Synchrony between flower opening and petal-color change from red to blue in morning glory, Ipomoea tricolor cv. Heavenly Blue

Abstract

Petal color change in morning glory Ipomoea tricolor cv. Heavenly Blue, from red to blue, during the flower-opening period is due to an unusual increase in vacuolar pH (pHv) from 6.6 to 7.7 in colored epidermal cells. We clarified that this pHv increase is involved in tonoplast-localized Na+/H+ exchanger (NHX). However, the mechanism of pHv increase and the physiological role of NHX1 in petal cells have remained obscure. In this study, synchrony of petal-color change from red to blue, pHv increase, K+ accumulation, and cell expansion growth during flower-opening period were examined with special reference to ItNHX1. We concluded that ItNHX1 exchanges K+, but not Na+, with H+ to accumulate an ionic osmoticum in the vacuole, which is then followed by cell expansion growth. This function may lead to full opening of petals with a characteristic blue color.

The AtNHX1 exchanger mediates potassium compartmentation in vacuoles of transgenic tomato

“…Our data strongly support the notion that NHX-type proteins serve this function. Recently, the K + / H + exchange activity of the ItNHX1 protein has been implicated in the increase of the vacuolar pH, K + accumulation, cell expansion and color transition from red to blue in petals of Ipomoea tricolor (Yoshida et al, 2009). Also, Arabidopsis nhx1 mutant plants had reduced leaf area and smaller epidermal cells (Apse et al, 2003), suggesting that these mutant plants do not supply the vacuoles with sufficient osmotic potential for the normal expansion of leaf cells, which largely depends on K + accumulation (Maggio et al, 2006).…”

How do vacuolar NHX exchangers function in plant salt tolerance?

“…6.6 to 7.7 that renders blue flowers. [31][32][33] The purple (pr) mutation of I. nil abolishes the activity of InNHX1, abrogates vacuolar alkalinization and impedes the color shift from red to blue in opening flowers. 32 Petunia hybrida flowers normally have a lower pH than Ipomoea flowers, and the color of wild-type flowers stays on the reddish (low pH) side of the color spectrum.…”

“…Modulation of pHv has been linked to K + /H + exchange but not to Na + /H + exchange. 33 In summary, a significant and growing body of evidence indicates that NHX antiporters play a fundamental, basic role in the plant cell. At regular growth conditions they contribute to K + uptake, capturing K + into vacuoles for cellular storage, turgor generation and pH regulation.…”

Ion Exchangers NHX1 and NHX2 Mediate Active Potassium Uptake into Vacuoles to Regulate Cell Turgor and Stomatal Function in Arabidopsis

“…Scattered evidence indicated that vacuolar Na + ,K + /H + antiporter (NHX)-type exchangers could serve this critical function in plant cells (Venema et al, 2002;Rodríguez-Rosales et al, 2008Yoshida et al, 2009;Jiang et al, 2010;Leidi et al, 2010). Studies in transgenic tomato (Solanum lycopersicum) showed that overexpression of the vacuolar At NHX1 or the endosomal Le NHX2 Na + ,K + /H + exchangers led to greater K + accumulation (Rodríguez-Rosales et al, 2008;Leidi et al, 2010).…”