Plant Sex Protein Identified at UC Riverside

Discovery Shapes Understanding of how Seeds are Created

Researchers at the University of California, Riverside have identified a protein that helps guide sperm to egg in flowering lily plants, a significant step forward in the field of plant reproduction.

Elizabeth Lord, professor of plant biology and a member of the Center for Plant Cell Biology at UC Riverside, authored the paper titled “Chemocyanin, a Small, Basic Protein from the Lily Stigma Induces Pollen Tube Chemotropism.” The paper appears this week in the Proceedings of the National Academy of Sciences.

Botanists have long known that, in flowering plants, the female organs play a role in guiding sperm-laden pollen tubes to the eggs found in ovules. But until now, they did not know exactly how. Lord’s team found that chemocyanin, a protein with a previously unknown function, effectively guided sperm-laden lily pollen tubes to the plant’s ovules, which hold the eggs from which come seeds in the lily. The protein works specifically in lilies. Tobacco pollen tubes were not similarly guided.

The paper also shows that chemocyanin was more effective when another protein found in the stigma of the lily, SCA, was present. Chemocyanin and other proteins such as SCA may unlock the network of signals involved in plant reproduction.

“The importance of understanding how plants reproduce is enormous for the future manipulation of crop and nursery plants,” said Lord. “There is a huge flower industry in California, and we know little about how seed set occurs in most flowering plants.”

Lilies are good examples because they cannot produce seed with their own pollen so they must be cross-pollinated with another variety, according to Lord. And while the industry grows Easter lilies from bulbs not seed, whenever they want to produce new varieties for the flower market or for gardens they have to produce seed by crossing varieties.

“This research has relevance for all flowering plants because we do not know yet how pollen tubes, which carry sperm cells, are guided to the egg cell in the ovary,” said Lord. “Our discovery of a protein from the pistil which acts to guide pollen tubes to the egg cell is a first for flowering plants.”

The protein, chemocyanin, is concentrated on the flower stigma, where pollen grains land on the flower. The pollen grains germinate on the stigma to form pollen tubes, which carry sperm cells, then pass through the female tissues starting from the stigma, ending up in the ovary, which contain ovules that contain eggs.

“You would be surprised to know that we don’t even know the identity of the molecules that attract human sperm cells to the egg,” Lord added.

Lord’s research team at UC Riverside included doctoral students Sunran Kim and Juan Dong; postgraduate researchers Jean-Claude Mollet and Sang-Youl Park; and academic coordinator in the Department of Chemistry, Kangling Zhang.