No abstract

Witchweeds (Striga spp.) greatly limit production of Africa’s most staple crops. These parasitic plants use strigolactones (SLs) – chemical germination stimulants, emitted from host’s roots to germinate, and locate their hosts for invasion. This information exchange provides opportunities for controlling the parasite by either stimulating parasite seed germination without a host (suicidal germination) or by inhibiting parasite seed germination (pre-attachment resistance). We sought to determine genetic factors that underpin Striga pre-attachment resistance in sorghum using the genome wide association study (GWAS) approach. Results revealed that Striga germination was associated with genes encoding hormone signaling functions e.g., the Novel interactor of jaz (NINJA) and, Abscisic acid-insensitive 5 (ABI5). This pointed toward abscisic acid (ABA) and gibberellic acid (GA) as probable determinants of Striga germination. To test this hypothesis, we conditioned Striga using: ABA, ABA + its inhibitor fluridone (FLU), GA or water. Unexpectedly, Striga conditioned with FLU germinated after 4 days without SL. Upon germination stimulation using sorghum root exudate or the synthetic SL GR24, we found that ABA conditioned seeds had above 20-fold reduction in germination. Conversely, FLU conditioned seeds recorded above 20-fold increase in germination and conditioning with GA caused at least 1.5-fold reduction in Striga seed germination. Germination assays using seeds of a related parasitic plant (Alectra vogelii) showed similar degrees of stimulation and reduction of germination by the hormones further affirming the hormonal crosstalk. Our findings have far-reaching implications in the control of some of the most noxious pathogens of crops in Africa.

BACKGROUND Pre‐attachment resistance to the parasitic plants Striga hermonthica and S. asiatica occurs in sorghum mutants designated low germination stimulant 1 (lgs1). However, only a few of these mutants have been identified and their resistance validated. Additionally, pre‐attachment resistance in sorghum beyond lgs1 mutants has not been explored. We used lgs1‐specific markers to identify new lgs1‐like mutants in a diverse global sorghum collection. The sorghum collection was also evaluated for pre‐attachment resistance against Striga using an in vitro assay that measured Striga germination activity and radicle growth. RESULTS From a total of 177 sorghum accessions, 60 recorded mean germination levels of below 42%, which is comparable with the previously identified lgs1‐like sorghum (SRN39 and IS9830) used as controls in this study. Furthermore, 32 of these accessions recorded Striga radicle lengths comparable or lower than the controls (0.42 mm). Thirty‐eight accessions contained the lgs1 mutation and although overall, lgs1 mutants had considerably reduced Striga germination, some low inducers of Striga germination were wild‐type for lgs1. Germination was positively but weakly correlated with radicle length pointing to additional radicle growth inhibitory activity. CONCLUSIONS lgs1 mutations, alongside other mechanisms for low Striga germination stimulation, are prevalent in sorghum, and poor Striga radicle growth is suggestive of host‐derived inhibition. As an outcome, our study makes available multiple Striga‐resistant sorghum with adaptability to diverse agro‐ecological regions in sub‐Saharan Africa making immediate deployment possible. © 2021 Society of Chemical Industry