Improved tissue culture conditions for the emerging C₄ model Panicum hallii

Panicum hallii Vasey (Hall's panicgrass) is a compact, perennial C₄ grass in the family Poaceae, which has potential to enable bioenergy research for switchgrass (Panicum virgatum L.). Unlike P. hallii, switchgrass has a large genome, allopolyploidy, self-incompatibility, a long life cycle, and large stature-all suboptimal traits for rapid genetics research. Herein we improved tissue culture methodologies for two inbred P. hallii populations: FIL2 and HAL2, to enable further development of P. hallii as a model C₄ plant.

The optimal seed-derived callus induction medium was determined to be Murashige and Skoog (MS) medium supplemented with 40 mg L^-1 L-cysteine, 300 mg L^-1 L-proline, 3% sucrose, 1 g L^-1 casein hydrolysate, 3 mg L^-1 2,4-dichlorophenoxyacetic acid (2,4-D), and 45 μg L^-1 6-benzylaminopurine (BAP), which resulted in callus induction of 51 ± 29% for FIL2 and 81 ± 19% for HAL2. The optimal inflorescence-derived callus induction was observed on MP medium (MS medium supplemented with 2 g L^-1 L-proline, 3% maltose, 5 mg L^-1 2,4-D, and 500 μg L^-1 BAP), resulting in callus induction of 100 ± 0.0% for FIL2 and 84 ± 2.4% for HAL2. Shoot regeneration rates of 11.5 ± 0.8 shoots/gram for FIL2 and 11.3 ± 0.6 shoots/gram for HAL2 were achieved using seed-induced callus, whereas shoot regeneration rates of 26.2 ± 2.6 shoots/gram for FIL2 and 29.3 ± 3.6 shoots/gram for HAL2 were achieved from inflorescence-induced callus. Further, cell suspension cultures of P. hallii were established from seed-derived callus, providing faster generation of callus tissue compared with culture using solidified media (1.41-fold increase for FIL2 and 3.00-fold increase for HAL2).

Aside from abbreviated tissue culture times from callus induction to plant regeneration for HAL2, we noted no apparent differences between FIL2 and HAL2 populations in tissue culture performance. For both populations, the cell suspension cultures outperformed tissue cultures on solidified media. Using the methods developed in this work, P. hallii callus was induced from seeds immediately after harvest in a shorter time and with higher frequencies than switchgrass. For clonal propagation, P. hallii callus was established from R1 inflorescences, similar to switchgrass, which further strengthens the potential of this plant as a C₄ model for genetic studies. The rapid cycling (seed-to-seed time) and ease of culture, further demonstrate the potential utility of P. hallii as a C₄ model plant.