Comparison of lignin deposition in three ectopic lignification mutants.

The Arabidopsis thaliana mutants de-etiolated3 (det3), pom-pom1 (pom1) and ectopic lignification1 (eli1) all deposit lignins in cells where these polymers would not normally be found. Comparison of these mutants provides an opportunity to determine if the shared mutant phenotype arose by perturbing a common regulatory mechanism in each of the mutants. The mutants were compared using a combination of genetics, histochemistry, chemical profiling, transcript profiling using both Northern blots and microarrays, and bioinformatics. The subset of cells that ectopically lignified was shared between all three mutants, but clear differences in cell wall chemistry were evident between the mutants. Northern blot analysis of lignin biosynthetic genes over diurnal and circadian cycles revealed that transcript abundance of several key genes was clearly altered in all three mutants. Microarray analysis suggests that changes in the expression of specific members of the R2R3-MYB and Dof transcription factor families may contribute to the ectopic lignification phenotypes. This comparative analysis provides a suite of hypotheses that can be tested to examine the control of lignin biosynthesis.

Light, the circadian clock, and sugar perception in the control of lignin biosynthesis.

Experiments were undertaken to investigate some of the mechanisms that may function to regulate lignin biosynthesis (lignification) in Arabidopsis thaliana. Northern blot analyses revealed that several genes encoding enzymes involved in the synthesis of lignin monomers displayed significant changes in transcript abundance over a diurnal cycle. Northern blot analysis also suggested that some of the changes in diurnal transcript abundance were likely to be attributable to circadian regulation, whereas others were likely to be attributable to light perception. Comparison of circadian changes in transcript abundance of lignin biosynthetic genes between wild-type plants and the sex1 mutant, which is impaired in starch turnover, suggested that carbon availability related to starch turnover might determine the capacity to synthesize lignins. This hypothesis was supported by the observation that the sex1 mutant accumulated fewer lignins than wild-type plants. Consistent with the relationship between carbon availability and lignin accumulation, analysis of dark-grown wild-type A. thaliana seedlings uncovered a role for sugars in the regulation of lignin biosynthesis. Analysis of lignin accumulation, as determined by qualitative changes in phloroglucinol staining, suggested that metabolizable sugars positively influence the abundance of lignins. Transcriptome analysis supports the hypothesis that sugars are not merely a source of carbon skeletons for lignification, but they also function as a signal to enhance the capacity to synthesize lignins.

The genetic control of lignin deposition during plant growth and development.

Lignins are complex, three-dimensional polymers embedded in the cell walls of specialised plant cells, where they play important roles in plant growth and development. Plants must possess mechanisms to coordinate lignin deposition so that its synthesis occurs at the appropriate time and place, in response to endogenous and exogenous cues. Here we consider the genetic basis of the control of lignin deposition. We focus on the transcriptional regulation of lignification, considering how the genes encoding the lignin biosynthetic pathway might be co-ordinately controlled, and the transcription factors that are likely to be involved. We also discuss the mechanisms regulating lignification that have been revealed by mutants with altered lignin deposition. We conclude that, while transcriptional regulation is a common feature in the control of lignification, there are many different regulators that may bring about this common mode of regulation. Contents Summary 17 I. Introduction 17 II. Transcriptional regulation of genes encoding lignin biosynthetic enzymes 19 III. Co-ordinate regulation of genes encoding lignin biosynthetic enzymes 21 IV. Mutants with altered spatial and temporal control of lignification 23 V. Conclusion 28 Acknowledgements 28 References 28.