Loss of PIF7 Attenuates Shade and Elevated Temperature Responses Throughout the Lifecycle in Pennycress

Pennycress (Thlaspi arvense) is an intermediate oilseed bioenergy crop under development for growth over winter in the Midwest. Crucial work remains to domesticate and optimize pennycress for incorporation into cropping systems and to increase its resilience to stresses. We found that increased planting density reduces biomass and hastens time to flowering and maturity in field-grown pennycress, both of which are associated with plant shade avoidance responses (SAR). We confirmed that pennycress elongates when exposed to foliar shade and elevated ambient temperatures (28°C), physiological responses controlled by the red-light photoreceptor PHYTOCHROME B. We applied knowledge of the PHYTOCHROME B signalling pathway from Arabidopsis thaliana to identify target genes to mutate to suppress shade-avoidance responses during interseeding and to alter tissue responses to elevated temperatures. Mutating PHYTOCHROME INTERACTING FACTOR 7 reduced organ elongation to competition and heat cues and retained a compact rosette when exposed to shade and/or elevated temperature. Crucially, yield and oil content were unaltered in pif7. Furthermore, indicators of plant health, including hue, chlorophyll indices, and root system architecture, were improved in the pif7 mutant compared with the wild type. This showed that plant architecture and physiological health can be uncoupled under competition and heat stress, supporting our efforts to attenuate morphological responses to environmental cues. We propose that mutating PIF7 is a strategy to reduce SAR, improve pennycress performance at high densities during interseeding establishment in standing crops, and address the impacts of a warming climate on plant architecture.