invaded and colonized the vascular bundle of all LA100 parts, including the roots, hypocotyl, and stem. Pathogen colonization continued to increase over time, leading to the complete wilting of plants by 14-17 dpi. In LA140, the Fomh16-GFP isolate colonized the roots and hypocotyl vascular system at 7 dpi. Nevertheless, this colonization was restricted in the hypocotyl and decreased significantly, and no fungal growth was detected in the vascular system at 21 dpi. Thus, the resistant genotype might trigger a robust defense mechanism. In addition, while the pathogen was present in LA140, the inoculated plants did not exhibit any symptoms until 28 dpi. Quantitative PCR was utilized to measure the Fomh16-GFP biomass in various parts of LA100 and LA140 at different time points. The findings indicated a positive correlation between the quantity of Fomh16-GFP DNA and disease development in LA100. Alternatively, a high amount of Fomh16-GFP DNA was identified in the roots of LA140. Nonetheless, no significant correlations were found between DNA amount and disease progression in LA140. Aqueous extracts from LA140 significantly reduced Fomh16-GFP spore germination, while no significant reduction was detected using LA100 extracts.IMPORTANCEFusarium wilt of Luffa, caused by Fusarium oxysporum f. sp. luffae (Folu), causes great losses in Luffa plants worldwide. This study used a green fluorescent protein (GFP)-tagged isolate of Folu (Fomh16-GFP) to investigate the infection progress and colonization dynamics of Fomh16-GFP in the resistant and susceptible Luffa genotypes, which could be important in understanding the resistance mechanism of Folu in Luffa plants. In addition, our work highlights the correlations between DNA amount and disease progression in resistant plants using real-time PCR. We observed a positive correlation between the quantity of Fomh16-GFP DNA and disease progression in LA100, while no significant correlation was found in LA140. These results could be valuable to further investigate the resistance mechanism of Luffa genotypes against Folu. Gaining a better understanding of the interaction between Folu and Luffa plants is crucial for effectively managing Fusarium wilt and enhancing resistance in Luffa rootstock and its varieties.

Proteomic Analysis of Fusarium oxysporum-Induced Mechanism in Grafted Watermelon Seedlings

Grafting can improve the resistance of watermelon to soil-borne diseases. However, the molecular mechanism of defense response is not completely understood. Herein, we used a proteomic approach to investigate the molecular basis involved in grafted watermelon leaf defense against Fusarium oxysporum f.sp. niveum (FON) infection. The bottle gourd rootstock-grafted (RG) watermelon seedlings were highly resistant to FON compared with self-grafted (SG) watermelon plants, with a disease incidence of 3.4 and 89%, respectively. Meanwhile, grafting significantly induced the activity of pathogenesis-related proteases under FON challenge. Proteins extracted from leaves of RG and SG under FON inoculation were analyzed using two-dimensional gel electrophoresis. Thirty-nine differentially accumulated proteins (DAPs) were identified and classified into 10 functional groups. Accordingly, protein biosynthetic and stress- and defense-related proteins play crucial roles in the enhancement of disease resistance of RG watermelon seedlings, compared with that of SG watermelon seedlings. Proteins involved in signal transduction positively regulated the defense process. Carbohydrate and energy metabolism and photosystem contributed to energy production in RG watermelon seedlings under FON infection. The disease resistance of RG watermelon seedlings may also be related to the improved scavenging capacity of reactive oxygen species (ROS). The expression profile of 10 randomly selected proteins was measured using quantitative real-time PCR, among which, 7 was consistent with the results of the proteomic analysis. The functional implications of these proteins in regulating grafted watermelon response against F. oxysporum are discussed.

