Jeongim Kim

One step conversion of lignin from intact biomass to two phenol products, improving access to aromatics and fuels.

The Arabidopsis thaliana YUCCA family of flavin monooxygenase proteins catalyses a rate-limiting step in de novo auxin biosynthesis. A YUCCA6 activation mutant, yuc6-1D, has been shown to contain an elevated free IAA level and to display typical high-auxin phenotypes. It is reported here that Arabidopsis plants over-expressing YUCCA6, such as the yuc6-1D activation mutant and 35S:YUC6 transgenic plants, displayed dramatic longevity. In addition, plants over-expressing YUCCA6 exhibited classical, delayed dark-induced and hormone-induced senescence in assays using detached rosette leaves. However, plants over-expressing an allele of YUCCA6, that carries mutations in the NADPH cofactor binding site, exhibited neither delayed leaf senescence phenotypes nor phenotypes typical of auxin overproduction. When the level of free IAA was reduced in yuc6-1D by conjugation to lysine, yuc6-1D leaves senesced at a rate similar to the wild-type leaves. Dark-induced senescence in detached leaves was accompanied by a decrease in their free IAA content, by the reduced expression of auxin biosynthesis enzymes such as YUCCA1 and YUCCA6 that increase cellular free IAA levels, and by the increased expression of auxin-conjugating enzymes encoded by the GH3 genes that reduce the cellular free auxin levels. Reduced transcript abundances of SAG12, NAC1, and NAC6 during senescence in yuc6-1D compared with the wild type suggested that auxin delays senescence by directly or indirectly regulating the expression of senescence-associated genes.

The biosynthesis of lignin, flavonoids, and hydroxycinnamoyl esters share the first three enzymatic steps of the phenylpropanoid pathway. The last shared step is catalyzed by 4-coumarate:CoA ligase (4CL), which generates p-coumaroyl CoA and caffeoyl CoA from their respective acids. Four isoforms of 4CL have been identified in Arabidopsis (Arabidopsis thaliana). Phylogenetic analysis reveals that 4CL1, 4CL2, and 4CL4 are more closely related to each other than to 4CL3, suggesting that the two groups may serve different biological functions. Promoter-GUS analysis shows that 4CL1 and 4CL2 are expressed in lignifying cells. In contrast, 4CL3 is expressed in a broad range of cell types, and 4CL3 has acquired a distinct role in flavonoid metabolism. Sinapoylmalate, the major hydroxycinnamoyl ester found in Arabidopsis, is greatly reduced in the 4cl1 4cl3 mutant, showing that 4CL1 and 4CL3 function redundantly in its biosynthesis. 4CL1 accounts for the majority of the total 4CL activity, and loss of 4CL1 leads to reduction in lignin content but no growth defect. The 4cl1 4cl2 and 4cl1 4cl2 4cl3 mutants are both dwarf but do not have further reduced lignin than the 4cl1 mutant, indicating that either 4CL1 or 4CL2 is required for normal plant growth. Although 4CL4 has a limited expression profile, it does make a modest contribution to lignin biosynthesis. Together, these data show that the four isoforms of 4CL in Arabidopsis have overlapping yet distinct roles in phenylpropanoid metabolism.