Developing Disease-Resistant Rice

Rice is a significant food source and reportedly feeds half of the human population. Avoiding the loss of crops due to disease is therefore a quest with significant positive consequences if successful. Two particularly troubling rice pathogens are Rice Blast and Bacterial Leaf-Blight.

Rice Blast (left) and Bacterial Leaf-Blight.

When plants are attacked they activate defence responses, one of which is the salicylic acid signalling pathway that protects against pathogens that feed on the plant (biotrophic pathogens). Somewhat similar to animal immune systems, a previous exposure to the pathogen can prime the plant’s defence response, conferring a greater resistance if subsequently attacked by the same pathogen. Therefore, if this signalling pathway could be activated prior to exposure to the pest, it should be resistant.

The authors of a recent article in the Plant Biotechnology Journal referred to previous studies that identified a gene (WRKY45) that regulated the salicylic signalling pathway and, when activated, has been shown to promote protection against both Rice Blast and Bacterial Leaf-Blight.

The common problem with increasing disease resistance in plants is that it causes problems with its other properties, most notably its growth and ability to withstand other stresses such as drought and salinity. This was seen in a previous study when the WRKY45 gene was over-expressed and resulted in substantial growth suppression.

Therefore, these researchers went on a search for a way of optimising disease resistance but also maintaining agronomic properties in the rice plants. To do this they found a series of weak but varying strength, constantly active, gene transcription promoters and inserted them upstream of the WRKY45 gene, and grew a heap of rice plants.

As controls to compare these plants against they had normal rice plants and plants that had a strong promoter in front of the WRKY45 gene. Plants with the strong promoter showed good resistance against the two pathogens and the normal rice plant didn’t resist disease.

The researchers the tested the 16 lines of plants with varying levels of WRKY45 gene expression as follows:

  1. They challenged the plants with the pathogens and removed 6 that little resistance to the pathogens.
  2. Of the remaining 10 plants, 8 showed resistance greater than the normal plant.
  3. The remaining 8 plants, they assessed the plant length, the number of panicles and the number of grains of each line and compared them to the normal plant and the one strongly expressing the resistance gene. One line in particular showed comparable traits in these areas to the normal plant. Testing showed that this plant was transcribing the resistance gene between 11% and 31% of the level the strong promoter was transcribing the gene.

Given the resistance shown combined with the retention of agronomic qualities in this line when tested in the lab, they were then subjected to some field testing. This time, additional traits were assessed including grain filling rate, 1000 grain weight and number of days to 50% heading and the researchers found that modified rice plants were only modestly effected in these traits and that some lines were better than normal plants in all traits but plant length.

To test the effect that the modified WRKY45 expression would have given different stresses, the plants were grown in different climates, different seasons and grown in soil with increased salinity and their agronomic traits compared to normal plants.

When grown in different seasons the modified line was comparable to the normal plants but flowered slightly later when grown under shorter day length. When subjected to seven days in 8 degree Celsius, the over-expressed line failed to recover while the normal and modified lines were able to recover and continue growth. Finally, a similar test was done subjecting the plants to 10 days growth in high salt soil. Most over-expressed lines died whilst the modified line was again comparable to the normal line.

Overall, the paper showed that expression of a protective gene can be optimised to reach a balance between protection against pathogens and still being a viable agronomic crop. ‘Vaccinating’ crops to reduce loss to disease will have a considerable effect on increasing food production.

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