Tolerance to heat stress in natural variants and CRISPR gene-edited high oleic acid lines of the oilseed plant pennycress (Thlaspi arvense).

Background/Question/Methods

Pennycress (Thlaspi arvense L.) is a cold-temperate and boreal-zone winter annual currently being rapidly domesticated for use as a multipurpose oil seed and cover crop, which could be grown during the off-season in the U.S. corn belt and provide significant environmental as well as economic benefits. In contrast to its extreme cold tolerance, pennycress is susceptible to heat stress, particularly during flowering. Comparison of natural variants from across their geographic range could allow the identification of traits linked to heat stress tolerance. In addition, CRISPR genome editing has allowed the development of “high oleic acid” lines with reduced unsaturation of their triacylglycerides. While these were developed for oil quality improvement, we hypothesized that they could allow improved heat tolerance as an ancillary benefit, due to greater oxidative stability and reduced membrane fluidity. In a pair of growth chamber experiments, high oleic acid lines and natural variants from northern and southern locations were subjected to progressively increasing temperatures. Pollen viability, lipid peroxidation, and seed yield following heat stress were measured.. Separately, effects of heat stress on male and female fertility were estimated through reciprocal crosses, to determine whether pistils or anthers were more vulnerable.

Results/Conclusions

In wild type pennycress, fertility declined rapidly with temperature, with only 50% pollen viability at 27 °C. As hypothesized, high-oleic acid pennycress lines showed 60% - 250% higher pollen viability at 28 – 30 °C compared to the corresponding wild type, but not at lower or higher temperatures. High-oleic lines also exhibited higher seed yields following a one-week heat stress at 32 °C. Plants from southern California had much higher pollen viability following heat stress than Alberta-collected plants. These effects were most likely not due to changes in lipid peroxidation levels, which did not differ between the linesHigh temperature stress (four hours at 32 °C) affected female fertility more strongly than male fertility. A one-week heat stress also reduced seed fitness in the next generation. These results indicate that natural variants can be investigated to discover heat-tolerant varieties and that lines with reduced fatty acid desaturation, in addition to improved seed oil quality, could confer improved heat stress tolerance as an additional benefit.