This study tested the hypothesis that a post-anthesis moderate soil drying can improve grain filling through regulating the key enzymes in the sucrose-to-starch pathway in the grains of rice (Oryza sativa L.). Two rice cultivars were field grown and two irrigation regimes, alternate wetting and moderate soil drying (WMD) and conventional irrigation (CI, continuously flooded), were imposed during the grain-filling period. The grain-filling rate and activities of four key enzymes in sucrose-to-starch conversion, sucrose synthase (SuSase), adenosine diphosphate-glucose pyrophosphorylase (AGPase), starch synthase (StSase), and starch branching enzyme (SBE), showed no significant difference between WMD and CI regimes for the earlier flowering superior spikelets. However, they were significantly enhanced by the WMD for the later flowering inferior spikelets. The activities of both soluble and insoluble acid invertase in the grains were little affected by the WMD. The two cultivars showed the same tendencies. The activities of SuSase, AGPase, StSase, and SBE in grains were very significantly correlated with the grain-filling rate. The abscisic acid (ABA) concentration in inferior spikelets was remarkably increased in the WMD and very significantly correlated with activities of SuSase, AGPase, StSase, and SBE. Application of ABA on plants under CI produced similar results to those seen in plants receiving WMD. Applying fluridone, an indirect inhibitor of ABA synthesis, produced the opposite effect. The results suggest that post-anthesis WMD could enhance sink strength by regulating the key enzymes involved, and consequently, increase the grain-filling rate and grain weight of inferior spikelets. ABA plays an important role in this process.

The WTH3 gene was obtained by a DNA fragment isolated by the methylation-sensitive representational difference analysis technique due to its hypermethylation in the human multidrug resistant (MDR) breast cancer cell line MCF7/AdrR. The WTH3 gene product is 89% and 91% identical to the human Rab6 and Rab6c proteins, but possesses an elongated C-terminal region which contains 46 extra amino acids. Nevertheless, we consider the WTH3 gene a new member of the Rab6 gene family. Semi-quantitative reverse transcriptase-polymerase chain reaction results showed that WTH3 was 15 and 4 times downregulated in MCF7/AdrR and MES-SA/Dx5, a human MDR uterine sarcoma cell line, as compared to their non-MDR parental cell lines. Permanent expression of the WTH3 transgene in MDR cell lines increased to varying degrees their sensitivity to several anticancer drugs, which included doxorubicin, taxol, vinblastine, vincristine, and etoposide, as compared to the control sublines transfected with the empty vector. Flow cytometry and fluorescence microscope experiments suggest that the WTH3 transgene stimulated the host's uptake and retention of DOX. Our results imply that the WTH3 gene plays a role(s) in MDR phenotype development in vitro.

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