CRISPR/Cas9-mediated mutagenesis of Clpsk1 in watermelon to confer resistance to Fusarium oxysporum f.sp. niveum

CRISPR/Cas9-mediated editing of Clpsk1 enhanced watermelon resistance to Fusarium oxysporum. The clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 system has proven to be an effective genome-editing tool for crop improvement. Previous studies described that Phytosulfokine (PSK) signalling attenuates plant immune response. In this work, we employed the CRISPR/Cas9 system to knockout Clpsk1 gene, encoding the PSK precursor, to confer enhanced watermelon resistance to Fusarium oxysporum f.sp. niveum (FON). Interactions between PSK and FON were analysed and it was found that transcript of Clpsk1 was significantly induced upon FON infection. Meanwhile, application of exogenous PSK increased the pathogen growth. Then, one sgRNA, which targeted the first exon of Clpsk1, was selected for construction of pRGEB32-CAS9-gRNA-Clpsk1 expression cassette. The construct was then transformed to watermelon through Agrobacterium tumefaciens-mediated transformation method. Six mutant plants were obtained and three types of mutations at the expected position were identified based on Sanger sequencing. Resistance evaluation indicated that Clpsk1 loss-of-function rendered watermelon seedlings more resistant to infection by FON. These results indicate that CRISPR/Cas9-mediated gene modification is an effective approach for watermelon improvement.

Wheat straw increases the defense response and resistance of watermelon monoculture to Fusarium wilt

BACKGROUND

Wheat straw is a rich resource worldwide. Straw return is an effective strategy to alleviate soil-borne diseases on monoculture watermelon. Previous studies focus on soil structure, physical and chemical properties; however, little is known about the molecular responses on host plant.

RESULTS

No significant difference on the population of Fusarium oxysporum f.sp. niveum race 1(Fon1) in rhizosphere soil was found between control (no addition of wheat straw) and the treated groups (addition of 1% (T1) or 2% (T2) wheat straw). RNA-Seq analysis showed that 3419 differentially expressed genes were clustered into 8 profiles. KEGG analysis revealed that phenylpropanoid biosynthesis and plant hormone signal transduction were involved in wheat straw induced response in monoculture watermelon. Genes in lignin biosynthesis were found to be upregulated, and the lignin and auxin contents were higher in T1 and T2 compared to the control. Lignin was also enriched and the Fon1 population decreased in watermelon roots treated with wheat straw. The enzyme activities of phenylalanine ammonia-lyase and peroxidase were increased.

CONCLUSIONS

Our data suggest that the addition of wheat straw enhances the defense response to Fon1 infection in watermelon through increasing lignin and auxin biosynthesis.

Treatment With Wheat Root Exudates and Soil Microorganisms From Wheat/Watermelon Companion Cropping Can Induce Watermelon Disease Resistance Against Fusarium oxysporum f. sp. niveum

Companion cropping with wheat (Triticum aestivum L.) can enhance watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] wilt disease resistance against Fusarium oxysporum f. sp. niveum. However, the mechanism of resistance induction remains unknown. In this study, the effects of microbial community dynamics and the interactions between wheat and watermelon plants, particularly the effect of wheat root exudates on watermelon resistance against F. oxysporum f. sp. niveum, were examined using a plant-soil feedback trial and plant tissue culture approach. The plant-soil feedback trial showed that treating watermelon with soil from wheat/watermelon companion cropping decreased watermelon wilt disease incidence and severity, increased lignin biosynthesis- and defense-related gene expression, and increased β-1,3-glucanase activity in watermelon roots. Furthermore, soil microbes can contribute to increasing disease resistance in watermelon plants. Tissue culture experiments showed that both exogenous addition of wheat root exudates and companion cropping with wheat increased host defense gene expression, lignin and total phenols, and increased β-1,3-glucanase activity in watermelon roots. In conclusion, both root exudates from wheat and the related soil microorganisms in a wheat/watermelon companion cropping system played critical roles in enhancing resistance to watermelon wilt disease induced by F. oxysporum f. sp. niveum.

Inside and Beyond Color: Comparative Overview of Functional Quality of Tomato and Watermelon Fruits

