Mechanisms of Extracellular NO and Ca2+ Regulating the Growth of Wheat Seedling Roots

Our previous studies suggested the cross talk of nitric oxide (NO) with Ca2+ in regulating stomatal movement. However, its mechanism of action is not well defined in plant roots. In this study, sodium nitroprusside (SNP, a NO donor) showed an inhibitory effect on the growth of wheat seedling roots in a dose-dependent manner, which was alleviated through reducing extracellular Ca2+ concentration. Analyzing the content of Ca2+ and K+ in wheat seedling roots showed that SNP significantly promoted Ca2+ accumulation and inhibited K+ accumulation at a higher concentration of extracellular Ca2+, but SNP promoted K+ accumulation in the absence of extracellular Ca2+. To gain further insights into Ca2+ function in the NO-regulated growth of wheat seedling roots, we conducted the patch-clamped protoplasts of wheat seedling roots in a whole cell configuration. In the absence of extracellular Ca2+, NO activated inward-rectifying K+ channels, but had little effects on outward-rectifying K+ channels. In the presence of 2 mmol L-1 CaCl2 in the bath solution, NO significantly activated outward-rectifying K+ channels, which was partially alleviated by LaCl3 (a Ca2+ channel inhibitor). In contrast, 2 mmol L-1 CaCl2 alone had little effect on inward or outward-rectifying K+ channels. Thus, NO inhibits the growth of wheat seedling roots likely by promoting extracellular Ca2+ influx excessively. The increase in cytosolic Ca2+ appears to inhibit K+ influx, promotes K+ outflux across the plasma membrane, and finally reduces the content of K+ in root cells.