Developing striga resistance in maize through maize-sorghum hybridization

Abstract

Striga hermonthica is an obligate hemi-parasite that belongs to the family Orobanchaceae. Crop infestations by this parasitic weed pose a great threat to agriculture in Sub-Saharan Africa. The most commonly affected crops include cereals such as sorghum, maize, and rice, which are important to subsistent livelihoods in the region through food production. Some of the control measures currently used include weeding and herbicide seed dressing using Imazapyr (StrigAway®) maize and crop rotation with non-host plants. For effective conrol, these strategies will need to be combined with other promising integrated Striga management approaches. This study focused on a Striga resistant sorghum cultivar N-13 whose mechanism of resistance is cell wall fortification by lignin synthesis. In the first approach, cloning of a key lignin biosynthesis gene, Hydroxycinnamoyl transferase (HCT) was carried out. HCT gene has a nucleotide sequence of 1489 bps and codes for an enzyme made of 496 amino acid sequence. The HCT protein consists of two domains, and the active substrate-binding site is located between these two. The probability of successfully hybridizing maize and sorghum was evaluated by reconstructing phylogenetic relationship of Sorghum bicolor HCT protein and its orthologs in the grass family. The Sorghum bicolor HCT protein was aligned with 7 other orthologs and revealed a 92% identity to Zea mays HCT protein. The second approach involved crossing maize inbred line E04 with a Striga-resistant sorghum cultivar N-13. A total of 9 out of 540 maize cobs formed embryos representing a 1.67% success rate. The embryos were rescued and cultured in vitro and after that regenerated. Molecular characterization of hybrids was done using the Polymerase Chain Reaction (PCR). The screening strategy involved amplification of the CEN38, a repetitive marker unique only to sorghum. Hybrid plants expressing CEN38 marker were screened for resistance to S. hermonthica using a soil free laboratory assay. The F1 hybrids had a 74.19% survival rate and a death rate of 25.81%. Sorghum cultivar N-13 served as a positive control for post-Striga germination resistance. The mean count, length and biomass of S. hermonthica plants growing on F1 and F2 hybrids roots were significantly lower compared to the sorghum cultivar N-13 and the susceptible maize inbred line E04 according to Tukey’s HSD test (p<0.05). The hybrids exhibited mechanical resistance against S. hermonthica similar to the sorghum cultivar N-13. This research shows that (i) lignin biosynthesis gene, HCT (1489 bps) from sorghum cultivar N-13 can be isolated and cloned into a vector. Therefore, this gene can be transformed into susceptible host plants to confer resistance to Striga spp. (ii) Maize-sorghum hybrid can be used as a strategy to alleviate yield loss associated with Striga infestation.