Sweetpotato Gene Research Articles

Sugar-responsible elements in the promoter of a gene for beta-amylase of sweet potato.

Expression of genes coding for sporamin and beta-amylase, the two most abundant proteins in storage roots of sweet potato, is coordinately inducible in atypical vegetative tissues by sugars. A sweet potato gene for beta-amylase (beta-Amy) with introns as well as a beta-Amy::GUS fusion gene composed of the beta-Amy promoter and the GUS coding sequence, both showed sugar-inducible expression in leaves of transgenic tobacco which occurred via a hexokinase-independent pathway. Analyses using various 5'-terminal and internal deletions of the beta-Amy promoter indicated that truncated promoters of beta-Amy containing a sequence between -901 and -820, relative to the transcription start site, and the basic promoter region can confer sugar-inducible expression. This 82 bp region contained the TGGACGG sequence that plays an essential role in the sugar-inducible expression of the truncated promoter of the sporamin gene. Deletion or base substitutions of this element in the truncated beta-Amy promoter abolished the sugar-inducible expression, the results suggesting that the TGGACGG element plays an important role in the coordinate induction of expression of genes for beta-amylase and sporamin by sugars.

RAPD variation in sweetpotato (Ipomoea batatas (L.) Lam) cultivars from South America and Papua New Guinea

The island of New Guinea is considered a secondary center on diversity for sweetpotato, because of its range of isolated ecological niches and large number of cultivars found within a small area. Information of genetic diversity in Papua New Guinea (PNG) sweetpotato is essential for rationalizing the global sweetpotato germplasm collection. Using random amplified polymorphic DNA (RAPD), we compared the genetic variation and genetic diversity in 18 PNG cultivars versus 18 cultivars from South America. The analysis of molecular variance revealed large genetic diversity in both groups of cultivars. The within-group (among individuals) variation accounted for 90.6% of the total molecular variance. However, the difference between PNG and South American groups is statistically significant, although it explained only 9.4% of the total molecular variance. The PNG cultivars are also less divergent than their South American ancestors as the mean genetic distance in PNG group is significantly smaller than that of South American group. The lower level of genetic diversity in PNG cultivars was also reflected by multidimensional scaling. This study shows that PNG cultivars, after many years of isolated evolution in an unique agro-ecological environment are substantially divergent from their ancestors in South America. The genetic diversity level in PNG cultivars is significantly lower than that in South American cultivars. It thus provides a baseline for continuing studies of genetic diversity in different sweetpotato gene pools.

Inhibitors of Protein Phosphatases 1 and 2A Block the Sugar-Inducible Gene Expression in Plants.

Genes coding for two major proteins of the tuberous root of sweet potato (Ipomoea batatas), namely, sporamin and [beta]-amylase, are inducible in leaves and petioles when they are supplied with high concentrations of sucrose or other metabolizable sugars, such as glucose and fructose, and the accumulation of a large amount of starch accompanies this induction. Three inhibitors of protein phosphatases 1 (PP1) and 2A (PP2A), namely, okadaic acid, microcystin-LR, and calyculin A, strongly inhibited the sucrose-inducible accumulation of mRNAs for sporamin, [beta]-amylase, and the small subunit of ADP-glucose pyrophosphorylase in petioles. However, these inhibitors did not have any major effect on the steady-state levels of mRNAs for catalase and glyceraldehyde-3-phosphate dehydrogenase, and the sucrose-inducible increase in the level of sucrose synthase mRNA was enhanced by okadaic acid. Inhibitors of PP1 and PP2A also inhibited sucrose-inducible expression of a fusion gene, consisting of the promoter of the sweet potato gene for [beta]-amylase and the coding sequence for [beta]-glucuronidase (GUS), in leaves of transgenic tobacco (Nicotiana tabacum). The inhibition was not due to inhibition of uptake and cleavage of sucrose, since okadaic acid also inhibited induction of the fusion gene by glucose or fructose. Addition of okadaic acid to leaves that had been treated with sucrose for 6 h inhibited further increases in GUS activity. These results suggest that the continuous dephosphorylation of proteins is required in the transduction of carbohydrate metabolic signals to the transcriptional activation of at least some sugar-inducible genes in plant.

The species ofIpomoea closely related to the sweet potato

The problem of the origin of the sweet potato, Ipomoea batatas (L.) Lam., has both geographic and biosystematic aspects. The geographical locality where the sweet potato first occurred, or was first domesticated, is generally accepted to be the New World. Hypotheses of an Oceanian or African origin have been discounted by Conklin (2). Nevertheless, the sweet potato was evidently well distributed throughout the Pacific islands and reached even New Zealand in pre-Columbian times (16). Spanish and Portuguese travelers are believed to have disseminated the sweet potato through the tropics so rapidly that the impression was created that the sweet potato was pantropical in distribution prior to discovery of the New World. Although the actual geographical origin and the subsequent spread of the sweet potato throughout the world have ethnological implications, the geneticist or plant breeder is much more concerned with the origin of the sweet potato from related species. This interest is not a matter of idle curiosity. Reconstruction of sweet potato-like plants from related species should provide a new method of introducing useful variation, including disease resistance, to the sweet potato gene pool. In recognition of this fact, considerable emphasis has been placed on the species of Ipomoea