Since the early 1930s or so, there have been a number of reports indicating that various agronomically important plant species, i. e., maize (Zea mays), sorghum (Sorghum bicolor),

and pearl millet (Pennisetum glaucum), can produce so-called brown-midrib mutants. The maize mutants (bm1-bm4) were spontaneous mutations (58-60) and were later shown to have lower lignin contents than wild-type lines (62), whereas both sorghum (227) and pearl millet (228) mutants were generated via chemical mutagenesis. For sorghum, out of the 19 individual mutant lines obtained, two (bmr12 and bmr18) had estimated lignin contents reduced by 42 and 45%, respectively, when compared to wild type (227). Since then a spontaneous mutation has also been identified in sorghum (bmr26) (229). To our knowledge, none of these mutants have yet found commercial application since their discovery, due to defects, such as increased brittleness, increased lodging, delayed growth, and flowering (230).

Some of these lines have been characterized as mutations in CAD (bml) (67) and COMT [ bm3 (66,231) and bmr12/bmr18/ bmr26(229)]. As noted earlier, our previous metabolic flux analyses (34) had suggested that under the conditions employed none of these steps would “normally” have a rate-limiting role in terms of carbon allocation to the monolignol/lignin pathway. Nor would modulation of F5H be anticipated to alter carbon (metabolic flux) allocation. Effects of manipulating each of these steps are thus discussed below.