However, ketone production is relatively inefficient and occurs only when enzyme levels are very high, as in pea seeds. Most 2-oxidases are specific for C19-GAs, and will accept the 3ȕhydroxy bioactive GAs and their non-3ȕ-hydroxylated precursors as substrates (Fig. 9). C20-GAs may also be 2ȕ-hydroxylated and Arabidopsis has been shown to contain GA 2-oxidases that are specific for these compounds (61). Such enzymes may be important for maintenance of GA homeostasis (see later) when levels of C20-GA precursors become very high.