Nonsubstrate-like signals

The classification into substrate-like and nonsubstrate-like signals is, however, not a strict one and certainly does not imply a principal difference in the mode of action. It is complicated by the following facts ... 

The plant hormone gibberellic acid (D 6.3 a nonsubstrate-like signal), for instance, reduces the biosynthesis of amaranthine, a betalain (D 22.1.4), by limitation of precursor supply, an effect normalized by the administration of L-tyrosine or l-DOPA,... 

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