By Helena Morales Johansson, Checkbiotech.org

In the near future it could be possible to reduce the loss of maize crop due to damage at low temperature with the help of a new finding by researchers in the United States.

Maize is of tropical Mexican origin and its importance as human nutrition, as well as food source for livestock, has expanded outside its Mexican origins. Due to the increased need of maize it is grown in other temperate zones such as Europe and the United States, where it has to withstand temperatures below 15°C. During spring, there can be sudden drops in temperature for some days enough to damage a majority of the crop. If the temperature goes below 0°C it is more deleterious since maize is a frost-sensitive plant.

Due to the sensitivity of maize, the researchers, Wang and his colleagues at Iowa State University in collaboration with the Massachusetts General Hospital and Harvard Medical School in Boston, focused on finding how to enhance cold resistance in maize. The study intended to see if maize had a mechanism of its own, that could be improved upon to increase its ability to survive at low temperatures. The researchers introduced a gene, called NPK1, from tobacco into the maize plants that has previously been shown to be important for survival of tobacco plants at low temperatures. NPK1 activates a chain reaction, known as a signaling cascade, which induces the expression of genes important for survival at low temperature.

To investigate the effect of NPK1 on maize plant survival at temperatures ranging from cold to sub-freezing that normally damages the maize plants Wang and his colleagues introduced a fragment of the tobacco derived NPK1 gene into maize. The tests demonstrated that maize plants containing NPK1 survived longer at low temperatures than maize plants lacking NPK1. Furthermore, introducing NPK1 in maize did not affect the growth of maize.

As the researchers searched for an explanation of how NPK1 protects maize from damage at low temperature, classic theory led them to sugars. In research literature, sugars are documented as anti-freezing agent with higher levels positively correlating with an increased freezing tolerance in plants. However, Wang and his colleagues did not find a correlation between high sugar levels and higher resistance to low temperatures. Thus, the complete mechanism of how NPK1 provides protection at low temperatures in maize is not yet clear, says Dr Wang.

However, as Dr Wang points out, “Almost all organisms have this gene and the signaling cascade is only upregulated upon stress. The protective activation of this cascade may be too slow in maize. By the time the defense is upregulated the organism is dead. The insertion of NPK1 in maize allows the activation of maize’s natural responses to stress in a much shorter time period than through natural acclimation.”

An added benefit is that, NPK1 is not only involved in protection at low temperatures but it also helps plants survive drought, saline soil conditions and soils polluted with heavy metals. These properties indicate its importance for general stress resistance, which is important in agriculture where the growing conditions can change dramatically within a short time range. For example, the use of low temperature resistant maize could reduce the loss of maize plants and decrease the costs of producing maize. In the long term, this improvement could result in sustained production of maize leading to lower prices for the customers.

Source:
Expression of an active tobacco mitogen-activated protein kinase kinase kinase enhances freezing tolerance in transgenic maize
Huixia Shou, Patricia Bordallo, Jian-Bing Fan, Joanne M. Yeakley, Marina Bibikova, Jen Sheen and Kan Wang
Plant Transformation Facility, Department of Agronomy, Iowa State University, Ames, IA 50011-1010;
Illumina Inc., 9885 Towne Centre Drive, San Diego, CA 92121; and
Department of Molecular Biology, Massachusetts General Hospital, and Department of Genetics, Harvard Medical School, Boston, MA 02114
Proceeding of the National Academies of Sciences U S A. 2004 Mar 2;101(9):3298-303
Communicated by Luis Herrera-Estrella, National Polytechnic Institute, Guanajuato, Mexico, December 9, 2003 (received for review October 9, 2003)

Cold acclimation is the major process that prepares plants for freezing tolerance. In addition to extensive transcription regulation by cold-inducible master transcription factors, oxidative stress signaling has been postulated to play a role in freezing tolerance. Activation of oxidative signaling through the expression of an active mitogen-activated protein kinase kinase kinase provided benefits in transgenic tobacco at freezing temperature bypassing cold acclimation. Because involvement of the mitogen-activated protein kinase cascade in oxidative stress signaling is evolutionarily conserved in eukaryotes from yeast to mammals, we tested the effect of expressing a heterologous tobacco mitogen-activated protein kinase kinase kinase (Nicotiana PK1), which can mimic H₂O₂ signaling, in a major cereal crop. We demonstrate that low-level but constitutive expression of the Nicotiana PK1 gene enhances freezing tolerance in transgenic maize plants that are normally frost sensitive. Our results suggest that a new molecular approach can be designed to genetically enhance freezing tolerance in important crops.