Drought Tolerance and Application of Marker-Assisted Selection in Sorghum

Sorghum is an important staple food crop in drought prone areas of Sub-Saharan Africa, which is characterized by erratic rainfall with poor distribution. Sorghum is a drought-tolerant crop by nature with reasonable yield compared to other cereal crops, but such abiotic stress adversely affects the productivity. Some sorghum varieties maintain green functional leaves under post-anthesis drought stress referred to as stay-green, which makes it an important crop for food and nutritional security. Notwithstanding, it is difficult to maintain consistency of tolerance over time due to climate change, which is caused by human activities. Drought in sorghum is addressed by several approaches, for instance, breeding drought-tolerant sorghum using conventional and molecular technologies. The challenge with conventional methods is that they depend on phenotyping stay-green, which is complex in sorghum, as it is constituted by multiple genes and environmental effects. Marker assisted selection, which involves the use of DNA molecular markers to map QTL associated with stay-green, has been useful to supplement stay-green improvement in sorghum. It involves QTL mapping associated with the stay-green trait for introgression into the senescent sorghum varieties through marker-assisted backcrossing by comparing with phenotypic field data. Therefore, this review discusses mechanisms of drought tolerance in sorghum focusing on physiological, morphological, and biochemical traits. In addition, the review discusses the application of marker-assisted selection techniques, including marker-assisted backcrossing, QTL mapping, and QTL pyramiding for addressing post-flowering drought in sorghum.

Identification and Validation of QTLs for Macronutrient Contents in Brown and Milled Rice Using Two Backcross Populations between Oryza sativa and O. rufipogon

Mineral malnutrition as a prevalent public health issue can be alleviated by increasing the intake of dietary minerals from major staple crops, such as rice. Identification of the gene responsible for mineral contents in rice would help breed cultivars enriched with minerals through marker-assisted selection. Two segregating populations of backcross inbred lines (BIL) were employed to map quantitative trait loci (QTLs) for macronutrient contents in brown and milled rice, BC1F5, and BC2F4:5 derived from an interspecific cross of Xieqingzao B (Oryza sativa) and Dongxiang wild rice (O. rufipogon). Phenotyping the populations was conducted in multiple locations and years, and up to 169 DNA markers were used for the genotyping. A total of 17 QTLs for P, K, Na, Ca, and Mg contents in brown and milled rice distributed on eight regions were identified in the BC1F5 population, which is explained to range from 5.98% to 56.80% of phenotypic variances. Two regions controlling qCa1.1 and qCa4.1 were validated, and seven new QTLs for Ca and Mg contents were identified in the BC2F4:5 population. 18 of 24 QTLs were clustered across seven chromosomal regions, indicating that different mineral accumulation might be involved in common regulatory pathways. Of 24 QTLs identified in two populations, 16 having favorable alleles were derived from O. rufipogon and 10 were novel. These results will not only help understand the molecular mechanism of macronutrient accumulation in rice but also provide candidate QTLs for further gene cloning and grain nutrient improvement through QTL pyramiding.

QTL underlying iron toxicity tolerance at seedling stage in backcross recombinant inbred lines (BRILs) population of rice using high density genetic map

Fe is a trace element considered to be essential for rice, and it drives several metabolic processes. Fe toxicity occurs due to excessive Fe ions (Fe2+) and which, disturb cellular homeostasis and dramatically reduces the rice yield. A set of 118 BRILs made from a cross of japonica cv.’02428’ and indica ‘Changhui 891’ was used with high density bin map constructed by using high quality SNP to identify the QTL for Fe toxicity tolerance. As a whole total of 23 QTL were identified for various seedling traits, 3 under control with phenotypic difference ranging from 14.21% to 62.46%, 11 QTL under stress with phenotypic difference ranging from 7.89% to 47.39% and 9 under stressed/control ratio with phenotypic variance ranging from 9.17% to 183.50%. LOD values of QTL ranging from 4.05 to 17.04 in control, 3.41 to 8.09 in stress and 2.84 to131.63 in stress/control ratio. Shoot length (SL), root length (RL), shoot fresh weight (SFW), root fresh weight (RFW), shoot dry weight (SDW), and root dry weight (RDW), were used to estimate the degree of Fe tolerance. Many stable QTL, qSSDW-4, qSSDW-6, qRSDW-4 and qRSDW-6 affecting SDW were detected and beside this some new QTL, qRSFW-1, qRRFW-10 and qRRDW-1 were successfully identified significantly contributing to Fe toxicity tolerance in rice. The results of current study indicated that these novel regions could be transferred via markers assisted section and QTL pyramiding to develop Fe resistant lines in rice.

Aggressiveness of diverse French Aphanomyces euteiches isolates on pea Near-Isogenic-Lines differing in resistance QTL

Aphanomyces root rot is a major disease in many pea-growing regions worldwide. The development of resistant varieties is necessary to manage the disease. Near isogenic lines (NILs) carrying resistance alleles at main validated quantitative trait loci (QTL) were developed by marker-assisted backcrossing (MAB). This study aimed to evaluate the aggressiveness of diverse French isolates of Aphanomyces euteiches on NILs carrying different resistance QTL. Fourty-three A. euteiches isolates from different French pea-growing regions were tested for aggressiveness on eight NILs carrying single or combinations of resistance QTL and two susceptible or resistant control lines, in controlled conditions. Three clusters of isolates, unrelated to geographical origin, were identified, including 37%, 56% and 7% of isolates with high, moderate and low average level of aggressiveness, respectively. Three groups of pea lines were also identified, including the pea resistant control line as moderately to highly resistant to all the isolates, five NILs carrying a major-effect resistance allele at QTL Ae-Ps7.6 with medium to broad range of effects on the isolates and three NILs carrying minor-effect resistance alleles with a narrow range of effects on the isolates. Results suggest that highly aggressive isolates naturally occur, that may be selected by partially resistant pea lines carrying QTL and increase the risk of erosion of QTL effect. QTL pyramiding strategies for a higher level and a broader range of effect of quantitative resistance on A. euteiches populations will be required for breeding for durable pea resistant varieties.

Stay-Green QTLs Response in Adaptation to Post-Flowering Drought Depends on the Drought Severity

Stay-green trait enhances sorghum adaptation to post-flowering drought. Six stay-green backcross introgression lines (BILs) carrying one or more stay-green QTLs (Stg1-4) and their parents were characterized under non-stress (W100: 100% of soil field capacity (FC)) and two levels of post-flowering drought (W75: 75% FC; W50: 50% FC) in a controlled condition. We aimed to study the response and identify the drought threshold of these QTLs under different levels of post-flowering drought and find traits closely contributing to grain yield (GY) under different drought severity. W50caused the highest reduction in BILs performance. From W100to W50, the GY of the recurrent parent reduced by 70%, whereas that of the BILs reduced by only 36%. W75and W50induce different behavior/response compared to W100. Harvest index contributed to the GY under the three water regimes. For high GY under drought transpiration rate at the beginning of drought and mid-grain filling was important at W75, whereas it was important at mid-grain filling and late-grain filling at W50. Stay-green trait can be scored simply with the relative number of green leaves/plants under both irrigated and stress environments. QTL pyramiding might not always be necessary to stabilize or increase the GY under post-flowering drought. The stay-green QTLs increase GY under drought by manipulating water utilization depending on drought severity.