Salt stress (both salinity and alkalinity/sodicity) is a worsening problem in inland areas in sub-Saharan Africa such as in Mali, Ethiopia, and Burundi because of the buildup of salt as a consequence of the excessive use of irrigation water with improper drainage, coupled with the use of poor-quality irrigation water or sodic soils developed from salt-bearing rocks. Over 20 million hectares of land suited to rice production in Asia are currently either underexploited or unexploited because of excess salt and other related soil problems, and in India and Bangladesh alone, productivity of more than 7 million hectares of rice land is adversely affected by salt stress. Salt-affected areas are predominantly inhabited by impoverished communities with fewer opportunities for food security and livelihood options.
THE STRASA SOLUTION
Identification of agronomic and physiological traits for reproductive stage tolerance
Two testing protocols were used to evaluate tolerance to reproductive stage salinity stress: (1) pot experiments and (2) artificial saline field in L5-L6 field. The project identified concentrations of Na and K, chlorophyll, malondialdehyde, ascorbate, and proline as the most effective components for screening tolerant and sensitive genotypes using the new phenotyping screening technique that uses the perforated pot method. Using this technique, the project screened a total of 100 lines from the 3K rice genome panel for tolerance to reproductive stage salinity in 2018. Filled/unfilled grain number and filled/unfilled grain weight were recorded. Chlorophyll assay was done while NaK analysis is on-going. A total of 50 genotypes from landraces/local varieties from the 3K germplasm and selected elite lines from Salinity IYT 2017DS were screened to confirm tolerance to reproductive stage salinity. Plant height, SES, tiller numbers, % spikelet sterility, and grain weight were collected as agronomic parameters. Results showed that the most tolerant genotypes as indicated by % yield reduction were CHINA 1039 MUTANT (DWARF)::IRGC 24168-2; TAN NONG::IRGC 56126-1; IR 96465-B-BGY 378-104-2-1; DAMODAR IRGC 127299:2013DS; and IR 45427-2B-2-2B-1-1::G1. The most sensitive genotypes with the highest % yield reduction (>95%) were IR29 AND IRRI 154 (sensitive check).
Advance selected families derived from MABC populations to BC2F4
In 2018DS, out of 24 lines screened for reproductive stage salinity tolerance in L5 and L6, six promising lines – IR117839-4-4-22-CMU 1-2-B,IR 17839-4-4-29-CMU 1-2-B, IR 117839-14-9-16-CMU 1-1-B, IR 117839-14-9-22-CMU 2-1-B, IR 117839-22-15-B-CMU 10-1-B, and IR117839-22-18-12-2 – derived from IRRI 154 saltol with yield and maturity ranging from 5 to 6 t/ha and 109 to 118 days, respectively, under normal condition and yield of 1.0-1.3 t/ha (EC > 9 dS/m) under salinized condition were selected. These lines were included for evaluation at the field hotspot in Ajuy, Iloilo, Philippines in 2018WS in moderate and high saline stress conditions. These six lines showed an average yield from 3.1 to 5.8 t/ha under normal condition and 1.4-3.5 t/ha under saline stress condition and tolerance to salinity stress in both seedling and reproductive stages. Moreover, 100 salinity tolerant lines were shared with the IRRI Africa Hub for evaluation for seedling stage and reproductive stage salinity stress.
