Effects of Sophora alopecuroides aqueous extract on the pathogenicity of Fusarium oxysporum and on the defense mechanisms of Hami melon seedlings

【Objective】The cultivation of Hami melon has a long history, establishing them as a traditional and high-value economic crop in the Xinjiang Uygur Autonomous Region. As a nationally recognized geographical indication product, the expansion of intensive agriculture has led to increased continuous cropping intensity of Hami melon, thereby exacerbating soil- borne diseases, most notably Hami melons fusarium wilt. Fusarium oxysporumf. sp. melonis (Fom) is a serious pathogen that causes crucial quality and yield loss, which is also a great threat to the green and sustainable development of the Hami melon industry in Xinjiang. In comparison with chemical fungicides, Sophora alopecuroides L. is rich in various alkaloids, flavonoids, and terpenoids, giving it a wide range of therapeutic potential. To investigate the potential of S. alopecuroides extract in controlling Fusarium wilt disease in Hami melon, the present study focus on clarifying the inhibitory mechanism of the S. alopecuroides extract on Fom and its regulatory effect on resistance of Hami melon seedlings.【Methods】Aqueous extracts of S. alo-pecuroides at different concentrations (5, 25 and 50 g ·L-1 ) were prepared to analyze their inhibitory effects on the growth and pathogenicity of Fom. Plate assays were conducted to investigate the suppression effects of the S. alopecuroides extract on the growth of Fom colonies and the expression of pathogenic-related functional genes. And qPCR assays were carried out to detect the copies of key genes related to biosynthesis of chitin synthetase (Fochs V), fusaric acid (FUBT), superoxide dismutase (SOD) and peroxidase (POD). Pot experiments were performed to explore the mitigation effect of the S. alopecuroides extract on the pathogenicity of Fom from the perspective of the physiological characteristics and defense enzyme activities of the Hami melon seedlings.【Results】Different concentrations of S. alopecuroides extract inhibited the growth of Fom. Low concentration of extract (5 g ·L-1 ) had no significant effect on Fom growth in the short term (3 days and 5 days). After 7 days of treatment, the colony diameter of Fom was 6.45 cm, significantly lower than that in the control group (6.60 cm). In treatment with 25 g·L-1 of extract, the Fom colony diameters decreased by 15.06%, 13.61% and 5.37% respectively, compared with the control group. Beside, high concentration of S. alopecuroides extract (50 g · L- 1 ) could inhibit the growth of Fom, with an inhibition rate ranging from 7.4% to 17%. Results from qPCR indicated that 50 g·L-1 of S. alopecuroides extract remarkably restrained the expression of genes related to the pathogenicity and antioxidant capacity of Fom after three days of treatment. After 3 days of treatment, with the increase of the S. alopecuroides extract, copies of Fochs V decreased from 1.26 × 109 copies·g-1 to 1.32 × 106 copies·g-1 , and copies of FUBT decreased from 7.94 × 108 copies·g-1 to 7.94 × 106 copies· g- 1 . Moreover, with 3 days treatment of 50 g · L- 1 S. alopecuroides extract, the POD copies were 1.62 × 105 copies· g-1 , a significant 21% reduction compared with the control. And the SOD gene copies were 5.37 × 104 copies · g- 1 , a significant 26% reduction relative to the control group. Results from the controlled pot experiments revealed that Fom had no effects on the height and real leaves number of Hami melon seedlings. Nevertheless, Fom could clearly suppressed the growth of Hami melon main roots (2.65 cm), compared to the control group (3.82 cm). Notably, 50 g · L- 1 of S. alopecuroides extract application alleviated the suppression of root growth by Fom, which promoted the development of the main roots to a length of 4.25 cm. Although Fom had no significant impact on peroxidase (POD) activity, Fom significantly inhibited the activity of catalase (CAT) and phenylalanine ammonia lyase (PAL). Besides, the inhibitory effects of Fom on superoxide dismutase (SOD) varied with exposure time, as indicated by the detection results from Hami melon leaves enzyme activity. With 3d treatment of Fom, the activity of SOD was clearly inhibited, but there was no clear difference between the treatment and control group. However, with the application of S. alopecuroides extract, the activity of the four aforementioned enzymes was restored to varying degrees. For example, the S. alopecuroides extract-only treatment did not significantly affect PAL activity in Hami melon seedlings leaves in 3 days. However, results from the cross-experiment revealed that under Fom stress, the PAL activity in the combined treatment groups was significantly higher than that in the Fom-only treatment group (1 488 U·g-1 ), even restoring the enzyme activity to a level comparable with the CK group (2 457.2 U·g-1 ). Similar results were also observed in the root vitality test. Both S. alopecuroides extract (86.3 U·g-1 to 135.8 U·g-1 ) and Fom (72.6 U· g-1 ) could significantly inhibit the root activity of Hami melons with three days treatment, in comparison with the control group (139.8 U· g-1 ). Cross-experiment results indicated S. alopecuroides extract did not alter the inhibitory effect of Fom on root viability in the short term (3 days). Dramatically, with increased exposure time, the extract significantly reduced the inhibitory effect of Fom on the root viability of Hami melons.【Conclusion】S. alopecuroides extract significantly suppresses the growth of Fom (P＜0.001) mycelial growth. And high concentration of S. alopecuroides extract could inhibit the expression of pathogenicity-related genes in 3 d treatment. Although Fom remarkedly suppressed the taproot growth of Hami melon, medium to high concentrations of the S. alopecuroides extract could alleviate this inhibition. Furthermore, the extract of S. alopecuroides could mitigate the Fom-induced suppression of SOD, CAT, POD, PAL, and root vitality in Hami melon seedlings.