52. Expression of the rice Wx gene in response to sugars

M. Kun I-eda1, Y. Sano1,2 and H.-Y. Hirano1,3

8657 Japan

     The Wx gene encodes an enzyme, ADPglucose starch glycosyl transferase, which catalyzes amylose synthesis in the rice endosperm. Carbon fixed during photosynthesis is transported as sucrose through phloem to seeds during their maturation. Since sucrose is used as a substrate for the starch synthesis panthway in which it is converted into ADPglucose, genes for starch synthesis are expected to be controlled by sucrose. In this study, we analyzed the effect of sugars on the gene expression of the Wx gene.

     Flowers at anthesis were marked on the lemma (about 10-20 seeds per panicle). After 7 days, the panicles including the marked seeds were cut from plants, put into test tubes that contained various sugars, and incubated at 28°C for 24 hours in the dark. Thus, we exogenously supplied various sugars using the panicle culture method. Total RNAs were extracted from the seeds 7 days after anthesis and analyzed by Northern blotting using Wx cDNA as the probe (Fig. 1). The results indicated that the levels of the Wx transcript were elevated when sucrose, glucose and fructose were supplied, as compared with water. In contrast, the Wx transcript levels from the seeds in the solution of mannitol were as low as those from the seeds incubated in water. Since mannitol or sorbitol are not metabolized in plant cells, these results suggest that the Wx gene is activated by the sugars that are metabolized in the cell. Next, we examined the effect of the concentration of sucrose on the Wx gene activation. The Wx gene transcripts were upregulated by increasing concentration of sucrose (up to 100mM). High concentration (300 mM) of sucrose, however, showed an inhibitory effect: that is, the level of Wx gene transcript in 300 mM sucrose is lower than that of in 50 or 100 mM sucrose, although it is higher than that of 0 mM sucrose (H20). Since the concentration of sucrose in the phloem is known to be 100 mM, our experiments appear to reflect the physiological condition in rice plants. Thus, sucrose transported to the endosperm may be one of the important factors that regulate the expression of the Wx gene.