subunits. As can be seen in the diagram shown to the right, these monolignols are derived directly from their corresponding aldehydes, except in the case of sinapyl alcohol - while several CCR homologs have been shown to act on sinapoyl-CoA in vitro, it is unclear whether this activity is biologically relevant and most current models of the lignin pathway do not include this reaction as a valid step. Recent studies indicate that many plant species have two distinct homologs of CCR with differential activity in planta. In some plants the two homologs vary primarily by substrate specificity. For example, CCR1 of the model legume Medicago truncatula shows strong preference toward feruloyl-CoA (typical of most CCRs), while the plant's CCR2 exhibits a clear preference for both p-coumaroyl- and caffeoyl-CoA. This second CCR, which is allosterically activated by its preferred substrates but inhibited by feruloyl-CoA, is thought to act as part of a shunt pathway toward coniferaldehyde that enhances the pathway's overall flexibility and robustness in different conditions. In other cases though, the two homologs vary primarily by expression pattern. In the model plant Arabidopsis thaliana, for instance, the CCR1 and CCR2 homologs both display higher activity toward feruloyl-CoA than other substrates, but CCR2 is only expressed transiently during bacterial infection. The homolog pair in switchgrass (Panicum virgatum) differs in both ways: CCR2 prefers p-coumaroyl- and caffeoyl-CoA and is only expressed under specifically induced conditions, while CCR1 prefers feruloyl-CoA and is expressed constitutively in lignifying tissue. Regulation of CCR expression is thought to occur primarily at the transcriptional level. In Arabidopsis thaliana, several of the required transcription factors for CCR expression have actually been identified, including MYB58 and MYB63, both of which are implicated generally in secondary cell wall formation. It has been shown that over-expression of these two transcription factors results in a 2- to 3-fold increase in CCR mRNA transcripts, though intriguingly, the up-regulation of genes further upstream in the monolignol pathway is even greater. Non-transcriptional regulation of CCR, however, can be important as well. In rice (Oryza sativa), for example, evidence suggests that the CCR1 homolog is an effector of Rac1, a small GTPase important for plant defense response. In this case, the Rac1 protein is proposed to activate CCR upon binding, leading to enhanced monolignol biosynthesis. Because Rac1 also activates NADPH oxidase, which produces peroxides critical for monolignol polymerization, overall lignin biosynthesis is enhanced as well. Biotechnological significance