Different Functions of IbRAP2.4, a Drought-Responsive AP2/ERF Transcription Factor, in Regulating Root Development Between Arabidopsis and Sweetpotato.

Plant root systems are essential for the uptake of water and nutrients from soil and are positively correlated to yield in many crops including the sweetpotato, Ipomoea batatas (L.) Lam. Here, we isolated and functionally characterized IbRAP2.4, a novel nuclear-localized gene encoding the AP2/ERF transcription factor, from sweetpotato. IbRAP2.4 was responsive to NaCl, PEG8000, ethylene, and Indole 3-acetic acid treatments. As revealed by electrophoretic mobility shift assay and dual luciferase assay, IbRAP2.4 could bind to both DRE and GCC-box elements and acted as a transcription activator. IbRAP2.4 overexpression significantly promoted lateral root formation and enhanced the drought tolerance in Arabidopsis thaliana, while it inhibited storage root formation in transgenic sweetpotato by comprehensively upregulating lignin biosynthesis pathway genes. Results suggested that IbRAP2.4 may be a useful potential target for further molecular breeding of high yielding sweetpotato.

Effects of exogenous phytohormones on chlorogenic acid accumulation and pathway-associated gene expressions in sweetpotato stem tips.

Sweetpotato (Ipomoea batatas [L.] Lam.) stem tips, which contain high concentrations of chlorogenic acid (CGA), are useful as a physiologically functional food to protect against some serious diseases. According to previous studies, exogenous application of phytohormones may be an effective agrotechnical measure to control CGA biosynthesis through the transcriptional regulation of pathway gene expressions. To understand the mechanism of CGA biosynthesis in sweetpotato, we investigated the effects of exogenous phytohormones on CGA metabolism in stem tips of sweetpotato. A significantly elevated CGA content was observed in salicylic acid (SA)-treated sweetpotato stem tips at 72 h, as well as in those subjected to abscisic acid (ABA) or gibberellic acid (GA) treatments. Dynamic expression change of seven enzyme genes involved in sweetpotato CGA biosynthesis were analyzed to determine correlations between transcript levels and CGA accumulation. As revealed by the differential expression of these genes under distinct phytohormone treatments, the regulation of specific pathway genes is a critical determinant of the accumulation of CGA in sweetpotato stem tips. We also found that several hormone-responsive sites, such as those for ABA, GA, SA, and jasmonic acid (JA), were present in the promoter regions of sweetpotato CGA biosynthestic pathway genes. Collectively, phytohormones can regulate the transcription of CGA synthesis-related genes and ultimately affect CGA accumulation in sweetpotato stem tips, whereas the regulatory differences are mirrored by cis-acting elements in the corresponding pathway gene promoters.

Effects of nitrogen level on structural and functional properties of starches from different colored-fleshed root tubers of sweet potato.

The nitrogen (N) influences the growth of sweet potato. However, it is unclear whether the different levels of N can affect starch physicochemical properties. In this study, 9 different colored-fleshed sweet potato varieties were planted in the same field with additional N fertilizer treatment of 0, 15 and 30 kg/ha. The physicochemical properties of starches from root tubers were measured. With increasing N level, the amylose content decreased in yellow-fleshed variety Sushu 16 and increased in white-fleshed variety Sushu 29 and purple-fleshed varieties Ningzishu 1 and 4, but did not significantly change in other varieties. The starch size decreased in purple-fleshed variety Ningzishu 1 and white-fleshed varieties Sushu 28 and Sushu 29 with increasing N treatment, but first increased then decreased in yellow-fleshed variety Sushu 16 and first decreased then increased in white-fleshed variety Sushu 24 and yellow-fleshed variety Sushu 25. The different levels of N treatment had no influence on protein content, crystalline structure, and gelatinization enthalpy of starch. The effects of N treatment on gelatinization temperatures and pasting viscosities of starches were determined by varieties and genotype backgrounds of sweet potato. The PLSR and PLS-DA were also carried out based on structural, thermal, and pasting parameters of starches.

De novo sequencing and comprehensive analysis of the mutant transcriptome from purple sweet potato (Ipomoea batatas L.).

Purple sweet potatoes, rich in anthocyanin, have been widely favored in light of increasing awareness of health and food safety. In this study, a mutant of purple sweet potato (white peel and flesh) was used to study anthocyanin metabolism by high-throughput RNA sequencing and comparative analysis of the mutant and wild type transcriptomes. A total of 88,509 unigenes ranging from 200nt to 14,986nt with an average length of 849nt were obtained. Unigenes were assigned to Gene Ontology (GO), Clusters of Orthologous Group (COG) and Kyoto Encyclopedia of Genes and Genomes (KEGG). Functional enrichment using GO and KEGG annotations showed that 3828 of the differently expressed genes probably influenced many important biological and metabolic pathways, including anthocyanin biosynthesis. Most importantly, the structural and transcription factor genes that contribute to anthocyanin biosynthesis were downregulated in the mutant. The unigene dataset that was used to discover the anthocyanin candidate genes can serve as a comprehensive resource for molecular research in sweet potato.