PHILADELPHIA - Biologists at the University of Pennsylvania have identified the first gene known to mediate the maturation of plants from a juvenile stage to adulthood. The discovery could lay the foundation for crops that repel pests by taking advantage of natural differences between younger and older plants, reducing farmers' reliance on pesticides while sidestepping the controversy surrounding produce engineered with the addition of genes from other species.
The work is detailed in the March 23 issue of the journal Science.
While versions of the newfound gene appear in species from yeast to humans, the Penn findings represent the first demonstration of function in a higher organism, in this case the plant Arabidopsis thaliana. The gene, called squint because mutant seedlings' pointy, elongated leaves resemble squinting eyes, is believed present in all flowering plants, including such valuable crops as corn, tomatoes and soybeans.
Capitalizing on the natural morphological and biochemical differences that characterize these crops at different stages of development could further curtail pesticide use, said lead author R. Scott Poethig, while avoiding the highly contentious practice of importing genes from other species, conventionally known as genetic engineering.
"Many pests find either juvenile or adult plants unpalatable, so tinkering with the genes that control plant development could render crops uninviting," said Poethig, a professor of biology in Penn's Plant Science Institute.
For example, Poethig said, mature leaves on corn and rice plants are more resistant to pests than their more tender counterparts, and only the juvenile, lowermost branches of birch, willow and aspen trees found in Arctic regions are distasteful to the snowshoe hares that might otherwise graze on them.
"Mutations like squint allow you to use a plant's natural resistance to disease, and other naturally occurring developmental traits, in different ways," Poethig said. "Instead of introducing a foreign gene from another species, one should be able to isolate mutations in squint-like genes that cause a normal, desirable trait to be expressed at a different time in development."
Squint encodes the protein Cyclophilin 40 (CyP40). CyP40's biochemical function is already known - in human beings, it's part of a complex that blocks receptors for hormones like estrogen and progesterone - but its physiological role in higher organisms has remained a mystery.
Poethig's work with plants mutant in squint indicates that CyP40 affects secondary characteristics of adult plants, like the shape and biochemical properties of leaves, but not sexual maturation or flowering. The very first leaves that appear on a squint mutant are toothed and angular, like mature leaves, rather than stubby and rounded like juvenile leaves. The timing of sexual maturity and flowering, though, is not affected.
"Many mutations affecting flowering time have been identified, and naturally occurring variation in flowering time is widely exploited in agriculture," Poethig said, "but I am not aware of a concerted effort to change the expression of juvenile and adult vegetative traits."
Among plants, Poethig noted that CyP40 helps explain why ivy looks the way it does: the lobed leaves seen growing on college campuses are characteristic only of the juvenile plant. A marked difference is also seen in oak and beech trees, where only the upper, adult branches drop their leaves in autumn. The lowermost, juvenile branches retain their dead leaves throughout the winter.
Poethig's co-authors are Tanya Z. Berardini, Krista Bollman and Hui Sun, all of Penn's Plant Science Institute. The work was funded by the National Institutes of Health and the National Science Foundation.