Marker Assisted Introgression of Striga Resistant and Drought Tolerant Quantitative Trait Loci into Popular Ethiopian Sorghum Cultivars
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Date
2021-03-30
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Addis Ababa University
Abstract
Sorghum (Sorghum bicolor (L.) Moench) is the fifth important dryland cereal crop worldwide, and a dietary staple food for more than half a billion poor and food-insecure people dwelling in the semi-arid tropics of Africa and Asia. Sorghum is one of the strategic crops in Ethiopia, which provides more than one third of the cereal diet and is almost entirely grown by subsistence farmers to meet needs for food, feed, income, beverage, house and fuel purposes. Despite the multiple uses, its productivity is constrained at least partly by the intense Striga infestation and frequent drought, which causes up to 100% crop loss. The possible approaches to overcome the two prevailing constraints include genetic manipulation of the crop and the growing environment. The use of host-plant resistance offers a potentially cost-effective and environmentally sound complementary management strategy for incorporation into integrated management of Striga and drought.
The studies were, therefore, designed to introgress Striga resistance and drought tolerance genes/quantitative trait loci (QTLs) into popular and farmer-preferred cultivars through marker assisted backcrossing (MABC). One-hundred-eighteen and 61 backcrossed lines were developed for Striga and drought, respectively. The 118 backcrossed lines and 15 parental lines were evaluated for Striga resistance based on low Striga seed germination stimulant activity in the agar gel assay (AGA) and under field condition. The AGA study was conducted at Holetta national agricultural biotechnology research center (NABRC) arranged in completely randomized design with four replications, while the field evaluation was conducted at two selected Striga hotspots and two non-infested locations layout in -lattice design with triplicates. The 61 drought tolerant converted and nine parental lines were evaluated under two-contrasting (optimum vis-à-vis stress) environments in -lattice design with three replications. The well-watered experiment was irrigated throughout the season, while the water-limited trial was irrigated well during the early growth stages with irrigation withheld at anthesis in order to allow moisture stress during grain-filling stage. The results from the AGA indicated that genotypes significantly differed (P<0.01) for their low production of Striga germination stimulant activity (lgs). The mean maximum germination distance (MGD) and germination rate (GR) ranged from 0.0 mm to 29.45 mm and 0.0% to 72.4% in the agar, respectively. Of the 118 backcrossed lines, 22.9% showed less than 10 mm of MGD and GR of <30%, revealing provision of low germination stimulant/strigolactones production (lgs). There were significant positive (r = 0.4-0.81) correlations showing the roles of these parameters as selection criteria in breeding for resistance. The existence of higher heritability (h2b = 77-83%) and genetic advance (GA = 62-93%) for the germination parameters indicated possibilities for improving resistance against Striga through selection.
The results from the field evaluation also revealed that genotypes significantly differed (P<0.01) for both agronomic traits and reaction to Striga. About 21.2% of the backcrossed lines were promising with relatively good level of resistance to Striga and yield ranging from 773.3 to 1220 kg ha-1. Grain yield showed highly significant negative associations with all the studied Striga characters indicating severe damage. The heritability and genetic advance ranged from 37.7 to 88.7% and 7.72 to 52.9%, respectively, in which their values are greater for agronomic than Striga resistance traits. The aspects of interaction with heterogeneity of natural Striga infestation, pathogenicity, and concomitant environmental effects may have increased genotype by environment interactions resulted relatively low heritability.
Genotypes that carry different QTLs showed different capacity to produce Striga germination stimulants in the AGA and agronomic performances under natural Striga infestation. The combined effect of both QTL (lgs2_SBI-05_60404021 and lgs_3_60629027) at a time showed lower Striga germination stimulant activity and better field resistance indicating existence of possible cumulative effects. The pre-attachment traits (MGD and GR) based on the AGA and emerged Striga under field condition showed significant positive relationships. This indicated that the AGA proved to be a useful, precise and fast
indirect selection method to screen for sorghum lines with the low production of Striga germination stimulants resistance mechanism.
The analysis of variance (ANOVA) for the stay-green (drought tolerant) experiment revealed that the developed backcrossed lines significantly differed for most of the agronomic and physiological characters. The mean grain yield of the genotypes under full-water (2069.2 to 5704.2 kg ha-1) and water-limited (923 to 4585 kg ha-1) conditions were 3633 and 1991 kg ha-1, respectively. Out of the 61 backcrossed lines, only 9.8% showed superior performance in yield ranging from 2831-4585 kg ha-1, under water-limited condition indicating the potential to withstand drought after anthesis. They were also characterized by high chlorophyll content, leaf area and greenness at physiological maturity. Relatively high heritability (35.4-79.5%; 35.1-76.3%) and genetic advance (1.7-35.5%; 1.4-42.7%) was obtained for most agronomic and physiological characters under full-irrigation and water-limited regimes, respectively, indicating that selection for such characters could be fairly easy. Grain yield showed significant positive association with chlorophyll content (r = 0.24 & 0.45), green leaves (r = 0.31 & 0.29) and leaf area (r = 0.29 & 0.27) both at booting and maturity under the former and later moisture regimes, respectively. This indicated that the presence of greater green leaf area with high chlorophyll content at booting and maturity contributed higher photosynthesis, better availability of food reserves for grain-filling and enhanced yield.
Of the 13 groups of QTLs, the cumulative effect of stg1+stg2+stg3a+stg4, stg1+stg2+stg3a+stg3b, stg1+stg3a+stg3b+stg4, and stg2+stg3b+stg4 showed overall superior performance in most of the agronomic and physiological characters under both moisture regimes.
Thus, the study provided useful evidence that the MABC transfer of lgs and drought tolerance genes from donor into popular cultivars have the potential to enhance tolerance/resistance to Striga and drought in sorghum, respectively, and could be used as noble tools supporting effective future sorghum breeding.
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Keywords
Marker Assisted, Introgression, Striga Resistant, Drought Tolerant, Quantitative Trait Loci, Popular Ethiopian, Sorghum Cultivars