A role for strigolactones in the genetic variation in tolerance to Striga infection in sorghum

The root parasitic weed Striga hermonthica is a major constraint to cereal production in sub-Saharan Africa. Tolerance to Striga—guaranteeing a reasonable yield despite infection—is potentially a valuable trait to curtail crop yield losses. However, the mechanisms underlying tolerance are not well understood, making selection for the trait difficult. The strigolactones, signalling molecules in root exudates of Striga hosts, play an important role in the Striga infection process as germination stimulants. However, there are indications that they may also play a role in tolerance. In the present investigation, we set out to study tolerance to Striga infection, and the possible role of strigolactones in this, using 20 sorghum genotypes displaying large genetic variation in strigolactone composition. To assess tolerance, we analysed the effects of Striga infection on sorghum on a number of morphological and physiological parameters. Striga infection resulted in genotype-dependent losses in biomass accumulation, a decrease in maximum rate of photosynthesis, carboxylation and photosystem II efficiency and an increase in leaf ABA concentration. The genotypes Aklamoi, Fakimustahi, Korokollow, Feterita Geshaish, Najad and Naten were identified to be sensitive, as they were severely affected in morphological and/or physiological parameters. In contrast, the genotypes Hariri, Tetron, Mogud, Wad Ahmed, Framida, Tabat and Wad Baco were identified to be tolerant, as they displayed only low to moderate responses in the morphological and/or physiological parameters. The genotypes SRN39, IS9830 and Tetron were identified as resistant as they exhibited low Striga emergence. Especially the latter was also less affected by Striga in morphological traits and can thus also be considered tolerant. Tolerant genotypes displayed a higher orobanchol to 5-deoxystrigol and/or sorgomol ratio in their root exudates. The possible relationship between strigolactones and tolerance, as well as the analysis of strigolactones and/or carboxylation efficiency as a tool for the selection of Striga tolerance are discussed.