Table III.

Observed and predicted labeling pattern of IAA

Precursor	[1′,2′-13C2]IAA	[2,3-13C2]IAA	Ratio
Trp	0.47	0.62	0.76
IGP	0.68	0.61	1.11
Indole plus acetate-derived metabolite	0.97	0.62	1.56
Indole plus PEP-derived metabolite	0.89	0.62	1.43
Observed	0.39	0.70	0.56

Predictions are made on the basis of metabolites derived from kernels labeled with [U-¹³C₆]Glc. The normalized abundance of [1′,2′-¹³C₂]IAA and [2,3-¹³C₂]IAA and the ratio [1′,2′-¹³C₂]IAA to [2,3-¹³C₂]IAA is given. Four hypothetical IAA biosynthetic pathways were analyzed for their plausibility. (a) Biosynthesis from Trp: [1′,2′-¹³C₂]IAA derives from [1′,2′,3′-¹³C₃]Trp (w3) plus [2′,3′-¹³C₂]Trp (w2). (b) Biosynthesis from IGP: [1′,2′-¹³C₂]IAA derives from [3,4,5-¹³C₃]Rib-P. (c) Biosynthesis from indole plus an acetate derived metabolite: [1′,2′-¹³C₂]IAA derives from [1,2-¹³C₂]acetyl-CoA. (d) Biosynthesis from indole plus a PEP or triose-P-derived metabolite: [1′,2′-¹³C₂]IAA derives from [1,2,3-¹³C₃]PEP plus [2,3-¹³C₂]PEP. For a biosynthesis from IGP [2,3-¹³C₂]IAA derives from [1,2-¹³C₂]Rib-P, for the other hypothetical pathways [2,3-¹³C₂]IAA derives from [2,3-¹³C₂]indole.