The role of metabolites in cassava (Manihot esculenta Crantz) resistance to Whitefly (Bemisia tabaci)
The whitefly (Bemisia tabaci) (Gennadias, 1889) is a major challenge in cassava production especially for most African regions. This is because most of the varieties used are susceptible to the pest resulting in severe leaf damage, viral disease transmission of which African Cassava Mosaic Disease (ACMD) and Cassava Brown Streak Disease (CBSD) have caused serious epidemics. Breeding for resistant varieties against the whitefly is thus among the key priorities for cassava improvement programmes. Unfortunately, much of breeding work has been focused on the viral diseases and not the causal vector. Consequently, there has been dramatic increase in cassava whitefly population densities on cassava suggesting the need to consider the vectors when breeding for resistance, particularly in identifying the sources of resistance and the mechanisms of resistance to the vectors. Research on several crops have demonstrated that primary and secondary metabolites may confer resistance/tolerance against pests. However, there is limited information regarding the biochemical defence mechanism and metabolites that may be involved in conferring resistance/tolerance to pests in cassava. This study, therefore, aimed at generating knowledge on the role of metabolites in mediating cassava resistance to B. tabaci. The specific objectives of the study were to i) determine resistance levels to B. tabaci in existing Uganda germplasm ii) identify the secondary metabolites associated with cassava resistance to B. tabaci, iii) determine changes that occur in secondary metabolites in cassava infested with B. tabaci, iv) determine influence of environment on secondary metabolite and protein content in cassava with different leaf damage levels, and v) determine the heritability and mode of gene action of secondary metabolite in cassava with different leaf damage levels by B. tabaci. Fifty genotypes were screened monthly in four locations (Arua, Kasese, Kamuli and Namulonge) at 30 to 180 days after planting (DAP) in 2015, using nymph count, whitefly count, leaf damage and sooty mould scores. The screening studies identified ten genotypes as resistant to B. tabaci, as they showed relatively low (< 50-100) nymph and whitefly count, < 2 leaf damage and < 1.5 sooty mould. These were among genotypes used for assessing metabolites associated with cassava resistance to whitefly (B. tabaci) as well as their interaction with some environmental factors in subsequent studies. To assess the genotypes’ ability to induce resistance metabolites, phloem sap was extracted biweekly on eight genotypes in Namulonge at 60 and 90DAP in 2016, a time where, significant (P < 0.05) variations for nymph count, whitefly count, leaf damage and sooty mould compared to between 30 and 60DAP were observed. Metabolites were determined spectrophotometrically using standard protocols. Resistant genotypes were found to have high amounts of metabolite compared to susceptible genotypes at 90DAP. Genotypes CS 1-144, UG 120251, UG 120257, UG 120124, UG 120191, UG 120133, had high B. tabaci population density at 90DAP and showed high amounts of salicylic acid and peroxidase. Production of metabolites; salicylic acid, peroxidase, tannins and flavonoids were shown to be induced in part by B. tabaci feeding, an indication of their involvement in resistance. High whitefly counts were considered to have contributed to increased resistance metabolites. The genotypes (eight) were further evaluated at Namulonge for metabolite changes that may occur over a time period (30-180DAP) which included the documented peak infestation time (90-120DAP) for two seasons in 2016. There were significant changes in metabolite quantities among genotypes from 30 to 180DAP, clearly associated to nymph, whitefly count and leaf damage. For example, there was high percentage increase in tannin across time with UG 120257 and CS 1-144 recording values of 45 and 69% respectively. CS 1-144 had 11.16% and UG 120257 had 0.98% for total phenolic content. Peroxidase and protein in NAM 130 had 133.6% and 84.4% and in CS 1-144 had 112.8 % and 82.6% increase respectively. In addition, the nymph count peaked between 60-120DAP, whereas leaf damage from 90 – 120DAP and most of the metabolites increased significantly at 60 to 180DAP. Meanwhile, nymph count and leaf damage reduced between 120-180DAP observed in resistant genotypes CS 1-144, NAM 130 and UG 120257. As a result, the reduced leaf feeding was suggestive of reduced insect infestation, reduced development period and either migration or death of the insects due to various mechanisms of the metabolites identified. Fifteen genotypes were evaluated monthly, under field condition, for six months (30-180DAP) in three locations (Kasese, Kamuli and Namulonge) for changes in the induced metabolites across two seasons in 2016, concurrently with variations in nymph count, whitefly count, leaf damage and sooty mould. The plants were evaluated in a randomized complete block design and data were analysed using additive main effect and multiplicative interaction (AMMI). It was observed that the measured traits on the tested cassava genotypes were influenced by environment, as well as genotype by environment (GxE) interaction. Temperatures varied across locations and showed that they may have influenced the various metabolite responses among the genotypes. However, based on AMMI, genotypic effect was greater than the environmental effect on the measured metabolites, an indication that genotypes contributed more to expression of metabolites compared to environmental factors. Genotypes UG 120257, UG 120191, UG 120251 and UG 120124 showed less environmental influence and relatively high amounts of tannin, flavonoid, total phenolic content and antioxidative capacity with low (< 2) leaf damage scores across all environments. An evaluation of 300-450 genotypes in Diallel mating design II was done in the second season of 2016 after which 45 family based S1 progenies and their corresponding parental genotypes (10) were selected and evaluated at Namulonge for two seasons in 2017. High broad and narrow sense heritability of metabolite traits were recorded, indicating the complexity in the traits and both additive and dominance gene action for the traits. The results suggested that both broad and narrow sense heritability were vital in secondary metabolite evaluation because the additive variance does not always give an adequate assessment of the influence of genetics on metabolites, therefore stressing the importance to consider the dominance variance in cassava. High heritability and additive variance estimates indicated that the parameters had high genetic variance, a higher frequency of genes controlling metabolite traits and the potential to improve metabolite traits with traditional breeding strategies. General combining ability was significant than specific combining ability, a result indicating possible early selection. Salicylic acid, peroxidase and phenolics conformed to 3:1, 9:7 and 27:37 phenotypic ratios, evidence of dominant epistatic gene action and that there were several genes influencing gene action of the metabolite response. From this study, genetic variations to whitefly (B. tabaci) were observed with ten genotypes identified as resistant, indicative of the possibility of selection of resistant genotypes for use in a breeding programme for cassava resistance to B. tabaci. It was concluded that, B. tabaci feeding induces metabolites such as salicylic acid, peroxidase and phenolic compounds and they play a role in resistance to B. tabaci. The propensity of the interactive association of the metabolites and their effect, for example on nymph counts, suggested that although not limited to, were involved in antinutritive, anti-digestive and toxic effects. In addition, among resistant genotypes, both constitutive and induced resistance were shown to have an effect on B. tabaci. Furthermore, the metabolite ecological performance was studied and discussed herein, and despite environmental influence, some genotypes showed stable performance. This indicated genotype adaptability in metabolite content and resistance, as a result could be considered useful for breeding plant resistance metabolites against B. tabaci. Finally, the metabolites were shown to be additively controlled, thus highly heritable and could easily be transferred into other genotypes. The study, thus provided a broad-spectrum platform upon which resistance to B. tabaci not only in cassava, can be a part of and assessed in an integrated strategy, as a means to enhance breeding for resistance to the pest.