Salinas, CA - Scientists have uncovered a crucial defense mechanism against seed rot in pea (Pisum sativum) The study sheds light on the role of proanthocyanidins (PAs), also known as condensed tannins, in conferring strong resistance to this common soil-borne disease.
High soil moisture content is typically the major environmental factor that increases disease severity; in wet weather or on poorly drained soils, decay may result in crop failure. Consequently, most growers of white-flowered pea use seeds treated with fungicides.
The study explored the effect of Mendel's (A-a) gene on seed rot resistance. The dominant form gives purple flowers-the recessive white flowers. It is well known that the gene also controls anthocyanin and related compounds in other parts of the plant, including the seedcoat.
The present work revealed purple-flowered plants have seed coats packed with PAs
toxic to the fungus, and the seedcoats have thick-walled cells. It is not surprising that
such seedcoats were strongly resistant to seed rot, even when modifying genes
removed seed coat anthocyanins.In contrast, white-flowered plants lacked PAs, had thin-walled cells, and were susceptible to seed rot. Strong resistance was a function of the seedcoat, not the embryo.
The contribution of the PAs and thick-walled cells to resistance was substantiated by studies with genes which modify the effects of A when it is dominant. One of these was the gene for flower color. When recessive it gives pink rather than the usual purple flowers.
Seeds borne on pink-flowered plants were a little less resistant to seed rot than seeds borne on purple-flowered ones. Chemical analyses showed pink was also associated with a distinctly different kind of PA in the seedcoat. Although the degree of difference was small, it provides further evidence of the relationship between PAs and resistance. It has been reported by others that there are two main kinds of PAs in peas. One kind is known to be higher in antioxidants but has the nutritional disadvantage that it ties up more iron. This is clearly important for countries where dry peas are a major source of protein, where iron deficiency is prevalent, and where colored peas are common. However, the present paper is the first to point out purple-flowering peas produce the PA said to bind the most iron, and pink-flowering produce the other. Pink flower color is a simple guide to select for binding less iron.
Fortunately, even seeds from pink-flowered peas have enough resistance to Globisporangium seed rot that seed treatment with chemical fungicides may prove to be unnecessary.
The research findings have broader implications for agriculture, particularly in the context of organic farming where certain chemical fungicides are restricted. Additionally, the increasing interest in the health benefits of phytochemicals associated with colored produce and the antioxidant power of proanthocyanidins adds value to colored pea varieties.
Lead author, Dr. Elmer Ewing, is Emeritus Professor at the Cornell University School of Integrative Plant Science Horticulture Section. Drs. Norman Weeden (Montana State University) and Ivan Simko (USDA Agricultural Research Service) co-authored this paper.