The quali-quantitative evaluation and the improvement of the levels of plant bioactive secondary metabolites are increasingly gaining consideration by growers, breeders and processors, particularly in those fruits and vegetables that, due to their supposed health promoting properties, are considered "functional." Worldwide, tomato and watermelon are among the main grown and consumed crops and represent important sources not only of dietary lycopene but also of other health beneficial bioactives. Tomato and watermelon synthesize and store lycopene as their major ripe fruit carotenoid responsible of their typical red color at full maturity. It is also the precursor of some characteristic aroma volatiles in both fruits playing, thus, an important visual and olfactory impact in consumer choice. While sharing the same main pigment, tomato and watermelon fruits show substantial biochemical and physiological differences during ripening. Tomato is climacteric while watermelon is non-climacteric; unripe tomato fruit is green, mainly contributed by chlorophylls and xanthophylls, while young watermelon fruit mesocarp is white and contains only traces of carotenoids. Various studies comparatively evaluated in vivo pigment development in ripening tomato and watermelon fruits. However, in most cases, other classes of compounds have not been considered. We believe this knowledge is fundamental for targeted breeding aimed at improving the functional quality of elite cultivars. Hence, in this paper, we critically review the recent understanding underlying the biosynthesis, accumulation and regulation of different bioactive compounds (carotenoids, phenolics, aroma volatiles, and vitamin C) during tomato and watermelon fruit ripening. We also highlight some concerns about possible harmful effects of excessive uptake of bioactive compound on human health. We found that a complex interweaving of anabolic, catabolic and recycling reactions, finely regulated at multiple levels and with temporal and spatial precision, ensures a certain homeostasis in the concentrations of carotenoids, phenolics, aroma volatiles and Vitamin C within the fruit tissues. Nevertheless, several exogenous factors including light and temperature conditions, pathogen attack, as well as pre- and post-harvest manipulations can drive their amounts far away from homeostasis. These adaptive responses allow crops to better cope with abiotic and biotic stresses but may severely affect the supposed functional quality of fruits.

Study on screening and antagonistic mechanisms of Bacillus amyloliquefaciens 54 against bacterial fruit blotch (BFB) caused by Acidovorax avenae subsp. citrulli

Bacterial fruit blotch (BFB) was a serious threat to cucurbitaceae crops. It was caused by the gram-negative bacterium Acidovorax avenae subsp. citrulli. Two hundred strains, which have the potential in controlling plant diseases in our laboratory's biocontrol strain library, were employed to this research to screen some antagonistic bacteria, which can efficiently control bacterial fruit blotch disease. Based on the results of antagonistic activity experiments, greenhouse tests and field trials, 5 of the test strains have high abilities to control BFB. One of the 5 bacteria strains has the highest potential to control BFB named 54. The biocontrol efficacy of 54 was up to 60%. To characterize the strain, we used series of methods to evaluate the bacterium, including morphology analysis, physiological biochemical test and biomolecular assay. We found that the bacterium 54 belongs to the species Bacillus amyloliquefaciens. The colonization test results showed that 54 had the highest colonization levels, and the density of the strain on leaves was up 10(5)colony forming units (CFU) per gram of leaf tissue. Our recent results show that B. amyloliquefaciens 54 can promote the plant growth due to raised the contents of available N, P, K and the leaf chlorophyll. The antagonistic bacterium 54 can significantly control the BF B by increasing the expression level of defense-related gene PR1 and the accumulation the hydrogen peroxide in the plant. The results of trail experiment was also verified this efficient results of bacterium. This is also the first report of B. amyloliquefaciens strain that is able to control BFB.

Development of the molecular methods for rapid detection and differentiation of Fusarium oxysporum and F. oxysporum f. sp. niveum in Taiwan

Fusarium wilt, caused by Fusarium oxysporum (Fo), is one of the most important fungal diseases worldwide. Like other plant pathogens, Fo displays specialized forms in association with its hosts. For example, F. oxysporum f. sp. niveum (Fon) is the damaging pathogen causing Fusarium wilt disease on watermelon, whereas F. oxysporum f. sp. cubense is the pathogen that infects banana. A rapid and reliable pathogen identification or disease diagnosis is essential for the integrated disease management practices in many crops. In this study, two new primer sets, Fon-1/Fon-2 and FnSc-1/FnSc-2, were developed to differentiate Fon and Fo, respectively. The PCR method using the novel primer sets has high sensitivity to detect Fon when the DNA concentration was as low as 0.01 pg or when the conidia number was as few as 5. In comparison with the published primer set, the Fon-1/Fon-2 primer set, derived from the sequence of OP-M12 random primer-amplified fragment, produced a 174 bp DNA fragment, and was more specific to Fon in Taiwan. In addition, with optimized PCR parameters, the molecular method using the Fon-1/Fon-2 primer set could directly detect Fon even when watermelon samples were collected in its early stage of disease development.