Deciphering of Pod Borer [Helicoverpa armigera (Hübner)] Resistance in Cajanus platycarpus (Benth.) Offers Novel Insights on the Reprogramming and Role of Flavonoid Biosynthesis Pathway

Abstract

Management of pod borer, Helicoverpa armigera in pigeonpea (Cajanus cajan L.), an important legume crop, has been a pertinent endeavor globally. As with other crops, wild relatives of pigeonpea are bestowed with various resistance traits that include the ability to deter the H. armigera. Understanding the molecular basis of pod borer resistance could provide useful leads for the management of this notorious herbivore. Earlier studies by our group in deciphering the resistance response to herbivory through multiomics approaches in the pigeonpea wild relative, Cajanus platycarpus, divulged the involvement of the flavonoid biosynthesis pathway, speculating an active chemical response of the wild relative to herbivory. The present study is a deeper understanding of the chemical basis of pod borer (H. armigera) resistance in, C. platycarpus, with focus on the flavonoid biosynthesis pathway. To substantiate, quantification of transcripts in H. armigera-challenged C. platycarpus (8 h, 24 h, 48 h, 96 h) showed dynamic upregulation (up to 11-fold) of pivotal pathway genes such as chalcone synthase, dihydroflavonol-4-reductase, flavonoid-3'5'-hydroxylase, flavonol synthase, leucoanthocyanidin reductase, and anthocyanidin synthase. Targeted LC-MS analyses demonstrated a concomitant increase (up to 4-fold) in naringenin, kaempferol, quercetin, delphinidin, cyanidin, epigallocatechin, and epicatechin-3-gallate. Interestingly, H. armigera diet overlaid with the over-produced flavonoids (100 ppm) showed deleterious effects on growth leading to a prolonged larval period demonstrating noteworthy coherence between over-accumulation of pathway transcripts/metabolites. The study depicts novel evidence for the directed metabolic reprogramming of the flavonoid biosynthesis pathway in the wild relative to pod borer; plant metabolic potential is worth exploiting for pest management.

Keywords: anthocyanins; flavonoids; herbivory; pigeonpea; wild relatives.

Figure 1

Dynamic response of flavonoid biosynthesis pathway genes in C. platycarpus under continued herbivory: (A) Pathway map depicting key genes (green color) upregulated in C. platycarpus compared to Cajanus cajan assessed earlier in the comparative proteome profiling during herbivory (obtained from proteome data; Rathinam et al., 2020 [33]. C4H: Cinnamate-4-hydroxylase, CHS: Chalcone synthase, CHI: Chalcone flavanone isomerase, F3H: Flavanone 3-hydroxylase involved in the anti-herbivore response of C. platycarpus; Log2FC in parenthesis indicates the upregulation of the respective genes in the wild relative based on differential proteomic analysis vis a vis cultivated pigeonpea (B) Experimental setup used to understand the dynamic changes in flavonoid pathway genes and metabolites under continued herbivory in the wild relative; (C) qRT-PCR analyses of the herbivore- challenged samples depicting the response of flavonoid biosynthesis genes in C. platycarpus.

Figure 2

Targeted LC-MS profiling of phenylpropanoids and flavonoids under continued herbivory in C. platycarpus. The compounds were quantified by developing calibration and multiple reaction monitoring (MRM) of authentic standards. The graph shows values ±SD of three biological replicates from each sample. * depicts the significant difference between each time interval at p < 0.05, from the student’s t-test.

Figure 3

Pathway mapping of herbivore-induced flavonoid pathway genes and metabolites in C. platycarpus. Heat maps indicate the dynamic expression of respective genes at different time points (8 h, 24 h, 48 h, 96 h) of herbivory. The red-colored font denotes the genes assessed for expression analyses in the present study. The blue-colored font denotes over-accumulated metabolites during herbivory.

Figure 4

Gene copy number assessment of selected flavonoid pathway genes in C. platycarpus. Genomic Southern analysis of 9 genes digested with HindIII, KpnI, and BamHI and probed with DIG-labelled gene-specific probes; P: positive control.

Figure 5

Diet overlay assay for the validation of selected flavonoids on H. armigera. Response of H. armigera larvae to artificial diet feeding assay incorporated with water-soluble flavonoids in 10 and 100 ppm concentrations. (A) Representative image of larvae that fed on flavonoids-incorporated artificial diet (B) Average length of larvae in cm; mean ± SE, n = 10 (C) Average larval weight in mg; mean ± SE, n = 10 (D) Average larval period in days; mean ± SE, n = 10. The larval length, weight, and duration were compared between control and respective treatments by student’s t-test; * p < 0.05; ** p < 0.001. WC: Water control; EC3G: Epicatechin-3-gallate; EGC: Epigallocatechin.

Figure 6

Performance of H. armigera larvae in artificial diet feeding assay incorporated with DMSO-soluble flavonoids (dissolved in WC, 1% DMSO, 1.5% DMSO, and 2% DMSO) in 10 and 100 ppm concentrations. (A) Representative image of larvae that fed on the selected flavonoids -incorporated artificial diet along with their respective controls (B) Average length of larvae in cm; mean ± SE, n = 10 (C) Average larval weight in mg; mean ± SE, n = 10 (D) Average larval period in days; mean ± SE, n = 10. The larval length, weight, and duration are compared between controls and respective treatments by student’s t-test; * p < 0.05; ** p < 0.001. WC: Water control; DMSO: Dimethylsulfoxide; Nar: Naringenin; Cya: Cyanidin; Mal: Malvidin; Del: Delphinidin; Pel: Pelargonidin.