A project undertaken at The University of Sydney, and supervised by Mary Byrne
Leaves can be thought of as the powerhouse of plants. They are three dimensional structures with cells and tissues specialised to carry out the critical function of photosynthesis, converting light into chemical energy for plant growth. Leaves are most often flattened structures. A look inside a leaf reveals that the arrangement of cells on the top side, which faces the sun, is optimised to capture sunlight and the cell arrangement on the bottom side, which faces away from the sun, is optimised for gas exchange. This leaf structure supports effective photosynthesis. In addition to this basic structure, plants possess an extraordinary diversity in the shape of leaves. Two extremes in shape are simple and compound leaves. Simple leaves have an undivided leaf surface. In compound leaves, the leaf surface is dissected into discrete units known as leaflets. Leaf shape can be made more elaborate through punctuated growth along the leaf margin, which for instance might produce serrations or lobes.
What determines leaf shape? Arabidopsis thaliana is a Brassicaceae species that has been used as a model system for studying plant biology because it is easy to grow and easy to use in genetic research. Arabidopsis has a simple leaf with serrations at the margin. In this study, we set out to investigate the genetic mechanisms regulating Arabidopsis leaf shape. Our focus was to determine the role of a family of TALE homeobox transcription factors in programming the serrations of Arabidopsis leaves. The TALE homeobox transcription factors are divided into two main groups based on the similarities and differences in protein sequence. There are 8 KNOX and 13 BLH TALE homeobox transcription factor genes in Arabidopsis. We found mutations in individual BLH genes do not affect leaf shape, suggesting these genes have overlapping and redundant functions. We confirmed this through looking at plants with mutations in multiple BLH and/or KNOX genes. Mutations in two BLH genes, SAW1 and SAW2, result in plants with more prominent serrations and these serrations are even more enhanced when combined with a mutation in one of the KNOX genes, KNAT3. Therefore, these genes normally function to limit serration growth. Remarkably the three genes are expressed on the top side of the leaf even though they function in growth at the leaf margin. We found that changing the domain where KNAT3 is expressed turns this gene from one that limits serration growth to one that promotes serration growth. Thus, leaf shape diversity might depend on the complement of genes expressed in leaves as well as where these genes are expressed. The more serrated leaves of the mutants result in just small increases in photosynthetic pathway products in controlled environmental conditions. Whether this has significance for growth and productivity remains to be determined.
