Root-Zone Salinity Alters Raffinose Oligosaccharide Metabolism and Transport in Coleus

Abstract

Exposure of variegated coleus (Coleus blumei Benth.) plants to a saline root-zone environment (60 mM NaCl:12 mM CaCl2) resulted in a significant decline in elongation growth rate over the 30-d experimental period. During the initial 5 to 10 d of exposure, mature source leaves showed strongly diminished rates of photosynthesis, which gradually recovered to close to the control rates by the end of the experiment. In green leaf tissues, starch levels showed the same transient decline and recovery pattern. Low starch levels were accompanied by the appearance of several novel carbohydrates, including high-molecular-weight raffinose family oligosaccharides (RFOs) with a degree of polymerization (DP) of 5 to 8, and an O-methylated inositol (OMI). New enzyme activities, including galactan:galactan galactosyltransferase, for the synthesis of high-DP RFOs and myo-inositol 6-O-methyltransferase for O-methylation of myo-inositol, were induced by salinity stress. Phloem-sap analysis showed that in the stressed condition substantially more sucrose than RFO was exported, as was the OMI. In white sink tissues these phloem sugars were used to synthesize high-DP RFOs but not OMIs. In sink tissues galactan:galactan galactosyltransferase but not myo-inositol 6-O-methyltransferase was induced by salinity stress. Models reflecting the changes in carbohydrate metabolism in source and sink tissues in response to salinity stress are presented.