Fusarium wilt disease is one of the major diseases causing a decline in watermelon yield and quality. Researches have informed that phytohormones play essential roles in regulating plants growth, development, and stress defendants. However, the molecular mechanism of salicylic acid (SA), jasmonic acid (JA), and abscisic acid (ABA) in resistance to watermelon Fusarium wilt remains unknown. In this experiment, we established the SA, JA, and ABA determination system in watermelon roots, and analyzed their roles in against watermelon Fusarium wilt compared to the resistant and susceptible varieties using transcriptome sequencing and RT-qPCR. Our results revealed that the up-regulated expression of Cla97C09G174770, Cla97C05G089520, Cla97C05G081210, Cla97C04G071000, and Cla97C10G198890 genes in resistant variety were key factors against (Fusarium oxysporum f. sp. Niveum) FON infection at 7 dpi. Additionally, there might be crosstalk between SA, JA, and ABA, caused by those differentially expressed (non-pathogen-related) NPRs, (Jasmonate-resistant) JAR, and (Pyrabactin resistance 1-like) PYLs genes, to trigger the plant immune system against FON infection. Overall, our results provide a theoretical basis for watermelon resistance breeding, in which phytohormones participate.

Sweetness or sugar content, represented by soluble solids contents (SSC), is a vital quality trait in watermelon breeding which can be assessed by the refractive index method. However, sampling watermelon juice out of the pulp is a process that is both labor-intensive and error-prone. In this study, we developed an automatic SSC measurement system for watermelon breeding to improve efficiency and decrease costs. First, we built an automatic cutting system to cut watermelons into precise halves, in which a laser rangefinder is used to measure the distance from the upper surface of the watermelon to itself, and thus, the diameter is estimated. The experiments showed a high correlation between the estimated diameters and the ground truths, with and . Then, we built an automatic Brix measurement system to obtain the Brix data from a central point on the watermelon’s section, where an image analysis procedure is applied to locate the testing point. This is then transformed to the camera coordination system, and a refractometer is driven by a 3-axis robotic arm to reach the testing point. Brix measurement experiments were conducted using three vertical gaps and four lateral gaps between the probe of the refractometer and the pulp. The result showed that the best parameters were a vertical gap of 4 mm and a lateral gap of 2 mm. The average accuracy reached 98.74%, which indicates that this study has the potential to support watermelon breeding research.

Watermelon fusarium wilt is a soil-borne disease that seriously affects watermelon yield. Our previous study found that salicylic acid (SA) plays an essential role in regulating watermelon roots against the Fusarium wilt (FW), but the molecular mechanism is still unclear. Therefore, in this experiment, we aim to explore the molecular mechanism of SA to manipulate disease resistance of the watermelon immune system based on transcriptome analysis, combined with chlorophyll fluorescence images, phytohormone content quantification, and RT‒qPCR verification. Our results demonstrated that exogenous SA activates watermelon plants against Fon1 infection at 3 dpi by inducing other phytohormones and enriching resistance genes to maintain the citrate cycle. Comparing analysis of differentially expressed genes (DEGs) identified that the CIPER10, CIMYB59, CIMYB44, CIPYL4, CIAGP31, and CIACO3 have key functions in SA signal transduction to active watermelon immune responses. The bioinformatics analysis indicated that SA has a cooperating network with abscisic acid (ABA), jasmonic acid (JA), and ethylene (ET) to manipulate the watermelon immune system against FW. Collectively, our findings provide significant insight into watermelon FW defense mechanisms and information for the breeding of new watermelon cultivars.