Reprogramming feedback strength in gibberellin biosynthesis highlights conditional regulation by the circadian clock and carbon dioxide

The phytohormone gibberellin (GA) is an important regulator of plant
morphology and reproduction, and the biosynthesis and distribution of
GA in planta is agriculturally relevant to past and
current breeding efforts. Tools like biosensors, extensive molecular
genetic resources in reference plants and mathematical models have greatly
contributed to current understanding of GA homeostasis; however, these
tools are difficult to tune or repurpose for engineering crop plants.
Previously, we showed that a GA-regulated Hormone Activated CAS9-based
Repressor (GAHACR) functions in planta (Khakhar et al.,
2018). Here, we use GAHACRs to modulate the strength of feedback on
endemic GA regulated genes, and to directly test the importance of
transcriptional feedback in GA signaling. We first adapted
existing mathematical models to predict the impact of targeting a GAHACR
to different nodes in the GA biosynthesis pathway, and then implemented a
perturbation predicted by the model to lower GA levels. Specifically, we
individually targeted either the biosynthetic gene GA20 oxidase (GA20ox)
or the GA receptor GID1, and characterized primary root length, flowering
time and the transcriptome of these transgenic lines. Using this approach,
we identified a strong connection between GA signaling status and the
circadian clock, which can be largely attenuated by elevated carbon
dioxide levels. Our results identify a node in the GA signaling pathway
that can be engineered to modulate plant size and flowering time. Our
results also raise concerns that rising atmospheric CO2 concentration are
likely to reverse many of the gains of Green Revolution crops.