Longer growing season, growth in colder regions possible
Demand for corn - the world's number one feed grain and a staple food for
many - is outstripping supply, resulting in large price increases that are
forecast to continue over the next several years. If corn's intolerance of
low temperatures could be overcome, then the length of the growing season,
and yield, could be increased at present sites of cultivation and its range
extended into colder regions.

Drs. Dafu Wang, Archie Portis, Steve Moose, and Steve Long in the Department
of Crop Sciences and the Institute of Genomic Biology at the University of
Illinois may have made a breakthrough on this front, as reported in the
September issue of the journal Plant Physiology.

Plants can be divided into two groups based on their strategy for harvesting
light energy: C4 and C3. The C4 groups include many of the most
agriculturally productive plants known, such as corn, sorghum, and sugar
cane. All other major crops, including wheat and rice, are C3. C4 plants
differ from C3 by the addition of four extra chemical steps, making these
plants more efficient in converting sunlight energy into plant matter.

Until recently, the higher productivity achieved by C4 species was thought
to be possible only in warm environments. So while wheat, a C3 plant, may be
grown into northern Sweden and Alberta, the C4 grain corn cannot. Even
within the Corn Belt and despite record yields, corn cannot be planted much
before early May and as such is unable to utilize the high sunlight of
spring.

Recently a wild C4 grass related to corn, Miscanthus x giganteus, has been
found to be exceptionally productive in cold climates. The Illinois
researchers set about trying to discover the basis of this difference,
focusing on the four extra chemical reactions that separate C4 from C3
plants.

Each of these reactions is catalyzed by a protein or enzyme. The enzyme for
one of these steps, Pyruvate Phosphate Dikinase, or PPDK for short, is made
up of two parts. At low temperature these parts have been observed to fall
apart, differing from the other three C4 specific enzymes. The researchers
examined the DNA sequence of the gene coding for this enzyme in both plants,
but could find no difference, nor could they see any difference in the
behavior of the enzyme in the test tube. However, they noticed that when
leaves of corn were placed in the cold, PPDK slowly disappeared in parallel
with the decline in the ability of the leaf to take up carbon dioxide in
photosynthesis. When Miscanthus leaves were placed in the cold, they made
more PPDK and as they did so, the leaf became able to maintain
photosynthesis in the cold conditions. Why?

The researchers cloned the gene for PPDK from both corn and Miscanthus into
a bacterium, enabling the isolation of large quantities of this enzyme. The
researchers discovered that as the enzyme was concentrated, it became
resistant to the cold, thus the difference between the two plants was not
the structure of the protein components but rather the amount of protein
present.

The findings suggest that modifying corn to synthesize more PPDK during cold
weather could allow corn, like Miscanthus, to be cultivated in colder
climates and be productive for more months of the year in its current
locations. The same approach might even be used with sugar cane, which may
be crossed with Miscanthus, making improvement of cold-tolerance by breeding
a possibility.

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