Down-regulation of theIbEXP1gene enhanced storage root development in sweetpotato

Abstract

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.

Comparative analysis of the root transcriptomes of cultivated sweetpotato (Ipomoea batatas [L.] Lam) and its wild ancestor (Ipomoea trifida [Kunth] G. Don)

“…The qRT-PCR results showed that the gene encoding expansin was more highly expressed in Ib than in It . By contrast, Noh et al [16] found that the expression of the expansin gene is inhibited during SR formation in sweetpotato. However, Firon et al [2] demonstrated that the expansin gene is involved in SR formation in sweetpotato.…”

“…Expression analysis during SR formation also identified a number of candidate genes [14–16]. For example, You et al [14] identified 22 differentially expressed genes by comparing early SRs and fibrous roots.…”

“…Several NAC family transcription factor genes are downregulated in SRs, and two NAM-like genes, as well as sporamin genes and genes involved in starch biosynthesis, are upregulated in SRs (compared to FRs) at six weeks after planting [15]. Noh et al [16] used antisense RNA interference to demonstrate the negative role of an expansin gene ( IbEXP1 ) in SR development; IbEXP1 suppresses the proliferation of metaxylem and cambium cells, and thus inhibits the initial thickening of SRs.…”

Root System Architecture and Abiotic Stress Tolerance: Current Knowledge in Root and Tuber Crops

“…Noh et al (2010) found that a MADS-box protein copy DNA, SRD1 enhances the proliferation of the metaxylem and cambium cells during the auxin-dependent initial thickening and growth of storage roots. Storage root development in sweetpotato is enhanced when an expansin gene (IbEXP1) is down-regulated (Noh et al, 2012), but lignin biosynthesis is inhibited as starch biosynthesis is enhanced during early storage root formation (Firon et al, 2013). Details on the molecular regulation of storage root formation in sweetpotato have been reviewed by Ravi et al (2009, 2014).…”

Recent Progress in Molecular Studies on Storage Root Formation in Sweetpotato (<i>Ipomoea batatas</i>)

“…Further analysis of gene expression and enzyme activity in the storage roots of transgenic plants suggested that SRF1 is involved in the regulation of carbohydrate metabolism during storage root formation through the modulation of vacuolar invertase gene expression. Noh et al (2013) reported functional characterization of an expansin gene (named IbEXP1) that is down-regulated in storage roots. The fibrous roots of transgenic sweetpotato plants with suppressed expression of IbEXP1 were thicker than those of the control plants, and showed an enhanced proliferation of metaxylem and cambium cells and reduced lignification in the central stele.…”