Gene discovery and MAS
Phenotyping capacity for early seedling stage salt tolerance has been enhanced at the AfricaRice Irrigated Lowland Breeding Unit in Senegal to more than 5,000 lines per year. Rapid and reliable evaluation of landraces for tolerance to salinity at the seedling stage was conducted at Mbe, Cote d'Ivoire using high-throughput phenotyping facilities. SSR and SNP analyses were done to confirm differences in alleles controlling tolerance in new donors and that of Saltol. The Phenomics of Rice Adaptation and Yield (PRAY) indica panel of 300 accessions screened at Ndiaye, Senegal for seedling and reproductive stage salt tolerance under field conditions at 4 dSm-1, and 60% of these accessions had a yield of <1.0 tons/ha. However, the top 5% of the accessions (14 entries) had yields of >3.0 tons/ha (3.06 - 4.7 tons/ha). Separately, three different mapping populations using new salt tolerance donors (Sahel 317 x Madina koyo, Sahel 329 x Madina Koyo, and Sahel 329 x NERICA-L-9) were phenotyped in three sites (Senegal, Cote d'Ivoire, and Serra Leone) at both vegetative and reproductive stages. Allele discovery work is ongoing using DArT seq with >50k SNPs. Ninety-six (94) salt-tolerant lines
from the IRRI HQ sourced from IRRI (30 lines), Philippines (23 lines), Bangladesh (3 lines), Myanmar (4 lines), Vietnam (1 line), India (3 lines), and Mozambique (30 lines), as well as five from Kenya as local checks were screened in Hola/Bura, from which 18 salt-tolerant lines were selected. Thirty-five (35) multi-QTL marker-assisted introgression lines (Saltol, Sub1 and AG) were received from IRRI and evaluated in both Senegal and Cote d'Ivoire, of which 14 lines have been selected for advancement. QTL discovery for salt tolerance is ongoing as part of a PhD thesis in collaboration with KAUST University in Saudi Arabia.
Large-scale control facility
The project established a semi-high-throughput facility for screening for seedling stage salinity tolerance in 2016 where 18,374 different genotypes/landraces from Sri Lanka, Indonesia, Myanmar, and India including the 3K rice genome panel were evaluated. A total of 112 accessions were found tolerant under EC 12 and EC 18 dS/m which will be used as new sources of tolerance for salinity stress. These can also be used as donor parents in breeding programs for salinity tolerance. The development of mapping population can help in the identification of new gene(s)/QTL(s) responsible for salinity tolerance.
Salt-tolerant rice varieties offer great potential to grow rice in marginal lands, which are usually left fallow particularly during the dry season because of high salinity. The area under this type of land is quite substantial—approximately 0.83 million hectares in Bangladesh and over 6.7 million ha in India. In West Africa, an estimated 1.5 million hectares of cultivable mangrove swamps are affected by salinity (Jones 1986).
Evaluation of multi-QTL marker-assisted introgression process
The protocol for the simultaneous introgression of multiple QTLs was developed and verified in several backgrounds, focusing on NSIC Rc222 and also in Makassane-Sub1, IRRI 171, IRRI 110, IRRI 139, and MS11. Key innovations include the use of the crossing methodology, allowing the production of over 2000 BC-F1 seeds from a single selected F1 plant. This enables the introgression of a large number of QTLs simultaneously, in no extra time, rather than just one or two QTLs. These methodologies have been adopted and were used in the QTL deployment group to speed up the delivery of other genes such as in priority disease resistance and grain quality loci. This milestone has significant synergy with the QTL fingerprinting tool, which gives insight into additional loci that can be targeted in the MAS populations.
Incorporation of diagnostic marker information into a QTL fingerprinting database tool
The project developed a diagnostic tool where causative alleles for 27 QTLs were identified and incorporated into a fingerprinting tool. Fingerprinting capacity expanded to 50 QTLs and counting, including many abiotic stress, biotic stress, grain quality, and agronomic QTLs. This tool provides breeders a comprehensive view of the major genes found in their elite material, which in turn enables better crossing decisions and more effective breeding.
Identification of candidate genes for at least two QTLs other than SalTol based on whole-genome sequencing
Genome sequences were obtained for six donors and five recipient lines used in the mapping of salinity tolerance QTLs and accurate marker systems for a range of QTLs were developed. However, results from the QTL validation studies showed many previously reported QTLs had dubious validity. Studies to verify these QTLs yielded at least one locus that was consistent across backgrounds and salinity stress levels; efforts were undertaken to deploy this into NSIC Rc222 as an elite recipient variety and this was taken to BC5 level. Candidate genes for this QTL were identified and several were cloned, including into binary vectors, but funding and time were insufficient to conduct transgenic validation. Fine-mapping is underway as an alternative strategy for identifying candidate genes for this locus.