Chunling Wu

Flavonoids are known to contribute to plant defense against herbivorous insects in a variety of systems, but their specific role in cassava (Manihot esculenta) resistance to the cassava mealybug (Phenacoccus manihoti) has remained unclear. Through expression profiling of nine flavonoid biosynthesis genes in high-resistance and high-susceptibility cassava varieties, chalcone synthase (CHS) was identified as the most differentially expressed candidate. Virus-induced gene silencing (VIGS) of CHS in the high-resistance variety C1115 led to a 67.5% reduction in CHS transcript levels, a 60.6% decrease in enzyme content, and a 29.7% decline in total flavonoid content. This was accompanied by the suppressed expression of downstream genes CHI and LAR. Silenced plants showed increased leaf damage and a higher pest index, effectively rendering the resistant phenotype susceptible. Furthermore, mealybugs feeding on these plants exhibited a 1.3-fold increase in their net reproductive rate. These results indicate that CHS-dependent flavonoid accumulation is one of the components required for resistance to P. manihoti in the high-resistance variety C1115, and identify CHS as a promising candidate target for marker-assisted breeding.

The use of resistant plants is recognized as an environmentally friendly measure for mite control. Oligonychus mangiferus, known as the mango red spider mite (MRSM), is a dangerous pest for mango production. To date, the resistance levels of the mango germplasms against the MRSM remain largely unknown. Furthermore, the environmental factors potentially influencing resistance performance have been seldom discussed. To fill those knowledge gaps, this study aimed to identify the resistance level of twelve mango cultivars against the MRSM. Based on three rounds of greenhouse and five seasons of field tests, cultivars with distinct resistant levels were identified. When exploring the climate impact, we found that for the susceptible cultivars, precipitation is the primary external environment factor altering the resistance performance, while temperature presents a secondary effect, and air humidity did not show a significant impact on MRSM resistance. By contrast, MRSM-resistant cultivars were not prone to be affected by changing climate conditions. Furthermore, yield tests indicated that the resistant cultivars can better reduce the yield losses compared with the susceptible ones. This study illustrated the climate element-driven effect on mango tree resistance performance against the MRSM, which can provide insight into insect pest management under changing climate conditions.

Reliable reference genes are quite important in calculating gene transcript levels by using real-time quantitative reverse transcription-PCR (RT-qPCR). Eotetranychus sexmaculatus is known as a dangerous mite causing significant yield reduction of rubber tree latex; however, selection of appropriate reference genes for validation of target gene expression in E. sexmaculatus has not been conducted yet. In the present study, nine candidate reference genes were analyzed for their expression stability in different life stages of E. sexmaculatus by using common algorithms including comparative ΔCq method, geNorm, NormFinder and BestKeeper. In addition, a comprehensive analysis software (RefFinder) was used to assign an overall final rank for each candidate gene. The results showed that β-actin and β-TUB were the best two reference genes and were subjected to evaluate expression of two protective enzyme genes (EsCu/ZnSOD and EsCAT1) in E. sexmaculatus. We found that the expression of EsCu/ZnSOD and EsCAT1 in E. sexmaculatus feeding on mite-resistant rubber tree germplasm was significantly lower compared with those feeding on mite-susceptible germplasm. These results will facilitate research in revealing molecular mechanisms underlying rubber tree resistance to the spider mite.

BACKGROUD: The red spider mite Tetranychus urticae is known as a worldwide pest in cassava production which can caused serious economic losses. Because no threshold levels are established for T. urticae in cassava cropping, many growers in China are applying excessive chemical applications per cassava cropping period. This is not only expensive, but also results in lower yields because of severe leaf damage and development of resistance. This study aims to develop an immediate threshold level for T. urticae control which could be determined in the laboratory and fit the field application requirements.RESULTS: The mite density of 25 mites/leaf was the threshold which caused significant decrease of photosynthetic pigment contents and protective enzyme activities in cassava leaves in the laboratory. Moreover, 25 mites/leaf was also the threshold density which resulted in significantly lower leaf damage and higher yield levels compared with a higher level of mites/leaf where calendar sprays were used.CONCLUSION: The mite density threshold that brought about significant physiological and biochemical changes in the laboratory basically coincided with the threshold that resulted in significant yield loss in the field. It is therefore concluded that the optimum threshold acaricide spray for T. urticae on cassava is 25 mites/leaf. © 2019 Society of Chemical Industry