Screened For Stress Tolerance And QTL Mapping In Japonica Backcross Lines

Abiotic stresses such as salinity, cold and heat are major threat to rice production. ElevenBC2F4backcross introgression populations derived from crosses between a japonica varietyChaoyou1as the recurrent parent and11donor parents collected from different countries werescreened for salt tolerance (ST) at the seedling and from tillering to maturity stages, heattolerance (HT) at flowering stage and cold tolerance (CT) at booting stage. The targetintrogression lines (ILs) were evaluated for yield and its related traits under normal and stresscondition to dissect physiological mechanisms of stress tolerance and map QTL for CT. Themain results were described as follows.1. Eleven BC2F4backcross introgression populations were screened for salt tolerance (ST)at seedling stage. One hundred and twenty-five (125) ILs were selected on the basis of singleplant SDS under salt water (140mmol·L-1NaCl).12,11,21and10ILs selected from four ofthe BC populations including donors Chouyou1/X21, Chouyou1/Q5, Chouyou1/Chhomrongand Chouyou1/Doddi were evaluated for morphological characteristics and ion concentrationsin replicated experiments under stress and normal conditions. The ILs had increased rootlength, root fresh weight, root dry weight but reduced shoot length, shoot fresh weight, shootdry weight under salt stress. The amplitudes of variations in the root traits were smaller thanthose of shoot traits, indicating that shoots were more sensitive to salt stress. With regard toionic effect, salt stress resulted in accumulation of Na+and reduction of K+in the whole plant.Compared to the recurrent parent, the increase of amplitude of Na+and the reduction of Kwere observably lower for the ILs. Coinstantaneous accumulation of20times more Na+in theshoot after salt stress was observed. Most of the salinity tolerant ILs improve their salinitytolerances through lower shoot Na+concentration and lower root to shoot Na+translocation.2. Eleven BC2F4backcross introgression populations were screened for salt tolerance (ST)at different developmental stages from tillering to maturity. One hundred and fifty (150) ILswere selected on the basis of single plant grain yield (GY) and spikelet fertility (SF) under saltwater (140mmol·L-1NaCl). Our results clearly showed that there was a rich hidden geneticvariation for ST in the donor lines and phenotypic selection under stress condition was apowerful way to identify transgressive segregaantis from the segregating BC populations.Further evaluation of126ILs selected from four BC populations in replicated experimentsunder stress and normal conditions identified10promising ILs with greatly improved ST and higher yield than CY1. Selection under salt stress for yield and SF, supplemented by acombination of desirable secondary traits, can greatly improve the efficiency and accuracy ofbreeding for high salt-tolerance. It is important to point out that three mechanisms includingosmotic adjustment, salt exclusion and tissue tolerance appeared to function together andaffect the same suite of ST related traits in the ILs; although their contributions to specific STtraits were very different for different ILs depending on the specific scenarios of stress.3. Eleven BC2F4backcross introgression populations were screened for heat tolerance(HT) at flowering stage. Total of124ILs were selected, based on single plant SF under hightemperature (38℃). The results showed that the ILs had better tolerance to heat (HT) than therecurrent parent, Chaoyou1, and the frequencies of plants with higher HT were higher in thepopulations derived from japonica donors were higher than those from indica ones,suggesting that japonica varieties were better sources of desirable genes for improving HT.Progeny testing of the124ILs under the HT stress and normal conditions revealed that64.5%(80) of the lines had better HT than the RP, as measured by SF. The124selected ILspresented wide segregations for other measured traits as well. Eight ILs with greatly improvedHT and higher yield than CY1were selected. HT plants selected from three pyramidingpopulations had an average SF of more than80%, which was significantly higher than the RPand the parents used in pyramiding. One hundred and six (106) plants with significant higherHT than the pyramiding parental lines were selected4. ILs from the eleven diverse donors were screened for CT at the reproductive stageunder the low temperature (LT) treatment created by irrigating the plants with cold water(19℃) at the reproductive stage. Three hundred and twenty four (324) BC2F5introgressionlines (ILs) were selected based on single plant SF. Progeny testing of the324ILs under thesimilar LT stress revealed that the efficiency of selection for CT was0.794. Further evaluationof116ILs selected from five BC populations in replicated experiments under LT and normalconditions identified18promising ILs with greatly improved CT and yield than CY1.Detailed comparisons between the ILs and CY1were conducted for yield and related traitsunder the LT stress and non-stress conditions to gain useful information and betterunderstanding of important issues related to breeding for CT using introgression breedingsuch as donor selection, selection efficiency, screening method and genetic drag fornon-target traits in the BC breeding process.5. ILs were assessed to identify cold-tolerance quantitative trait loci (QTL) from thedonors including Chhomromg, Doddi and Fengaizhan1in the genetic background of japonicacultivar Chaoyou1. X2test was used for QTL identification for the CT-selected ILs, meanwhile, random populations were assessed to identify CT-QTL by single marker analysismethod. Total of29QTL associated with CT were detected by the selected ILs and16identified by single marker analysis. The CT-QTL were classified into two types, one are eightQTL affecting trait difference for seed setting rate of stress to control, which distributedchromosomes3,5~8and the alleles at the QTL improved trait stability under stress andactually contributed to CT. The second type is the three common QTL detected by theselected ILs from different populations with less negative chance of QTL detection, whichdistributed chromosomes2,3and9. Furthermore, most of the QTLs identified in this studywere consistent with those documented. Therefore, QTL detection using selected ILs based onwell-known X2test is a simple and effective. previously by other studies.. The use ofintrogression populations in QTL identification and selection for outstanding lines providesan efficient way for integrating gene discovery and practical breeding and as a result speed upthe application modern molecular techniques in breeding for complex traits.The present study indicated that transgressive segregations were observed for ST, HTand CT in most of the backcross populations, indicating that all the11donors possesfavorable ‘hidden’ alleles for tolerances to the studied stresses. There were significantdifferences among populations in selection efficiency, reflecting stress tolerance depends notonly on donor itself but also on different combinations of the recurrent parent and the donorparent. The stress tolerant ILs will be partially a platform and valuable materials for favorableallele mining and improvement of stress tolerance by pyramiding of favorable genes.