Time-lapse imaging unravels leaf shape evolution
Researchers have used a combination of time-lapse imaging and genetic manipulation to identify a gene that shapes the very different leaves of two closely related plants.
Looking at angiosperm leaves, Daniela Vlad from the Department of Plant Sciences, University of Oxford and colleagues were intrigued as to why the Cardamine hirsuta has dissected leaves while its relative, Arabidopsis thaliana, has simpler, oval-shaped leaves.
The model organism Arabidopsis thaliana (left) next to its relative Cardamine hirsuta (hairy bittercress, right). Note the simple leaves of A. thaliana versus the dissected leaves of C. hirsuta. [Image courtesy of Angela Hay and Miltos Tsiantis]
Genetic screening revealed that a leaf-specific gene known as RCO guides development of dissected leaf patterns, sculpting developing leaflets by repressing growth at the flanks.
To study the impact of the gene, Vlad and colleagues imaged developing leaflets with fluorescently-labelled cell nuclei, using confocal laser scanning microscopy at 24 hour time intervals.
As Vlad explains, the researchers used MorphoGraph X software to analyse these time-lapse confocal images, extracting information on organ surface, segmenting cells and computing aerial expansion.
Seedling of the emerging model organism C hirsuta visualized using confocal microscopy and morphographix software. [Image courtesy of Daniel Kierzkowski, Richard Smith and Miltos Tsiantis]
Observations revealed that the RCO gene suppresses growth at certain locations along the perimeter of a developing C hirsuta leaflet, leading to the more complex, dissected leaves.
Meanwhile, cell expansion and proliferation in these leaflet regions of A thaliana was faster, filling up the space and yielding rounder leaves.
Confocal micrographs of developing leaf primordia of C hirsuta (dissected leaf shape) and A thaliana (simple leaf shape). [Image courtesy of Daniel Kierzkowski and Miltos Tsiantis]
The researchers believe that while the C hirsuta, with its more complex, dissected leaves has inherited RCO, this gene is now actually missing in its close relative, the Arabidopsis thaliana.
Time-lapse imaging of Cardamine hirsuta developing leaves using Confocal Laser Scanning Microscopy. The cell nuclei are fluorescently labelled. [Video courtesy of Carla Galinha and Miltos Tsiantis]
"RCO evolved in the Brassicaceae family through gene duplication and was lost in A, thaliana, contributing to leaf simplification in leaves," says Vlad.