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.