Seol Ah Noh

SRD1 is involved in the auxin-mediated initial thickening growth of storage root by enhancing proliferation of metaxylem and cambium cells in sweetpotato (Ipomoea batatas)

A sweetpotato (Ipomoea batatas cv. ‘Jinhongmi’) MADS-box protein cDNA (SRD1) has been isolated from an early stage storage root cDNA library. The role of the SRD1 gene in the formation of the storage root in sweetpotato was investigated by an expression pattern analysis and characterization of SRD1-overexpressing (ox) transgenic sweetpotato plants. Transcripts of SRD1 were detected only in root tissues, with the fibrous root having low levels of the transcript and the young storage root showing relatively higher transcript levels. SRD1 mRNA was mainly found in the actively dividing cells, including the vascular and cambium cells of the young storage root. The transcript level of SRD1 in the fibrous roots increased in response to 1000 μM indole-3-acetic acid (IAA) applied exogenously. During the early stage of storage root development, the endogenous IAA content and SRD1 transcript level increased concomitantly, suggesting an involvement of SRD1 during the early stage of the auxin-dependent development of the storage root. SRD1-ox sweetpotato plants cultured in vitro produced thicker and shorter fibrous roots than wild-type plants. The metaxylem and cambium cells of the fibrous roots of SRD1-ox plants showed markedly enhanced proliferation, resulting in the fibrous roots of these plants showing an earlier thickening growth than those of wild-type plants. Taken together, these results demonstrate that SRD1 plays a role in the formation of storage roots by activating the proliferation of cambium and metaxylem cells to induce the initial thickening growth of storage roots in an auxin-dependent manner.

Down-regulation of the IbEXP1 gene enhanced storage root development in sweetpotato

The role of an expansin gene (IbEXP1) in the formation of the storage root (SR) was investigated by expression pattern analysis and characterization of IbEXP1-antisense sweetpotato (Ipomoea batatas cv. Yulmi) plants in an attempt to elucidate the molecular mechanism underlying SR development in sweetpotato. The transcript level of IbEXP1 was high in the fibrous root (FR) and petiole at the FR stage, but decreased significantly at the young storage root (YSR) stage. IbEXP1-antisense plants cultured in vitro produced FRs which were both thicker and shorter than those of wild-type (WT) plants. Elongation growth of the epidermal cells was significantly reduced, and metaxylem and cambium cell proliferation was markedly enhanced in the FRs of IbEXP1-antisense plants, resulting in an earlier thickening growth in these plants relative to WT plants. There was a marked reduction in the lignification of the central stele of the FRs of the IbEXP1-antisense plants, suggesting that the FRs of the mutant plants possessed a higher potential than those of WT plants to develop into SRs. IbEXP1-antisense plants cultured in soil produced a larger number of SRs and, consequently, total SR weight per IbEXP1-antisense plant was greater than that per WT plant. These results demonstrate that SR development was accelerated in IbEXP1-antisense plants and suggest that IbEXP1 plays a negative role in the formation of SR by suppressing the proliferation of metaxylem and cambium cells to inhibit the initial thickening growth of SRs. IbEXP1 is the first sweetpotato gene whose role in SR development has been directly identified in soil-grown transgenic sweetpotato plants.

Overexpression of sweetpotato expansin cDNA (IbEXP1) increases seed yield in Arabidopsis.

Abstract Results of transcriptome analyses suggest that expansin genes play an active role in seed development and yield, but gain- or loss-of-function studies have not yet elucidated the functional role(s) of the expansin gene(s) in these processes. We have overexpressed a sweetpotato expansin gene (IbEXP1) in Arabidopsis under the control of cauliflower mosaic 35S promoter in an attempt to determine the effect of the expansin gene in seed development and yield in heterologous plants. The growth rate was enhanced in IbEXP1-overexpressing (ox) plants relative to wild-type Col-0 plants during early vegetative growth stage. At the reproductive stage, the number of rosette leaves was higher in IbEXP1-ox plants than that in Col-0 plants, and siliques were thicker. IbEXP1-ox plants produced larger seeds, accumulated more protein and starch in each seed, and produced more inflorescence stems and siliques than Col-0 plants, leading to a 2.1–2.5-fold increase in total seed yield per plant. The transcript level of IbEXP1 was up-regulated in response to brassinosteroid (BR) treatment in sweetpotato, and the transcript levels of three BR-responsive genes, fatty acid elongase 3-ketoacyl-CoA synthase 1, HAIKU1 and MINISEED3, were also increased in IbEXP1-ox Arabidopsis plants, suggesting a possible involvement of IbEXP1 in at least one of the BR signaling pathways. Based on these results, we suggest that overexpression of IbEXP1 gene in heterologous plants is effective in increasing seed size and number and, consequently, seed yield.

Production of ketocarotenoids in transgenic carrot plants with an enhanced level of β-carotene

One of the drawbacks in inducing the synthesis of ketocarotenoids in transgenic plants is the accompanying decrease in β-carotene level, especially in transgenic plants overexpressing only a β-carotene ketolase. In an attempt to overcome this drawback, we isolated a β-carotene ketolase (HpBkt) cDNA from Haematococcus pluvialis and generated transgenic carrot plants overexpressing HpBkt cDNA under the control of the ibAGP1 promoter and its transit peptide sequence. Semi-quantitative reverse transcriptase-PCR with total RNAs extracted from taproot tissues of five lines of the generated HpBkt-transgenic carrot plants revealed variable levels of the inserted HpBkt transcript, ranging from very low to high. All of the HpBkt-transgenic carrot plants tested produced astaxanthin, resulting in a color change from the light-orange coloration of wild-type taproots to a dark-orange–reddish coloration in the taproots of the HpBkt-transgenic plants. The astaxanthin content was not correlated with the HpBkt transcript level. Overexpression of HpBkt caused an increase in the transcript levels of the endogenous carotenogenic genes, including phytoene synthase 1 (PSY1), phytoene synthase 2 (PSY2), lycopene β-cyclase 1 (LCYB1), and β-carotene hydroxylase 1 (CHXB1), which resulted in elevated β-carotene levels in the HpBkt-transgenic plants (range 1.3- to 2.5-fold) compared to the wild-type plants. These results demonstrate that HpBkt-overexpressing carrot plants under the control of the ibAGP1 promoter and its transit peptide are capable of both newly synthesizing ketocarotenoids and enhancing their β-carotene level.