Sucrose-Induced Accumulation of β-Amylase Occurs Concomitant with the Accumulation of Starch and Sporamin in Leaf-Petiole Cuttings of Sweet Potato

Abstract

ABSTRACT,8-Amylase of sweet potato (Ipomoea batatas L.), which constitutes about 5% of the total soluble protein of the tuberous root, is absent or is present in only small amounts in organs other than the tuberous roots of the normal, field-grown plants. However, when leaf-petiole cuttings from such plants were supplied with a solution that contained sucrose, the accumulation of 8-amylase was induced in both leaf and petiole portions of the explants. The sucrose-induced accumulation of fl-amylase in leaf-petiole cuttings occurred concomitant with the accumulation of starch and of sporamin, the most abundant storage protein of the tuberous root. The accumulation of fl-amylase, of sporamin and of starch in the petioles showed similar dependence on the concentration of sucrose, and a 6% solution of sucrose gave the highest levels of induction when assayed after 7 days of treatment. The induction of mRNAs for f-amylase and sporamin in the petiole could be detected after 6 hours of treatment with sucrose, and the accumulation of j6-amylase and sporamin polypeptides, as well as that of starch, continued for a further 3 weeks. In addition to sucrose, glucose or fructose, but not mannitol or sorbitol, also induced the accumulation of fl-amylase and sporamin, suggesting that metabolic effects of sucrose are important in the mechanism of this induction. Treatment of leaf-petiole cuttings with water under continuous light, but not in darkness, also caused the accumulation of small amounts of these components in the petioles, probably as a result of the endogenous supply of sucrose by photosynthesis. These results suggest that the expression of the gene for f-amylase is under metabolic control which is coupled with the expression of sink function of cells in the sweet -otato.

Starch-accumulating Sweet Potato Callus Tissue Devoid ofβ-Amylase but with Two Starch Phosphorylase Isozymes

“…We therefore conclude that to see BA as a storage protein in callus cultures cannot be justified based on the argument forwarded by Nakamura et al for leaf-petiole cuttings. 4 ) Based on the results of studies on the inhibitory action of BA toward SP,3) and also the preliminary indication that BA may share the same subcellular loci with SP,2) we tended to postulate that BA could be a modulator of starch accumulation in the sweet potato root. Findings described in this paper render this view unlikely, and also support the finding of Oba, cited as a personal communication in Nakamura's paper.…”

“…1 -3) Based on this, we had postulated that both SP and BA may act in the modulation of starch accumulation in the sink organ. Nakamura et al 4 ) have demonstrated that when leaf-petiole cuttings were fed with a solution of sucrose or other metabolizable sugars, BA was coinduced in a large amount concomitantly with starch and the storage protein sporamin. Since BA is not present in ordinary leaf-petiole cuttings but is present in large amounts in tuberous roots, they suggested that the expression of BA gene is under metabolic control, which would be coupled to the expression of storage function of sweet potato cells.…”

“…The physiological role that BA may play in the plant metabolism is, however, sti1llargely unknown. 4 ) To prepare amyloplasts from sweet potato roots for the study of starch metabolism, we have met difficulties in that the large starch granules in root amyloplasts cause disruption of the organelle membrane even when pro top lasts are used as the source material. It would be helpful to establish a tissue culture in which the starch accumulation can be manipUlated so as to obtain small starch-granulecontaining amyloplasts to facilitate their separation.…”

Hexokinase as a sugar sensor in higher plants.

“…For instance, the maize CAB5 gene responds to 1 to 10 mM of glucose in <3 hr (Jang and Sheen, 1994), and de novo protein synthesis is not required (J.-C. Jang and J. Sheen, unpublished data). However, other genes display a slow response (>48 hr) and require high sugar levels (Nakamura et al, 1991;Mason et al, 1992;Koch et al, 1995). It is possible that distinct sugar-signaling pathways might be responsible for quick and slow sugar responses.…”

An Arabidopsis mutant showing reduced feedback inhibition of photosynthesis

“…(1994) proposed that sugar-induced expression of VSPB, CHS, PIN2 and patatin genes is activated in part by accumulation of sugar phosphates and the concomitant reduction of cellular phosphate levels. This group includes also genes coding for sporamin and 13-amylase in sweet potato (Ipomoea batatas) (Hattori et al, 1991;Nakamura et al, 1991). The signal transduction pathway for the induction of these genes by sucrose involves protein phosphorylation and dephosphorylation and Ca 2÷-signaling (Ohto and Nakamura, 1995;Takeda et al, 1994).…”