Photosynthesis is dependent on sunlight. Plants will use too much sun to grow taller. The three plants in the image on the left were grown under full sun. They have many branched roots. The plants to the right, on the other hand, were grown in full shade. They have fewer branches because they are trying to grow a longer stem so that they can reach more light. Credit: Daniele Rosado / Pedmale lab / CSHL, 2021
Plants that are too shaded will redirect their resources to get enough light. The plant's ability to produce crop yield and root growth is affected as it focuses on getting taller and attempting to reach the sun above its neighbors.
For organisms that depend on photosynthesis, shade avoidance is a crucial survival strategy. However, it can be a problem for farmers says Professor Ullas Pedmale of Cold Spring Harbor Laboratory (CSHL). It reduces the density at which growers are able to plant their crops and, therefore, lowers yields.
Pedmale says that because of the competition for sunlight, there is a lot of crowding. Therefore, they have to grow taller. He added:
"The plants put all their energy into trying to grow taller but it's now at the cost of biomass, which is what we depend on. It also comes at the expense to the roots so they don't grow."
For decades, researchers have studied the shade response of plants in an effort to find ways to make them thrive at higher densities. While most of the research on plants' shade response has been focused on changes in leaves and stems, Pedmale's group wanted to investigate what happens underground when a plant is denied light. This is a critical issue as plants with weak root systems are more susceptible to flooding, droughts, and hurricanesthreats.
Daniele Rosado, a postdoctoral researcher, and others from Pedmale compared the roots of tomato plants to those of the model plant Arabidopsis. The shade-grown plants had hundreds of genes that respond to stress, including many that encode proteins called WRKYs that regulate gene expression.
To prove that they inhibit root growth, Pedmale's research team engineered plants with WRKY genes that remained active regardless of environmental conditions. Plants with high levels of certain proteins, such as WRKY, grew stunted roots even in bright conditions. However, the plant's stems grew at a normal pace. They also discovered that the roots' shade response may be affected by the plant hormone, ethylene.
Pedmale hopes that revealing the role of these proteins in shade avoidance will allow researchers to develop better high yielding plants. Plants that can survive in more crowded environments, withstand extreme weather and possibly reduce global warming by absorbing carbon dioxide from the atmosphere into extensive root systems.
Learn more about how plants can be good neighbours in times of stress
Further information: WRKY transcription and ethylene signaling modify root growth during the shade avoidance reaction, Plant Physiology (2021). WRKY transcription and ethylene signaling alter root growth during the shade-avoidance response (2021). DOI: 10.1093/plphys/kiab493