Starch biosynthetic genes and enzymes are expressed and active in the absence of starch accumulation in sugar beet tap-root

Table 1 Starch biosynthetic enzyme activity in soluble protein extracts from parsnip and sugar beet tap-roots

	PGM	AGPase	SS	SBE
Root tissue and developmental stage	(Units converting 1 μmole G1P to G6P, μg soluble protein^-1, min^-1)	(nmol ADP-glucose, μg soluble protein^-1, min^-1)	(nmol ADP-glucose converted to starch, μg soluble protein^-1, min^-1)	(nmol G1P converted to branched starch, μg soluble protein^-1, min^-1)
Parsnip 48 DAP (light)	0.09 ± 0.02a	0.006 ± 0.002a	0.46 ± 0.20a	0.33 ± 0.01ab
Parsnip 48 DAP (dark)	0.12 ± 0.06a	0.007 ± 0.003a	0.55 ± 0.18a	0.38 ± 0.03ab
Parsnip 61 DAP (light)	0.12 ± 0.06a	0.006 ± 0.002a	0.68 ± 0.30a	0.52 ± 0.10a
Parsnip 61 DAP (dark)	0.11 ± 0.06a	0.007 ± 0.003a	0.71 ± 0.10a	0.47 ± 0.02a
Sugar beet 41 DAP (light)	0.09 ± 0.02a	0.007 ± 0.001a	0.43 ± 0.13a	0.20 ± 0.07b
Sugar beet 41 DAP (dark)	0.11 ± 0.03a	0.008 ± 0.001a	0.41 ± 0.06a	0.22 ± 0.04b
Sugar beet 54 DAP (light)	0.09 ± 0.02a	0.006 ± 0.001a	0.38 ± 0.06a	0.29 ± 0.17ab
Sugar beet 54 DAP (dark)	0.07 ± 0.004a	0.007 ± 0.001a	0.50 ± 0.16a	0.22 ± 0.08b

Samples were taken from parsnip tap-roots harvested 48 and 61 days after planting (DAP) and sugar beet tap-roots harvested 41 and 54 DAP. Enzyme activity of phosphoglucomutase (PGM), ADP-glucose pyrophosphorylase (AGPase), starch synthase (SS) and starch branching enzyme (SBE) were measured on crude extracts. Data are means ± SD of values from extracts derived from different homogenates of 3 pooled roots (n = 3). Columns sharing the same letters were not significantly different according to Tukey’s test (P = 0.05).

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