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Salinity is the second most important abiotic stress after drought that hampers rice production, especially in south and Southeast Asia. Breeding approach supplemented with molecularmarkers-assisted selection is the most promising approach in terms of efficiency to increase the productivity under salt-affected soils. Thirty-day-old rice seedlings of 300 F recombinant-inbred lines derived from a cross between the salt sensitive, IR29 (indica), and a salt tolerant, Hasawi (aus), were used to identify quantitative trait loci (QTLs) linked to salinity tolerance. One hundred and ninety four polymorphic SNP markers were used to construct a genetic linkage map involving 142 selected RILs that covered 1441.96 cM genome with an average distance of 7.88 cMbetween loci. Twenty new QTLs (LOD > 3) were identified on chromosomes 1, 2, 4, 6, 8, 9 and 12 using composite interval mapping with R as high as >20% with LODvalue of 7.21. Many earlier studies reported big qSaltol for seedling stage salinity tolerance in rice is on short arm of chromosome 1 but none of the QTL in our study was on qSaltol or nearby position, therefore, Hasawi conferred salinity tolerance in RILs due to novel QTLs. It is suggested to fine map the novel QTLs so that the level of salinity tolerance could be further enhanced by pyramiding of the different QTLs in one genetic background through marker-assisted selection.

Rice yellow mottle virus (RYMV) is a major biotic constraint for rice production in Africa. The resistance-breaking ability of Tanzanian RYMV strains and phylotypes (S4lm (Tz526), S4lv (Tz516), S4ug (Tz508), S5 (Tz429, Tz445), S6c (Tz486) and S6w (Tz539)) were tested by inoculating rice cultivars with RYMV1 resistant alleles (Gigante (rymv1-2), Tog12387 (rymv1-3), Tog5681 (rymv1-3), Tog5438 (rymv1-4), Tog5672 (rymv1-4+rymv2) and Tog5674 (rymv 1-5)) in a screen house. The results revealed multiple resistance-breaking strains and phylotypes on resistant cultivars Gigante, Tog12387, Tog5438 and Tog5681. However, the resistance breakdown was highly variable depending on the strain used, and disease severity ranged from 11% -75.3%. The virulence potential of RYMV phylotype S4lm (Tz526) was similar to phylotype S6w (Tz539). The impact of strains and phylotypes on yield and its components in rice cultivars revealed highly significant differences (P ≤ 0.001). The lowest percent plant height reduction (2.8%), number of tillers per plant (2.5%), 1000 grain weight (2.7%), spikelet sterility (3.5%) and yield (5%) was recorded in rice cultivar Gigante inoculated with RYMV phylotype S6c (Tz486). Phylotype S6c (Tz486) despite being less virulent compared to other strains, its virus titer in rice cultivar Gigante (1.833) was higher than S5 (Tz429, Tz445) inoculated on Tog5674 (0.171, 0.207) and S6w (Tz539) inoculated on Tog5681 (0.283). The resistant-breaking strain S5 (Tz445) multiplied in resistant rice cultivar Tog5674 without inducing visible symptoms but showed positive reaction to ELISA with low virus titer. The strain S5 overcame wide range of resistant alleles including rymv1-2, rymv1-3, rymv1-4 and rymv1-5 resistance, with exception of rymv1-4 + rymv2. The current results gave a new perspective for future identification of resistance-breaking mutations through sequencing of the RYMV genome in infected rice cultivars and mutagenesis of an infectious viral clone useful for future RYMV resistant breeding programs.How to cite this paper: Hubert, J., Lyimo, H

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Summary The host response to Meloidogyne javanica infection of selected Oryza sativa and NERICA (New Rice for Africa) genotypes that are commonly grown in Tanzania and East Africa was examined. The O. sativa genotypes ‘Komboka’ and ‘Supa’ appeared to be partially resistant. A further experiment showed that both genotypes are also partially resistant to M. graminicola infection. The host response of ‘Komboka’ and ‘Supa’ to M. javanica and M. graminicola was further examined and compared in indoor growth chamber conditions. The genotypes ‘TOG5674’, ‘TOG5675’, ‘CG11’ and ‘CG14’ (both O. glaberrima) were included as the resistant reference genotypes for M. graminicola and the genotype ‘UPLRi-5’ (O. sativa) was included as the susceptible reference genotype for both species of root-knot nematodes. Meloidogyne graminicola was more aggressive on ‘Komboka’ and ‘Supa’ than M. javanica. Significantly less root galling was observed on ‘Komboka’ and ‘Supa’ than on ‘UPLRi-5’. In ‘Komboka’ and ‘Supa’ significantly fewer second-stage juveniles (J2) were able to penetrate the roots, to develop into adult females and to reproduce compared with ‘UPLRi-5’. Differential emigration of J2 from the roots of ‘Komboka’ and ‘Supa’ compared with ‘UPLRi-5’ contributed to the observed partial resistance in these genotypes to M. graminicola and M. javanica. Nematodes that successfully penetrated and developed in ‘Komboka’ and ‘Supa’ showed aberrant phenotypes. ‘Supa’ and ‘Komboka’ may be recommended for use by farmers in M. javanica and M. graminicola-infested fields.

This study aimed at optimizing the immunocapture (IC) and simple-direct-tube (SDT) -reverse transcriptase polymerase chain reaction (RT-PCR) techniques for detection ofThe results also show that these techniques are rapid methods for characterization of RYMV strains and may be recommended for use soon after periodical surveys to quickly identify new strains for breeding purposes at low cost. However, SDT protocol was easier and faster than IC and it was also cost-effective in terms of reagents for the detection of RYMV.

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