Striga resistance in SAP low germination stimulant 1 sorghum
Following on knowledge that sorghum genotypes that exude primarily orobanchol are poor inducers of Striga germination, and thereforeStriga resistant, we performed two bioassays routinely used to measure Striga resistance. In the first assay, we used root exudates extracted from roots of sorghum to stimulate germination ofStriga seeds and determined the germination frequency (Figure 3a). Our analysis showed that most of the SAP lgs1 accessions were ineffective in stimulating Striga germination as displayed by their low Striga seed germination frequencies (Figure 3b). Most notably, PI585295, PI533976 and PI655979 had germination frequencies of less than 40 %, two-fold lower than that of the resistant control SRN 39. Expectedly, wild type LGS1 and the susceptible check Shanqui Red had high germination frequencies (Shanqui Red, 62% and PI533839, 73.6%). Surprisingly, PI656054, a wild type LGS1, and a 5-deoxystrigol producer had a notably low germination frequency (31.1 %) but high MGD alluding to the presence of other pre-attachment resistance mechanisms independent of the LGS1 region. There is a growing body of circumstantial evidence that implicates other hormones, particularly abscisic acid (ABA) in regulation of Striga seed germination. It is hypothesized that; some sorghum genotypes emit large amounts of ABA and this could inhibit germination of the Strigaseeds (Mallu et al. 2022). To explore if ABA was responsible for the low germination activity of SAP LGS1, we quantified the concentration of ABA in the root exudates of SAP accessions (Figure S3). However, our results did not show any correlation between germination stimulation and ABA levels in the root exudates, leaving the pre-attachment resistance mechanism of SAP LGS1 PI656054 unclear.
The second assay, agar gel assay involves co-cultivation of sorghum with preconditioned Striga embedded in agar. In this assay, resistant host roots stimulate less germination of parasite seeds and only do so for seeds closest to them (Supplementary Figure 2). This germination ability is measured using the metric of maximum germination distance (MGD) that computes the maximum germination distance between sorghum rootlets and the three most distant germinated Striga seeds. Resistant hosts therefore have low MGD while susceptible hosts have high MGDs. We found that all 12 lgs1 SAP sorghum had notably lower MGDs relative to the wild type LGS1 controls and the susceptible check Shanqui Red (Figure 3c). Notably, the MGDs of 6 SAP lgs1genotypes were comparable to that of the resistant control, SRN39.
Considering that the overall resistance of a host is a function of its ability to stimulate germination (pre-attachment resistance) and to block host attachment (post-attachment resistance), we performed pot experiments to determine Striga emergence for the SAP lgs1genotypes (Figure 3e). Results showed that 5 SAP lgs1 genotypes had lower or comparable number of emerging Striga as shown in Figure 3d (PI655979, count =11.75, PI656040, count =6, PI533576, count= 5.75) with the resistant check SRN39 (count = 16). Additionally, SAP LGS1 accession PI645054 had low Striga infestation (14.5), further supporting germination assays described above. Notably, some SAPlgs1 genotypes had high Striga emergence, an indication of weak post-attachment resistance in those genotypes.