Plastic vs. Plants: Mulch method changes tomato’s gene activity

A suite of at least 10 genes in a tomato plant behaves differently depending on the farmer’s mulch-and-fertilizer routine, according to an unusual analysis.

MULCH MATTERS. Two fields of Sunbeam tomatoes planted at the same time and photographed on the same day. In a season with adequate rainfall, tomatoes mulched with black plastic and given standard fertilizer (above) developed more leaf diseases and aged faster than the same variety planted in mown vetch and given half the fertilizer (below). Dave Clark

Dave Clark

Earlier work showed that when researchers mulch with a layer of mown vetch instead of the conventional black plastic, tomato plants live longer and develop less fungal disease, says Autar Mattoo of a Department of Agriculture laboratory in Beltsville, Md. He and his colleagues now report insight into the underlying plant chemistry.

Among the genes that kick into higher activity in vetch-mulched fields are two for plant defense and two for regulation of aging, the researchers say in an upcoming Proceedings of the National Academy of Sciences. “This kind of molecular analysis had not been done before,” says Mattoo.

The vetch system has been around for decades among farmers practicing environment-friendly agriculture. Over the winter, the farmers grow hairy vetch, a member of the bean family, and they mow it in the spring. They then set out tomato plants in the matted vetch, which discourages weeds and boosts nutrients in the soil.

Beltsville researchers compared vetch-mulched fields given half the normal dose of commercial fertilizer with plastic-mulched fields given a full dose of fertilizer. In years with sufficient rainfall, the vetch-treated fields yielded a Sunbeam tomato crop bigger than that of fields receiving conventional treatment. Although the plastic mulch provided the crop a little sooner, the vetch mulch offered other benefits, including reduced erosion, decreased signs of fungal disease on leaves, and delays in plant aging. These benefits appeared during two seasons but not in a third, when there was a severe drought.

To search for a mechanism behind the disease and aging benefits, Mattoo, Vinod Kumar of the same lab in Beltsville, and their colleagues studied proteins and genes in tomato plants grown in the two mulch-and-fertilizer systems. Among their analyses, the researchers used a technique that highlights differences in gene activity.

Among the genes that kicked into higher activity in vetch-mulched plants are a gene for chitinase, an enzyme that chews up the cell walls of attacking fungi, and a gene for osmotin, another defensive compound. The researchers also found extra activity for receptors for cytokinins. These plant hormones, which travel from the roots to the rest of the plant, are regulators of aging.

Mattoo has many ideas to explain the difference in the effects of the two crop-growing methods. For instance, he points out that nutrients can influence gene activity and that vetch-mulched tomatoes typically produce especially robust root systems that take up soil nutrients efficiently.

Plant physiologist Thomas Sinclair of the USDA in Gainesville cautions that the patterns of gene activity reported so far are “correlations, not cause and effect.” He adds that whatever the physiology turns out to be, “I’ll guarantee you it’s going to be very complicated.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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