The Jander Lab uses genetic and biochemical approaches to study plant-insect interactions and plant amino acid metabolism. We employ the small crucifer Arabidopsis thaliana (Arabidopsis) as a model system for most of our research.
Myzus persicae (green peach aphids) feeding on an Arabidopsis flower stalk.
Plant-insect interactions
The phloem-specific feeding style of aphids poses unique challenges, both to the host plants, which must mount defense responses against herbivores that do little overt damage, and to the aphids, which must adapt to a nutritionally imbalanced and potentially toxic food source. Myzus persicae (green peach aphids) are important agricultural pests and also feed readily from Arabidopsis. We are studying plant recognition of M. persicae salivary components, Arabidopsis gene expression changes induced during aphid feeding, and plant secondary metabolites that contribute to aphid defense. On the aphid side of the interaction, our goals are to identify M. persicae enzymes that detoxify plant metabolites, as well as aphid gene expression changes that are induced in response to plant defenses.
Plant amino acid metabolism
Unlike plants, which can synthesize the complete repertoire of protein amino acids, humans and other animals must obtain certain essential amino acids in their diets. However, despite the nutritional importance of plant-derived amino acids, many aspects of plant amino acid metabolism remain uninvestigated. Recent research in the lab has been focused on studying the function two plant enzymes, threonine aldolase and homocysteine methyltransferase, which were discovered in a mutant screen for Arabidopsis lines with altered seed amino acid content. Whereas threonine aldolase mutations increase seed threonine levels, homocysteine methyltransferase mutations increase seed methionine accumulation. Manipulation of these two enzymes could be used to improve the nutritional value of crop plants in which threonine and/or methionine are limiting for mammalian diets.
Understanding how plants protect themselves from insects could lead to the development of crop plants such as canola or cabbage that produce naturally-occurring insect deterrents. Such an advance could significantly reduce the amount of man-made, chemical insecticides released into the environment.
Georg Jander is studying a mechanism by which Arabidopsis plants fend off attacking caterpillars. It involves chemicals produced by the plant – called glucosinolates – which actually attract certain insects, but when these chemicals are broken down by enzymes in the plant or in the insect gut, smaller molecules are released that are natural insect repellents.
It’s known that white cabbage butterflies avoid laying eggs on glucosinolate-producing plants where caterpillars are already feeding. Although earlier work at BTI proved that intact glucosinolates attract egg-laying cabbage butterflies, Jander theorized that a glucosinolate breakdown product was acting as the deterrent. The molecule, called indole-3-acetonitrile, is produced when an enzyme in the caterpillar’s gut reacts with indole glucosinolates in the plant tissue to reduce their toxicity. When the caterpillar regurgitates on the plant, the molecule is left behind. Jander theorized that this molecule signals cabbage butterflies that caterpillars are present,which deters them from laying eggs.
Working with Arabidopsis plants and white cabbage butterflies, Jander’s lab proved that indole-3-acetonitrile in fact deters egg laying. They also found that some Arabidopsis plants naturally produce this molecule in the absence of caterpillar feeding. As a result, Jander believes that nitrile production by the plant is an adaptive mechanism to reduce the caterpillar population on the plant by reducing the number of eggs laid on it.
In other Arabidopsis plants, glucosinolates are converted into indole-3-carbinol when insects feed on them. This molecule reacts with other compounds in the plant to produce new molecules that are toxic to many insects, but attract egg-laying cabbage butterflies. Since indole- 3-actonitrile and indole-3-carbinol are produced from the same starting material, plants seem to face a defensive tradeoff concerning which one to produce. Jander theorizes that a plant’s habitat and the prevalent insect threat determine which molecule is produced in greater abundance.
Picture this: You can't move, and small insects are eating you alive. That's the nightmare scenario many plants cope with everyday. One way they do is with chemical weapons.
Glucosinolates, for example, sit benignly inside a cell until it's punctured—usually by predator feeding. They then mix with certain enzymes from other cells, reacting to yield sharp-tasting, sometimes toxic compounds such as those that give mustard and horseradish their distinctive tanginess.
Because of this phenomenon, “a radish doesn’t taste like a radish until you bite into it,” explains Georg Jander. In one project, he studies interactions of green peach aphids with Arabidopsis to learn more about how plants deploy glucosinolates—and how insect pests get around them.
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Aphids on Arabidopsis flower stalk.
Recently, Jander’s lab let aphids snack on a certain Arabidopsis variety and analyzed plant extracts to find which glucosinolates the plant responded with. Although this variety can make 20 different glucosinolates, they found that aphid feeding caused the plant to produce only some of these. The lab also fed aphids sugar solutions containing different glucosinolates to find which affected their reproduction. Of all the glucosinolates in the Arabidopsis strain’s arsenal, only those induced by aphid feeding had a significant negative effect on the insects.
These results seem to indicate that the plant can somehow tell aphids apart from other pests and react accordingly. Its other glucosinolates may be intended for other insects or hungry animals. Jander plans to test both these hypotheses and find out more about how plants differentiate between predators and communicate the need for defensive responses. In the future, such knowledge could help agricultural scientists make crops better equipped to ward off insect herbivores.
For college students, tight finances often motivate a diet of ramen noodles. For farmers faced with a tough bottom line, though, skimping on livestock feed isn't an option: while using just one crop for fodder would be cheap and simple, their animals need all of the essential amino acids to stay healthy. Since any one plant can't provide all of the amino acids in sufficient quantities, farmers fill in the gaps with supplements, at a cost of over a billion dollars a year in the United States.
Developing crops with elevated levels of needed amino acids could cut down on those costs but to engineer fortified foods, scientists need to know more about the pathways plants use to make the compounds. (Like animals, plants need 20 amino acids to make vital proteins, but unlike animals, most plants can make them all). To better understand the pathways, and how tinkering with them might affect other processes in plants, Georg Jander's lab is trying to make an Arabidopsis plant that contains more threonine. In Arabidopsis seeds, the enzyme threonine aldolase normally converts most threonine into another amino acid, glycine.
Jander's lab members tried knocking out a gene for threonine aldolase, but found this killed the plant. They then tried making a plant that had no threonine aldolase but extra threonine deaminase, an enzyme that converts threonine into yet another amino acid. The doubly-mutated plant appeared healthy, indicating that excess threonine (not lack of glycine) probably killed the plants with just threonine aldolase knocked out. If the lab can find a way to make healthy, threonine-rich Arabidopsis, other researchers might be able to apply that knowledge to threonine-deficient crops such as soy and rice, making healthy eating a little bit cheaper.
