These analyses have been most useful in predicting the outcome of various manipulations in the lignin-forming pathway. That is, previous studies, whereby monolignol 1 and 3 formation could be induced in loblolly pine (Pinus taeda) cell suspension cultures, enabled us to gain important insights into factors controlling metabolic flux to both p-coumaryl (1) and coniferyl (3) alcohols (34, 35). Thus, by increasing levels of available sucrose, the monolignol-forming pathway could be induced, with the cells secreting the monolignols (in the presence of an H2O2-scavenger) into the culture medium.

The data (based on both measuring various pathway metabolite levels and transcript pro­files) provided quite informative insights: the first was that (regulation of) carbon allocation to the pathway was controlled upstream through the amounts of Phe (6) being made avail­able, as well as through the differential activities of both cinnamate-4-hydroxylase (C4H) and p-coumarate-3-hydroxylase (pC3H). Furthermore, metabolite analyses also indicated that formation of both p-coumaryl (1) and coniferyl (3) alcohols could be differentially in­duced, suggesting the existence of distinct metabolic control over segments (i. e., H versus G) within the monolignol/lignin forming processes through differential modulation of pC3H activity. Beyond the hydroxylation steps, other downstream enzymatic steps (see Figure 7.1) were not considered to be rate limiting, at least under the conditions employed in the studies. [Of course, any enzymatic step becomes rate limiting if abolished or “knocked out.”] Tran­scriptional profiling data of each of the known steps involved in monolignol biosynthesis (available at that time) also appeared to support this analysis and interpretation (35).