SPECIES-DEPENDENT PATTERNS OF FRUCTAN SYNTHESIS BY ENZYMES FROM EXCISED LEAVES OF OAT, WHEAT, BARLEY AND TIMOTHY

Excised leaves of oat, wheat, barley and timothy were illuminated cont inuously for 24 h and shown to accumulate both sucrose and fructan. Th e fructan complements of the four species were compared with the well- characterized pattern from leaves of L. temulentum L. by thin-layer ch romatography (TLC). Wheat and barley had similar patterns of oligosacc harides, accumulating isokestose, kestose and a range of oligofructans up to an apparent degree of polymerization (DP) of 10. Oat leaves acc umulated neokestose in addition to kestose and isokestose. The oligofr uctans of oat also extended up to DP 10, though the pattern was more c omplex than that of wheat and barley. Timothy leaves accumulated fruct an predominantly of apparent DP > 16 and little smaller oligofructan. Analysis of concentrated carbohydrate extracts showed isokestose, kest ose and a ladder of oligofructans of apparent DP in the range 4 to > 2 0 in induced timothy leaves. The results showed that leaves of each gr ass accumulated a distinct and species-specific pattern of fructan. Co ncentrated enzyme preparations from illuminated leaves of each species were incubated at 30-degrees-C with 400 mol m-3 sucrose at pH 4.6 in the presence of 16 mol m-3 pyridoxal hydrochloride. The preparations c atalyzed the de novo net synthesis of trisaccharides and larger, oligo fructans at rates approximating to rates of soluble carbohydrate accum ulation in the tissue. On TLC, the enzyme products exhibited a marked resemblance to the complement of native fructan in the source tissue. The data demonstrate that the species specificity of the fructan compl ement can be explained by the properties of synthetic enzyme(s) alone, and is not dependent upon structural features of intact cells or tiss ues. The established view of fructan synthesis holds that the polymeri zing enzyme, fructan: fructan fructosyl transferase (FFT), is inhibite d by sucrose. The enzymes from all five grass species manufactured fru ctans of DP > 3 in the presence of high concentrations of sucrose (abo ve 200 mol m-3). Hence the properties of these grass fructan polymeras es differ from those of FFT.