Sequential uptake of aldoses over fructose and enhanced phosphate solubilization in Rhizobium sp. RM

Rhizobium sp. RM solubilized tri-calcium phosphate (TCP: 324-463 µg ml^-1) and rock phosphate (RP: 36-46.58 µg ml^-1) in the presence of common rhizospheric sugars-glucose, arabinose, xylose and their combinations. Fructose, though did not support RP solubilization individually, surprisingly solubilized significantly higher phosphate when combined with aldoses. The highest TCP (644 µg ml^-1) and RP (75 µg ml^-1) solubilization was achieved in fructose + glucose combination. Presence of gluconate, malate and oxalate in culture supernatant indicated functioning of periplasmic glucose oxidation, the non-phosphorylative arabinose dehydrogenase pathway and the tricarboxylate (TCA) cycle, respectively. Aldoses, when present together, were co-utilized (monoauxic growth) however, when added with fructose, prevented the uptake of fructose yielding a typical diauxic growth. This presented an unusual sequential utilization of aldoses over a ketose (fructose) in strain RM. The prevention of fructose uptake by aldoses was investigated through real-time expression of key genes coding fructose transport proteins and initial enzymes of sugar metabolism. Fructose was actively transported via fructose-specific ABC transporters as suggested by upregulation of frcB and frcC only in fructose and fructose growth phases of fructose + aldose combinations. The probable route of initial fructose metabolism involved either fructokinase and/or xylose isomerase, as confirmed by enzyme activities. The upregulation of hfq and hprK genes only in aldose phase of fructose + aldose combinations suggested their possible involvement in governing the preferential utilization. The novel aspects of this study are enhanced organic acid mediated P solubilization in fructose + aldose combinations and a rare hierarchy of aldoses over fructose which is possibly regulated at the level of fructose transport and fructokinase. KEY POINTS: • Sugars when provided in different dual combinations, supported enhanced P solubilization from complex phosphate sources like TCP and RP in Rhizobium sp. RM. • Transcriptional status of genes in cells of RM when grown in different individual sugars and their combinations suggested that fructose might be a less preferred carbon source and hence was utilized after aldoses with the possible regulation by Hfq and HPrK. • First study to present a unique phenomenon of sequential utilization of aldoses (glucose, arabinose and xylose) over fructose in a concentration-independent manner in Rhizobium sp. RM. and to present the effect of dual combinations of sugars on organic acid mediated P solubilization trait of rhizobia.