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How Plants Make Do in Tough Times

4 August 1999 7:00 pm
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BALTIMORE--When resources are scarce, plants dole out precious sugars to the cells that need them most. This rationing, scientists have discovered, is influenced by hormones called cytokinins. The finding, reported here last week at the annual meeting of the American Society of Plant Physiologists, may help explain how single-celled organisms evolved into multicellular plants.

Plants produce the complex sugar sucrose through photosynthesis and ship it via a network of tissue called the phloem. Once sucrose arrives in a cell, either the enzyme invertase or sucrose synthase chops it into usable bits. Both enzymes come in "feast" or "famine" forms, depending on the amount of available sucrose. In either case, invertase, present in growing tissues such as root tips, leaf buds, and especially in developing seeds or kernels, provides twice as much usable sugar.

Plant physiologist Karen Koch and her colleagues at the University of Florida, Gainesville, examined whether plant hormones tip the balance of sucrose-eating enzymes and thereby influence how the plant distributes its resources. They bathed corn root tips and cell cultures in cytokinins, plant hormones released by dividing cells. In the presence of cytokinins, the hungrier the cells were, the more invertase they produced. In contrast, the activity of sucrose synthase--an enzyme that helps build complex molecules--remained steady regardless of whether cytokinins were circulating in the mix.

This hormonal control system helps explain how plants direct needed energy to crucial locations such as roots and seeds, says plant physiologist Larry Noodén of the University of Michigan, Ann Arbor. "This makes a lot of sense," he says. Such a signaling system would have offered an evolutionary advantage, Koch adds, because it would allow a group of cells to send a long-distance signal to prioritize which cells get scarce resources.