Nocturnal gibberellin biosynthesis is carbon dependent and adjusts leaf expansion rates to variable conditions

Optimal plant growth performance requires that the presence and action of growth signals, such as gibberellins (GAs), are coordinated with the availability of photo-assimilates. Here, we studied the links between GA biosynthesis and carbon availability, and the subsequent effects on growth. We established that carbon availability, light and dark cues, and the circadian clock ensure the timing and magnitude of GA biosynthesis and that disruption of these factors results in reduced GA levels and expression of downstream genes. Carbon-dependent nighttime induction of gibberellin 3-beta-dioxygenase 1 (GA3ox1) was severely hampered when preceded by reduced daytime light availability, leading specifically to reduced bioactive GA(4) levels, and coinciding with a decline in leaf expansion rate during the night. We attributed this decline in leaf expansion mostly to reduced photo-assimilates. However, plants in which GA limitation was alleviated had significantly improved leaf expansion, demonstrating the relevance of GAs in growth control under varying carbon availability. Carbon-dependent expression of upstream GA biosynthesis genes (Kaurene synthase and gibberellin 20 oxidase 1, GA20ox1) was not translated into metabolite changes within this short timeframe. We propose a model in which the extent of nighttime biosynthesis of bioactive GA(4) by GA3ox1 is determined by nighttime consumption of starch reserves, thus providing day-to-day adjustments of GA responses.

Automated summary

The presence and action of growth signals, such as gibberellins, need to be coordinated with the availability of photo-assimilates for optimal plant growth performance. Disruption of factors like carbon availability, light and dark cues, and the circadian clock can lead to reduced levels of gibberellins and expression of downstream genes, impacting growth. Nighttime biosynthesis of bioactive GAs appears to be determined by the consumption of starch reserves, allowing for day-to-day adjustments of GA responses.