Size of seed depends on mom

U. TEXAS-AUSTIN (US) — Seed size is controlled by small RNA molecules inherited from a plant’s mother, a discovery that has implications for agriculture and understanding plant evolution.

“Crop seeds provide nearly 70 to 80 percent of calories and 60 to 70 percent of all proteins consumed by the human population,” says Z. Jeff Chen, professor of plant molecular genetics at the University of Texas at Austin. “Seed production is obviously very important for agriculture and plant evolution.”

Published in Proceedings of the National Academy of Sciences, the research provides the first genetic evidence that seed development is controlled by maternally inherited “small interfering RNAs,” or siRNAs,  known to control a number of aspects of growth and development in plants and animals.

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For the study, the researchers used Arabidopsis, a rapidly growing flowering plant in the mustard family.

In this case, siRNAs influence the development of a seed’s endosperm, which is the part of the seed that provides nutrients to the developing plant embryo, much like the placenta in mammals. The endosperm is also the source for most of the nutritional content of the seed for humans and animals.

Despite the importance of the endosperm, little has been known about the molecular mechanisms that govern its growth.

In flowering plant seeds, the embryo is formed by fusion of one paternal and one maternal genome, while the endosperm combines one paternal and two maternal genomes. This process of embryo and endosperm formation is known as “double fertilization.”

When a female plant with a duplicate genome (known as a tetraploid) is crossed with a male plant with a normal genome (called a diploid), not only is there an increase in the maternal genome in their offspring’s seed endosperm, but there is also an associated increase in maternal siRNAs.

Those maternal siRNAs decrease the expression of genes that lead to larger endosperm growth, meaning that the siRNAs create smaller seeds.

“Now we understand that siRNAs play a large role in sensing maternal and paternal genome imbalance and controlling seed development, and that maternal control is important,” Chen says.

Chen and colleagues, including David Baulcombe at the University of Cambridge, are working to find out how exactly siRNAs regulate gene expression in the endosperm and embryo and how they control seed size.

The new findings will enable scientists to develop biotechnological tools for improving seed production and crop yield.

But Chen cautions that “bigger isn’t always better.” In fact, in his experiments, seeds lacking the control of the maternally inherited siRNAs grew so large that they collapsed.

Chen’s research is funded by the National Science Foundation Genetic Mechanisms program and a Fulbright Award he received to do research with Baulcombe, who is widely recognized for his pioneering and seminal research discovering the role of siRNAs in gene silencing in plants.

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