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Why rice can’t get along with its neighbors?

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Why rice can’t get along with its neighbors?

Post by Sk on Mon Jul 20, 2015 6:58 pm

Plants can’t pack up and leave when they don’t
like their new neighbors, but some plants,
including rice, can produce chemical inhibitors
that weed out the competition. Researchers in the Jander laboratory at BTI (Boyce
Thomson Institute for Plant Research) have
discovered a new compound in certain rice
varieties that may slow the growth of nearby
plants. “Rice is one of the best-studied plants in
the world, but still, there are a lot of compounds in there that are novel,” said BTI Professor Georg
Jander. “b-tyrosine was completely unknown as a
rice metabolite or even as a plant metabolite.” Jander suspects that rice plants use b-tyrosine for
allelopathy—a phenomenon where a plant gives
off chemicals into the surrounding soil that stop
competing organisms from encroaching on their
territory. Some rice varieties are especially good at
suppressing weeds, and other allelopathic
compounds from rice have been well studied.
Nevertheless, weeds can be a major problem,
particularly for upland rice, which is not grown in
flooded paddies. Therefore, breeding that increases rice allelopathy may be one way that
farmers can deal with this issue. Researchers in the Jander laboratory first
discovered b-tyrosine while looking for new
defense compounds in rice. When Yan tried to
classify the compound’s defensive properties, he
discovered that b-tyrosine did not deter insects, at
least not at concentrations that would normally be found in the plant. Aphids, black cutworm
larvae and sugarcane borer larvae all survived on
diets spiked with b-tyrosine. But when they tried
to grow a type of bacteria that commonly infects
crops, called Pseudomonas syringae, they found
that even low concentrations of the compound could reduce bacterial growth. Rice plants make
b-tyrosine in the leaves, where it may help
prevent bacterial infections. Perhaps most striking, b-tyrosine could reduce
root growth in multiple different plant species.
Rice varieties that generate b-tyrosine, as well as
other grasses, were immune to its effects, but the
compound was especially effective against dicots
—flowering plants that have two embryonic leaves when they sprout, including lettuce,
tomatoes and oak trees. The researchers used genetic mapping to show
that b-tyrosine biosynthesis is encoded on rice
chromosome 12, where they discovered the
responsible gene, called TAM1. It encodes
tyrosine aminomutase, an enzyme that converts
a-tyrosine into b-tyrosine. But not all rice varieties are made equal.
Researchers found b-tyrosine in a majority of the
japonica, or short-grain, varieties that they tested,
but the compound was absent in long grain
indica and aromatic varieties. Humans
domesticated rice twice, and the researchers found that one ancestral rice species had TAM1
and passed it on to its descendants, while the
other didn’t. In future experiments, the researchers plan to test
the ability of b-tyrosine-producing rice to inhibit
growth of its neighbors. They’ll compare
Nipponbare, a variety with TAM1, to the same
variety but with a mutation that blocks b-tyrosine
production, to see how well the compound inhibits the growth of nearby plants in soil. The
researchers know that rice excretes b-tyrosine, but
they don’t yet know if it accumulates in the soil at
concentrations that will deter weeds. “What we need to consider is, are these
physiologically-relevant concentrations? We think
we’re in the range where we might be doing
damage to other plants,” said Jander. However,
they believe that b-tyrosine is “one piece of the
puzzle of what makes rice able to inhibit the growth of other kinds of plants.”
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