The striping phenotype in mutator stocks: A viral etiology?

Several years ago I gave Dr. Michael Freeling a couple of stocks of Mu to use in his research project. He later reported that plants of the Mu stocks I furnished him, when grown at Berkeley, frequently showed yellow or white striping. These original stocks were outcrosses of Mu to a standard line. The progeny of selfs of striped plants segregated for a high proportion of striped plants as well as other pigment mutants, runts, etc. Outcrosses of striped plants to normals frequently would have striped plants in the progeny, in seedlings as well as mature plants. Because striping was only very rarely observed in my nursery, I asked Dr. Freeling to furnish me with some samples of his striped stocks to grow in Iowa. These were grown in 1978. I did not observe striping in any of his material in the field that year but the plants were self-pollinated and outcrossed to one of my standard lines. The progenies of the selfs and outcrosses were seedling tested but no striped seedlings were observed.

Dr. John Mottinger was on leave last school year in Dr. Freeling's laboratory and had opportunity to observe the striped plants. Some had a phenotype that suggested they might have a viral infection so he tried to transmit the striping phenotype to normal (non-Mu) plants by infection. He reported at the 1980 Maize Genetics Meeting that an occasional stripe was observed in some of the infected plants.

This past summer (1980) I was on leave at the University of California, Davis, where I grew part of my nursery. I also sent material to Dr. Freeling at Berkeley to grow for me. The Berkeley material included Dr. Freeling's original striped stocks (made at Ames) and outcrosses of them. Also included were several of my mutator stocks and lines that had lost Mu activity. At Davis I also grew some of Dr. Freeling's striped stocks.

I have never consistently observed striping in my mutator stocks at Ames. An occasional plant is observed that has a single stripe typical of a sector due to the mitotic loss of a gene or mitotic mutation but certainly nothing that would resemble a viral infection.

The material grown at Davis did not differ from what I had observed in my stocks at Ames. Even the Freeling striped stocks did not have an obvious striping phenotype.

The material grown at Berkeley, however, did have a high proportion of striped plants. Most of the striping was of the mitotic sector type, frequently with numerous sectors per plant. Some plants had a "striate-like" phenotype that might be expected of a viral infection. The sector type of striping was found in all of the Mu stocks planted (i.e., Dr. Freeling's and mine). The non-Mu lines and standard lines did not show the striped phenotype. Also some plants had a blotched phenotype in which there were numerous large irregular areas of pale green tissue on the leaves. This phenotype was observed in some Mu stocks but was also common in families of my Q60 standard. The Mu stocks with blotches were those that had been crossed to the Q60 standard.

It is obvious that Mu stocks are susceptible to striping when grown at Berkeley but not at Ames or Davis (or Hawaii where I have my winter nursery). The one obvious difference in growing conditions found at Berkeley and not at the other locations where Mu stocks have been grown is the temperature over the growing season. Berkeley's growing conditions are considerably cooler. Many cool, foggy and/or cloudy mornings (or complete days) are observed. Temperatures rarely get into the eighties for any extended period of time. These conditions must be favorable for the expression of the striping phenotype. Plant pathologists have told me that cool temperatures frequently enhance the expression of viral symptoms in plants. These observations along with Dr. Mottinger's infection experiments suggest the presence of a virus in mutator stocks.

Electron microscopic studies of Mu stocks from Ames, and striped stocks from Berkeley have revealed rare rod-like bodies of dimensions similar to those of other plant viruses. However, plant virologists are not certain these bodies are indeed viruses. One was of the opinion that they were microtubules. Thus, additional tests must be made to confirm the presence of a virus.

Outcrosses were made of the striped and non-striped plants at Berkeley. The plants were evaluated for the degree of striping and classified into five classes from non-striped to heavily striped (i.e., non-striped, +, ++, +++ and ++++). outcrosses of plants from each of these classes will be grown next summer to determine if there is any relationship between the degree of striping and the mutation frequency. If the striping is caused by a virus, and if there is a relationship between the severity of the infection and the amount of striping, and if (a big if) the virus is responsible for the mutator activity, then there may be a direct relationship between the mutation frequency in an outcross and the amount of striping observed in the tested parent.

One additional experiment is underway which is designed to test for viral involvement in Mu. Extracts of mutator plants were rubbed on leaves, dusted with silicon carbide powder. If a virus is involved, the standards might be infected and become mutator plants which would transmit mutations to their outcross progeny. Outcrosses of these "infected" plants are being grown this winter in Hawaii.

Donald S. Robertson


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