Developing of new stress-tolerant rice varieties for the Mediterranean region

Abstract

[eng] Rice (Oryza sativa) is the most important crop for human direct consumption, but its yield and production are strongly affected by biotic and abiotic stresses. Rice is the most salt-sensitive cereal and in addition, salinity is a rising problem around the world reinforced by the climate change effects such as rise of the sea level, soil degradation and water scarcity. Regarding biotic stresses, the apple snail (Pomacea sp.) is one of the worst introduced pest that affects rice production. It has been detected for first time in Europe in Spain, in 2009 in Ebro River Delta, destroying rice fields at seedling stage. Additionally, rice blast, caused by an ascomycete called Pyricularia oryzae, is a disease that strongly affects the rice production worldwide causing yield losses that range from 15% to 50%, even using specific fungicides. The main objective of this thesis is to develop new stress-tolerant rice varieties for the Mediterranean region, by the introgression of the Saltol QTL and new blast resistance genes to Mediterranean local varieties. A molecular marker assisted backcross scheme (using KASP technology) was followed to introgress the salt tolerance traits. The Saltol donor varieties were FL478 and IR64-Saltol, two salt-tolerant Asiatic indica rice lines, while the recurrent parental lines were PL12 and PM37, two Mediterranean japonica rice lines. BC3F3 plants were tested for salt tolerance in hydroponic assays. The standard evaluation system (SES) described by IRRI was used to evaluate the lines. Relative chlorophyll content (RCC, measured with a SPAD), fresh weight and plant length was also recorded. Additionally, two consecutive years of field assays were performed in Ebro River Delta using different foreign and local lines (and their hybrids), to evaluate their general performance and rice blast resistance. From the 4 crosses combination performed between both Saltol donors and both Mediterranean recurrent parents, PL12 x FL478 (LP cross) and PM37 x IR64-Saltol (MS) were selected to proceed with the whole backcrossing process, to determine the return to the recurrent parent genome and to obtain the homozygous Saltol BC3F4 seeds. For the hydroponic assays, 54 BC3LPF3 lines were initially tested since a high variability between lines was observed. From them, some lines like LP-3, LP-15 or LP-17, performed similar or even better in the SES than the salinity donor FL478 line. The RCC data showed again a high variability between lines, although RCC did not correlate with SES results. The data analysis was hindered due to the differences scored between replicates and the fact that much of them were totally dead at the end of the assay. A certain degree of heterozygosity may explain the variability found between replicates during the SES evaluation. The fresh weight (FW) and the plant length in both shoot and root was strongly affected by the salinity treatment. However, the reduction was higher in shoot than in root. Finally, the blast tolerance field assays were severely affected by the rice stem borer Chilo suppressalis, both years. The evaluation for rice blast tolerance was really difficult and no outstanding line was selected to proceed. In conclusion, the Saltol QTL has been successfully introgressed in two Mediterranean japonica rice varieties, although more replicates of the hydroponics assays must be performed to confirm and select the most salt tolerant obtained lines. These lines will be tested in 2018 and 2019 in field assays, under salinized and no salinized conditions. Regarding rice blast resistance field assays, no conclusive results were achieved. More field assays must be done, and other lines should be tested. In order to reduce C. suppressalis infestations, more phytosanitary actions should be taken in the future.