As a student, most of us are first introduced to leaves as the 'kitchen of green plants that use
sunlight and carbon dioxide to make food'. This is mostly true. The primary function of leaves in most cases is to synthesize complex carbohydrates that can serve as an energy source for the plant.
Leaf structure is most often optimized to maximize the amount of incident sunlight, in order to make all that 'food'. This structure is in fact a result of the fastidious coordination of different molecules within the cells of leaves, directed by its genetic code.
The genetics involved in a process such as structure formation is exceedingly complex. Every leaf of every plant of a species looks perfectly alike. But of course, there are anomalies; it is such 'mistakes' that help scientists discover the faulty genes responsible for the anomaly. This consequently yields valuable information about the role of the gene when it is not faulty. Much of the research at Prof Utpal Nath’s lab at the Indian Institute of Science is focused on this intriguing aspect of plant science.
In leaves, growth is regulated by a 'proliferation zone' at its base, consisting of actively dividing cells; and an 'arrest zone' at its tip, consisting of cells that are not dividing but simply expanding in size. It is the harmonic coordination of these two processes-- cell division and cell expansion --that ultimately determine the final shape and size of a leaf. These processes are governed by genes and their products. The process of genetic regulation to achieve a precise 3D conformation is yet to be understood.
Arabidopsis thaliana, a small member of the cabbage family, is the most popular guinea pig when it comes to research on plants. A lot is already known about the genetics of this plant, in terms of which gene does what and how. And now, Prof. Utpal Nath at IISc and his team have isolated and characterized a novel mutant– tarani. This genetic fault is so named because its presence causes the leaves of the Arabidopsis plant to curl up like a boat; 'tarani' in Sanskrit means boat.
Leaves are generally flat. There is good reason for this. A thin flat surface ensures optimal capture of sunlight. Curled leaves are a case of genetic oddity. “Take, for instance, lettuce leaves,” says Prof. Nath, “they are a common example of genetic mutants when it comes to flatness of leaves”.
Leaf flatness is regulated by the transition zone that separates the proliferation zone from the arrest zone. Cells in the central transition zone continue to divide for longer, while cells towards the periphery quit division and begin expansion. Studies in Prof. Nath’s Lab with the tarani mutant have shown that the leaves of the tarani mutants possess a larger proportion of dividing cells at the centre when compared to the normal leaves. When cells at the centre continue to divide disproportionately compared to the periphery, the outcome is a curved upright boat shaped leaf deprived of its usual flatness. This newly characterized mutation has been mapped to the region between 7.1 and 7.5 Mb on chromosome 3 of the plant. Research at Prof. Nath’s Lab has proved that this stretch of DNA is responsible for maintaining 'flatness' of leaves in Arabidopsis.
Further studies would be necessary to determine the identity of the gene and elucidate the exact mechanism by which this gene contributes to development structure of leaves in Arabidopsis. This in turn could provide clues that lead to a better understanding of how the genetic code orchestrates the formation of something as simple as leaf, in flawless replicates.
About the study:
The paper appeared in the Journal of Experimental Botany (online first version) on February 24, 2015. It is freely accessible online at http://jxb.oxfordjournals.org/content/early/2015/02/23/jxb.erv015.abstract