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A trick of the light: Optimizing light quality conditions speeds plant growth

Model identifies optimal light conditions to manipulate the circadian clock.
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Speed breeding is a novel agricultural technology that can create improved crops twice as fast as conventional breeding. This method shortens breeding generation time by tricking the plant’s rigid circadian clock by manipulating light duration and temperature.

While the speed breeding method has mainly been conducted under white LED lights, previous studies have demonstrated that exposure to different light qualities can also promote plant growth and development. Yet, the optimal light quality and the associated photoperiod is unknown due to complex interactions between multiple photoreceptors and proteins controlling plant growth.

In a paper recently published in in silico Plants, Dr. Mathias Foo, Assistant Professor in the School of Engineering at the University of Warwick, and associates used plant computational modeling to understand the effects of qualities and photoperiod on plant growth.

Plants sense and respond to light using receptors, phytochromes and cryptochromes, which regulate internal circadian clock components. The clock generates biological rhythms and exports them to downstream-regulated genes to coordinate developmental events throughout the life of the plant.

The authors used modeling to identify the molecular mechanisms within the circadian clock that are sensitive to different light qualities and affect plant growth. First, they created a new model by incorporating a light quality function with a simple plant circadian clock model.

The circadian clock model has four groups of genes with multiple interlocked loops and light inputs, allowing it to respond to a wide range of light/dark duration treatments. The model included light-responsive variable called protein P as a stand-in for phytochromes that affect the circadian clock. Protein P is light-color independent. To study the impact of red and blue lights, the authors replaced protein P with a light quality function comprised of three photoreceptors, phytochrome A, phytochrome B and cryptochrome 1, which are sensitive to red and blue light, respectively.

This study focused on hypocotyl growth. The hypocotyl is the seedling stem located above the root and underneath the seed leaves. The hypocotyl is the main growing part of a plant seedling, so it serves a relevant proxy for relating light qualities to plant growth.

The authors included one other element in their model. One of the best characterized light-controlled developmental processes is seedling photomorphogenesis (in the light) and skotomorphogenesis (in the dark). Photomorphogenesis is characterized by the inhibition of hypocotyl and stem elongation, open cotyledons, chloroplast differentiation and the accumulation of chlorophyll, and leaf expansion. Conversely, skotomorphogenesis is characterized by long hypocotyls and elongated stems, closed cotyledons with apical hooks, unexpanded leaves, and undifferentiated plastids and chloroplasts. The switch from skotomorphogenesis to photomorphogenesis is regulated by the light signaling center COP1 (CONSTITUTIVE PHOTOMORPHOGENIC1/SUPPRESSOR OF PHYA-105 E3 ligase complex).

Foo explains the importance of the inclusion of COP1 in their model:

“It was recently found that the signaling cascades of red and blue light-activated photoreceptors compete with downstream transcription factors for binding to COP1. This could lead to more hypocotyl elongation under red than blue and red/blue light. We therefore included COP1 in our model investigating light quality. This is the first time this interaction has been included in a model of plant growth.”

The signaling cascades of red and blue light-activated photoreceptors compete with downstream transcription factors for binding to COP1. This could affect regulation of the circadian clock therefore hypocotyl elongation.
The signaling cascades of red and blue light-activated photoreceptors compete with downstream transcription factors for binding to COP1. This could affect regulation of the circadian clock therefore hypocotyl elongation.

The new model predicted that red and blue light receptors, phytochrome and cryptochrome, competitively bind with COP1 under mixed light condition (i.e. red and blue), resulting in more hypocotyl elongation under red light condition than under mixed light condition. To validate these results the authors grew Arabidopsis plants under red, blue, or red/blue light for three different photoperiods for 10 days and measured their hypocotyl length. The simulated prediction was confirmed with the experimental data.

Experimental measurements of hypocotyl length confirm simulated hypocotyl length under red, blue and red/blue light.
Experimental measurements of hypocotyl length and simulated hypocotyl length.

Foo concludes:

“Our model found that optimal light quality and duration can speed plant growth. This model can be used to assist experts in focusing on certain promising set of light qualities and photoperiods combinations, ultimately leading to drastically cutting the experimental time and resources.”

READ THE ARTICLE:

Miao Lin Pay, Dae Wook Kim, David E Somers, Jae Kyoung Kim, Mathias Foo, Modeling of Plant Circadian Clock for Characterizing Hypocotyl Growth under Different Light Quality Conditions, in silico Plants, 2022;, diac001, https://doi.org/10.1093/insilicoplants/diac001

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