Discussion
Expanding the genetic basis of sorghum to cope with parasitism byStriga , one of the biggest constraints to cereal production in
SSA, could have far-reaching impact towards alleviating food insecurity.
An efficient way to achieve Striga resistance is to harness
advances in genomics and exploit the vast genetic diversity of sorghum.
Building on previous successful identification of Strigaresistant sorghum from global collections (Kavuluko et al. 2021;
Mallu et al. 2021, 2022), we sought to identify and characterize
new sources of resistance. Our motivation for selecting the SAP
collection was obtaining sorghum with increased resistance toStriga parasitism combined with additional traits that accrue
benefits to smallholder farmers in Africa. The SAP is also extensively
sequenced providing vital data for downstream resistance breeding. Based
on our analysis and previous work (Boatwright et al. 2022), the
SAP comprises all major botanical races of sorghum (caudatum, kafir,
guinea, and durra) and a mixed population believed to have originated
from bicolor – one of the earliest races to undergo domestication
(Harlan & Stemler, 1976).
We adopted a simple methodology for discovery of Strigaresistance alleles in sorghum based on PCR screening using two sets of
primers – making it applicable in basic laboratories without the need
for sophisticated equipment and resources for advanced molecular
screening. Our approach hinged on identifying sorghum genotypes
harboring a chromosomal deletion in the sorghum LOW GERMINATION
LOCI 1 (LGS1 ) region which makes some genotypes inefficient in
stimulating the germination of the parasitic plant Striga (Gobenaet al. 2017). Mutants devoid of this region, termed lgs1genotypes are Striga resistant because they primarily exude the
low potent SL, orobanchol, compared to their susceptible wild type
counterparts (LGS1 genotypes) that exude 5-deoxystrigol which is more
potent. We found 12 SAP lgs1 accessions; among them two had been
previously described (Bellis et al . 2020). This represented 3.5
% of our screened population and importantly, 67 % of thelgs1 -like genotypes originated from Africa. This finding is
important and significant because it points to increased prevalence ofStriga resistance in sorghum domesticated in Africa, which can be
attributed to its adaptation to the parasite as both Striga and
sorghum have their natural distribution ranges in SSA.
SAP lgs1 accessions bore the expected phenotype of the referencelgs1-like sorghum SRN39 whose hallmark is the production of high
proportions of the SL orobanchol relative to 5-deoxystrigol (Gobenaet al. 2017). Germination assays further affirmed the expected
low germination stimulation of lgs1 -like sorghum genotypes. Our
results were consistent with previous work that have reported
germination efficiencies of 38 % in lgs1 -like genotypes (Malluet al. 2021). Unexpectedly, PI656054, an LGS1 accession,
showed notably low germination induction. Our hypothesis of involvement
of ABA in mediating the low germination of this genotype was
inconclusive as we did not find any correlation between germination and
ABA content. One possibility is the production of another less potent SL
that was not tested in this in this study. Determination of the actual
mechanisms will be an interesting subject of further investigation.Striga seed germination stimulation assayed by agar gel method
that measures MGD, the maximum germination distance which host root can
stimulate Striga germination in vitro agar culture showed
concurrence of our results with previous reports. For example, Gobenaet al . (2017) found the MGD of SRN39 to be 1mm. In pearl millet,
the Striga resistant 29AW had a MGD of 7.96 mm (Dayou et
al. 2021).
To further validate the resistance of SAP lgs1 accessions, we
performed pot experiments where sorghum was planted inStriga- infested soil and the number of emerging Strigacounted. Our results showed that most SAP lgs1 had low emergence.
We would like to point out PI655979, PI656040, and PI533576, that
consistently showed low germination stimulation in the bioassay and low
emergence in pot experiments. We would also like to point out PI 656054,
the SAP LGS1 that we described earlier as having lowStriga germination induction. Because Striga emergence in
pot experiments is a function of both pre- and post-attachment
resistance, all these genotypes represent good candidates for further
field evaluation.
The final aspect of our study was to evaluate SAP lgs1 lines
under field infestations. Consistently, SAP lgs1 lines that
showed low germination stimulation and emergence in the laboratory
bioassays endured low Striga infestation under field conditions.
The genotypes PI533976, PI656094 and PI655979, had Strigaemergence numbers comparable to the resistant check SRN39. A closer look
at the number of Striga emergence in SAP lgs1 showed
concurrence with other Striga resistant sorghum. For example, the
number of emerging Striga in Framida and IS9830 were like those
of SAP lgs1 PI533976, PI656094 and PI655979. Serendipitously
discovering PI656054, an LGS1 sorghum with pre-attachment
resistance that appears to be independent of the SL signaling pathway,
opens new avenues for Striga resistance studies. Once validated
phenotypically, the accession can be used as donors to incorporate new
diversity into breeding lines.
Another important Striga management strategy is tolerance –
ability of a crop to produce yield even under Striga infestation
(Mwangangi et al. 2021). This trait is well exemplified by N13, a
popular Striga resistant and tolerant durra sorghum from eastern
Africa (Rodenburg, Bastiaans, Weltzien & Hess 2005). Identification of
potentially tolerant SAP lgs1 has important implications because
researchers are now advocating for a combination of resistance and
tolerance in an integrated Striga management approach. We found
that most SAP lines could produce yields even under reasonableStriga infestation. Particularly, SAP lgs1 accessions:
PI656094 and 656096 are good candidates for deployment as tolerant
accessions. Although the mechanism of Striga tolerance has not
been fully explored, one can extrapolate a hypothesis based on the
adverse effects of Striga parasitism on growth retardation and
yield loss. ABA is a critical hormone for controlling plant responses to
water limitation, inducing stomatal closure to limit water loss through
transpiration (Mittelheuser & van Steveninck 1969). Striga are
insensitive to ABA, resulting in higher transpiration rates than their
hosts and the maintenance of a water potential gradient favoring
nutrient and water transfer to parasites (Fujioka et al. 2019b
a). One possibility is that host insensitivity to ABA could be extended
to imply Striga tolerance if host plants control the water
potential gradient under Striga parasitism via ABA signaling.
Because there is no validation data for this hypothesis, for now it
remains a subject for further investigation.
Field evaluation provided a further opportunity to study flowering time
and yields of SAP lgs1 lines. Regarding flowering time, all the
accessions were within the time considered as early maturity in sorghum.
This is important because early maturity helps cope with Strigaby reducing infestation. This is also important as a drought coping
mechanism, another major constraint for sorghum production in SSA.
To conclude, we: (i) describe
a simple Striga resistance allele mining assay that could be
adapted for allele discovery in other sorghum collections or
populations, particularly in Africa where sorghum is prone to be
enriched for Striga resistance, (ii) identify 12 newStriga resistant SAP lgs1 accessions that can be
integrated in breeding programs in SSA, and (iii) demonstrate that
identified sorghum accessions provide the additional advantage of early
maturity and tolerance to Striga parasitism.