Background: Soil amendments that can respond to light such as photocatalytic nanoparticles, polymers sensitive to light, and quantum dots are increasing as a way to manipulate rhizosphere function via optical and physicochemical modifications. Their capacity to affect plant circadian cycles could provide new avenues for precision agriculture in the face of evolving environmental conditions.
Objectives: Researchers investigated the impact of light reactive soil amendment on circadian rhythmicity in Arabidopsis thaliana through three objectives: (1) determine how the light responsive soil amendments impacted circadian rhythmicity; (2) quantify the impact on circadian rhythms by measuring the different parameters of the circadian rhythms by a luciferase reporter systems; and (3) evaluate the physiological and molecular response of the plants to the soil amendments, including stress tolerance and gene expression responses.
Methods: Circadian rhythm, amplitude, and phase monitoring was performed using bioluminescent reporters with luciferase (LUC) fused to the promoters of circadian clock genes (CCA1, LHY, and TOC1). Soil amendments included the addition of TiO₂ and ZnO nanoparticles; azobenzene-based light-sensitive hydrogels; and quantum dots. Circadian responses were measured under controlled photoperiods and changes in transcriptome profiles were analyzed with RNA sequencing.
Results: Cabbage seedlings grew faster when treated with TiO2 (34% faster than untreated) and flowering occurred 4.2 +/- 0.7 days sooner than controls under long day conditions. Light sensitive polymers affected the red:far red ratio of light in the root zone by changing the way light scattered in the root zone and this would activate phytochrome B signalling, which would result in the repression of TOC1. RNA-seq analysis identified significant differences in gene expression with 248 genes related to clock function being significantly upregulated while 193 genes were significantly downregulated during the evening. The TiO2 treatment also reduced the pH of the rhizosphere (6.2 compared to 7.1 in controls) and modified the dynamics of soil organic carbon. Together these changes increased drought tolerance by approximately 22%, likely due to abscisic acid (ABA) – circadian cross-talk.
Conclusions: Soil treatments that respond to light might influence circadian rhythm of plants, via changing the rhizosphere's optical and chemical characteristics, resulting in better growth and resilience against stress. This presents an opportunity for applying them to precision ag and sustainably managing crops for climate change impacts. More work needs to be done in determining how these treatments could affect the environment on a long-term basis.