ASPB Research Points to Biotech Potential

The American Society of Plant Physiologists recently changed its name to the American Society of Plant Biologists and celebrated the event during its summer meeting in Providence, R.I. More than 1,400 scientists, including members of the Canadian Society of Plant Physiologists, heard and presented results on everything from genomics to pathogenesis to pattern formation, and some of the work had strong biotechnological implications. Here's a sample of what transpired: Salt of the Earth A little salt may taste great on a sliced tomato, but a lot of it in the soil spells doom for the parent plants. Just 65 mM NaCl can reduce crop yields by 50 percent, according to Eduardo Blumwald of the University of California at Davis. California's farms, like many others worldwide, are threatened by soil salinization, in this case due to irrigation. Says Blumwald, "This is not just a Middle East problem." In a minisymposium on membrane transport, Blumwald reported on research aimed at helping crops grow in salty soils. Along with Hong-Xia Wang, now at the University of Toronto, Blumwald cloned several genes encoding Na+/H+ antiport proteins from Arabidopsis thaliana, a member of the mustard family. Antiports exchange protons, which plants secrete in abundance using a pump powered by photosynthesis, for sodium ions, which end up either outside the cell or inside storage organelles called vacuoles. When Wang and Blumwald inserted one of the genes into tomato plants, antiport activity in isolated leaf vesicles skyrocketed. The plants grew well on 200 mM NaCl and were as large as untransformed controls. Unfortunately, naturally salty tomatoes aren't in the cards. Sodium levels remained low inside transformed tomato fruits, which were as sweet as wild type tomatoes. Blumwald estimates that by planting transformed canola--a cousin of Arabidopsis that grows at high density--farmers could extract up to 4000 kg of salt per 10,000 square meters of high saline land per year, thus reclaiming it for other crops. Bittersweet Sticky Stuff Ever notice that some plants, like tobacco, are sticky to the touch? That's because their leaves are covered with glands called trichomes, which exude gooey complex polysaccharides and other substances. Some of those natural chemicals, including terpenoids, kill insects such as aphids that might otherwise chow down on the nutritious greens. Up to 17 percent of the dry weight of tobacco leaves consists of trichome exudate. If researchers enhanced the insecticidal properties of trichomes, farmers could reap the harvest of reduced costs from conventional pesticides. That's just what George Wagner and colleagues at the University of Kentucky had in mind when they made copy or cDNA from tobacco trichome messenger RNA. The team discovered that one of the cDNAs encodes a cytochrome P450 protein that converts a terpene called CBT-ol into its dihydroxy form, CBT-diol. By transforming tobacco plants with an antisense version of the gene, they turned off the reaction, thereby increasing CBT-ol in exudate. It just so happens that resistance to aphids correlates with high CBT-ol, and indeed, the revamped exudate killed more of the insects. Likewise, transformed tobacco plants were far less infested. A Nickel's Worth of Zinc Plants in the mustard family, or Brassicaceae, are important to botanists and the consuming public alike. Not only does the Arabidopisis belong to the group, so do cabbage, brocolli, cauliflower, and canola. Recently, plant biologists set their sights on another pet mustard. The genus Thlaspi includes weedy species like S. goesingense and S. caerulescens that hyperaccumulate metals such as nickel and cadmium, allowing the plants to grow on contaminated sites. The metal ions end up in cellular storage compartments called vacuoles, thus preventing damage. Out of sight, out of mind. A better understanding of how certain Thlaspi plants take up so much metal could be used to transform other plants that would remediate toxic waste sites and alleviate essential ion deficiencies in the human diet. With that in mind, a team led by Michael Persons of Purdue University recently cloned genes for metal tolerance proteins, or MTPs, from T. goesingense. MTP genes encode cation efflux transporters that move metal ions into vacuoles. The genes are also present in Arabidopsis--hyperaccumulation in Thlaspi apparently resides in greater gene expression in these species. When inserted into metal-sensitive yeast mutants, the genes correct the defect. In a symposium entitled 'Plant Physiology 2001 and Beyond: Breaking the Mold,' Leon Kochian of the US Department of Agriculture's Agricultural Research Service at Cornell University in Ithaca, N.Y., reported similar work on Thlaspi caerulescens. Leaves of T. caerulescens accumulate zinc and cadmium ions at levels up to 50,000 ppm. Kochian's team found five zinc transporter, or ZPT genes in Thlaspi, three of which don't seem to down regulate in response to high zinc levels. In other words, they stay active even in the presence of lots of metal, when transport proteins shouldn't be needed. Barry A. Palevitz (palevitz@dogwood.botany.uga.edu) is a contributing editor for The Scientist.

ASPB Research Points to Biotech Potential

Salt of the Earth

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