Dr. Claudio Stasolla and Dr. Robert D. Hill, Plant Science, University of Manitoba
SOYBEAN PRODUCTION IN the traditional U.S. growing areas is known to be sensitive to prolonged flooding. The effect is more pronounced in clay soils due to the slower drying of these soils after water levels recede. With the high proportion of clay soils in Manitoba, yield losses due to heavy rainfall or field areas with poor drainage are recognized as a significant problem for the farmer. The funding we have received over the past three years was to address this issue by examining how manipulation of a specific group of plant proteins, phytoglobins (Pgbs), can be used to influence plant behaviour in response to excessive moisture and, thereby, improve tolerance to the stress. In conjunction with the work on excessive moisture, preliminary studies focusing on the function of Pgbs in relation to reduced water availability will also be presented.
PHYTOGLOBINS (PGBS): MODULATORS OF PLANT RESPONSES TO EXCESSIVE MOISTURE
Phytoglobins act much like an electrical switch: when they are present in a plant cell, certain events are turned on as a result of an environmental stimulus. For example, root waterlogging; when they are absent in that cell, it responds differently to the same stimulus. We have found that the tolerance of plants to many of the common environmental stresses is dependent upon the expression of Pgbs in specific cells. In our earlier work, we could increase tolerance of corn to excessive moisture by increasing Pgb levels. Plants with high levels of Pgbs retained photosynthetic capacity during flooding while plants with reduced Pgbs levels had poor retention of photosynthetic activity. The beneficial effects of Pgbs were not only restricted to the shoot system, but also to the roots, which were able to grow and survive flooding. Spurred by these observations and corroborated by similar results in other species, we have studied the ability of Pgbs to improve tolerance of soybeans to flooding stress.
PGBS CAN BE USED TO PREDICT PLANT BEHAVIOUR TO EXCESSIVE MOISTURE
Effective methods to assess traits in lines are essential to the development of cultivars with improved characteristics. Towards this end, we have measured the Pgb levels as a function of a recovery rate of a group of soybean lines. Levels of Pgb directly correlate with the ability of the plant to recover after exposure to excessive moisture. Soybean plants characterized by high levels of Pgb in either shoot (Fig. 1A) or roots (Fig. 1B) have higher recovery rates after being fully submerged in water for seven days. Varieties with low levels of Pgbs showed the lowest recovery rate.
To further validate the protective role of Pgbs during excessive moisture conditions we generated soybean lines in which Pgb levels were experimentally altered. As expected, the recovery rate of plants over-producing Pgb was significantly higher than those under-producing the same protein. A similar improvement was also observed when the same plants were waterlogged (water level maintained 2 cm above soil surface) for 10 days. Plants over-producing Pgbs grew better, had a higher photosynthesis rate, and produced a higher number of adventitious roots (roots formed at the base of the shoot), a strategy employed by plants to cope with excessive moisture. Adventitious roots produced above the water level replenish the root system with oxygen, which is depleted in tissues below water.
Several studies have documented negative repercussions of altering stress-related proteins on agronomic traits. Contrary to this, our results showed no detectable abnormalities. Rather, preliminary studies revealed that seed number per plant is enhanced when Pgb is over-produced.
CAN PGBS ALSO EXERCISE A PROTECTIVE ROLE DURING DROUGHT CONDITIONS?
Continual root growth and production of lateral roots under drought conditions are key strategies which allow plants to “explore” the soil environment and reach the water table. Using Arabidopsis, a plant model system, we have previously demonstrated that high levels of Pgbs in the root tissue protect cells from dying during water stress, thus encouraging the growth of the root system. In soybeans, root growth retardation as a result of water depletion is attenuated in plant over-producing Pgb, and aggravated in plants under-producing Pgb (Fig. 2A). Under drought conditions, plants over-producing Pgb also develop more lateral roots relative to plants with reduced levels of Pgb (Fig. 2B). Based on these results, it is suggested that high levels of Pgbs can enhance plant behaviour to water stress.
WHERE DO WE GO FROM HERE?
This study has unequivocally demonstrated that Pgbs could be used as a tool to predict plant behaviour to excessive moisture, and its manipulation can alter plant performance and increase tolerance during both full submergence and flooding, as well as drought. The uniqueness of these result lies in the fact that tolerance to water stress (excessive or limited water conditions) can be achieved by the alteration in the level of a single protein, thus suggesting the role of Pgb as a “master regulator” of plant responses to stress. Our results indicate the high probability of using this information to develop soybean cultivars with improved flooding or drought tolerance for commercial production in the Canadian prairies. This can be accomplished using either traditional breeding techniques, using the evaluation protocol developed during the course of this research, or through the application of modern gene engineering and gene editing techniques.
Work presented in this document was conducted by Dr. Huang and supported by MPSG.
Figure 1. Correlation between level of phytoglobin (Pgb) (relative expression) and recovery rate of soybean plants submerged in water for seven days.
Figure 2. Root growth and number of lateral roots of two-day old soybean seedlings exposed for five days to water limiting conditions.
Figure 3. Seedling morphology in the same soybean lines exposed to drought for natural Pgb level wild-types (A), Pgb over-producers (B) and Pgb under-producers (C).