| To investigate the genetic background and genes for rice root growth under different water supply, three research strategies were performed, including QTLs analysis for upland condition induced root growth; mapping and expression analysis of genes for cell wall extensibility and cell proliferation in rice root tips under different water supply; and detecting upland condition induced genes in root tips using cDNA-AFLP analysis. The results are summarized as follows:1. QTLs and epistatic effect on SRL, MARL, ARN, TRDW, R/S and DSRL were detected based on an RI population consisting of 140 lines, derived from a cross between a tropic japonica variety Azucena and indica varietiy, ER.1552. Nine QTLs and twenty-six pairs of epistatic loci for the six traits were identified. Among these, four QTLs detected for SRLS, MARLS and TRDWS, explained about 11%, 20% and 23% of total variations, respectively. Two QTLs were found for SRLP and TRDWP, and each accounted for 11% and 13% of total variations. Three QTLs detected for DSRL, explained about 31% of total variations in DSRL. Only epistatic loci were found for MARLP, ARNP, R/SP and R/SS, which explained about 12% to 61% of total variations, respectively. No identical QTL or epistatic loci were found for the root traits in both solution and paper culture. The results indicate that there is a different genetic background responsible for rice seedling root growth under lowland and upland conditions.2. In order to further understand the relationship between cell extension, proliferation and root elongation in rice seedlings under different water supply, genes for cell wall extension (expansins, Xets, Egases and extensin) and the gene for cell proliferation (cyc07) were mapped using the same population as above, and their expression in root tips were analyzed.OsEXPl was maped on chromosome 4 linked with RG190, where some QTLs have been reported for different root/shoot/leaf traits under upland condition in rice. The expression of OsEXPl in root tips under upland condition was higher than under waterlogging condition, which indicated that OsEXPl accelerated root elongation under upland condition. OsEXP2 was mapped on chromosome 1, in the interval flanked by RZ730 and RG109, where some QTLs have been detected for SRLP, DSRL and otherdifferent root traits under upland condition in rice. The separation oiOsEXP2 genotype was associated with segregation of SRLP in the RI population. Northern blot analysis suggests OsEXP2 constitutively express in root tips. For OsEXP3, no polymorphism was detected between the parents. OsEXP4 was located on chromosome 5, in the interval flanked by RZ70 and RZ67, where QTLs have been observed for root traits under upland condition and root penetration ability in rice. OsEXP4 was possibly responsible for enhanced root development under upland condition. Different genes for expansin may be associated with different development stage of roots under upland condition.Three Xet-related genes, S3412, S2134 and S3187, were mapped in this case. S3412, which is believed to be contributable to the seminal root length under upland condition, was mapped on chromosome 8 linked with Al 8Al 120. Its expression in root tips was enhanced by upland treatment. Both S2J34 and S3187 were mapped on chromosome 11 at same loci linked with RG103. The results of Northern blot showed that the expression of S2134 in root tips was suppressed by upland treatment, and no difference was found in the expression of S3187 between the two conditions. Analysis of phylogenetic tree indicated that S2134 and S3187 belonged to one family while S3412 to another. Xet are encoded by a multi-gene family. It is possible that different Xet genes may control different physiological course.One EGase gene clone, S2647, was mapped on chromosome 4 linked with RG163, where QTLs have been reported for root traits under upland condition. But there was no difference in the expression ofS2647 in root tips between upland and...
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