Copper makes the pods explode

Researchers identify genes controlling the mechanical structure of exploding pods

Plants have developed many strategies to widely distribute their seeds. Some disperse their seeds to the wind, while others entice animals and birds to eat their seed-filled fruits. And some rare plants – like the watercress Cardamine hirsuta – have developed explosive pods that propel their seeds in all directions. In their new study published in PNAS, Angela Hay and colleagues – from the Max Planck Institute for Plant Breeding Research in Cologne, Germany – study which genes control the mechanical structure of these explosive pods. Their findings show that a key micronutrient – copper – is essential for establishing an accurate lignin pattern in the pods. Lignin is an abundant plant polymer found in lignocellulose, the main structural material of plants. It is present in the walls of plant cells and is responsible for the rigidity of wood.

The pods of C. hirsuta consist of two long valves. When the seeds are ready to be dispersed, these valves quickly separate and roll up, throwing the seeds over a large area. The secret to the explosive nature of these pods is their unique mechanical design, which includes three rigid lignin rods connected by hinges. These hinges are crucial for the explosive release of the potential energy stored in the nacelle. To create these hinged structures, lignin is deposited in a precise pattern in a single layer of pod cells, called the endocarp.

As Hay explains, “The mechanical design that allows these pods to explode depends on the lignin being deposited in a precise pattern in this single layer of cells. Little is known about what controls this pattern of lignin deposition, and so we set out to identify the genes that control this process. We found three genes needed to lignify the cell wall in exploding pods. These genes code for enzymes, called laccases, that polymerize lignin. When plants of C. hirsuta lack all three laccase genes, they also lack lignin in this specific cell type.

The research team also discovered another gene, called SPL7, required for the lignification of C. hirsuta pods. This gene codes for a protein that regulates copper levels in plants. The researchers found SPL7 in a mutant screen. Mutant plants lacking this gene also lack lignin in the endocarp cell walls. Without lignin, they would no longer be able to disperse their seeds widely. These effects were reversed when the SPL7 mutant plants were grown in high copper soil, but not when grown in low copper soil. SPL7 therefore helps C. hirsuta plants acquire enough copper to develop fully explosive pods, especially when copper levels are low.

But how does copper affect the mechanical structure of these explosive pods?

Interestingly, laccases are copper-binding proteins that depend on copper for their function. “The link between these two discoveries is copper,” says Hay. “Plants need SPL7 to cope when there is too little copper in the soil, and laccases need to bind copper for their enzymatic activity. Since lignin is critical to the mechanics of pod explosion and copper-requiring laccases regulate this lignification, this makes seed dispersal dependent on the control of copper levels by SPL7.

These findings provide important new information about the genes and cellular processes that generate these extraordinary explosive structures. They also shed new light on the role of copper in this process and on the lignification process itself, which remains little known. One reason for this is that large gene families are involved in lignin polymerization in plant cell walls. Determining how each gene is involved is therefore a challenge, but one that could be addressed using approaches reported in this study, such as CRISPR/Cas9 gene editing and conditional gene expression.

Copper deficiency in soil affects plants and trees in different ways and is solved by using copper fertilizers. This is a particular problem for forestry, as low levels of copper can weaken trees due to poor lignification. “Our work establishes a molecular link between copper and lignin via SPL7 and laccases. This knowledge could inspire new approaches to sustainable forest management,” says Hay.

These results could also be important for the more sustainable production of biofuels in the future. Lignified cell walls pose a challenge for biofuel production because they resist degradation and therefore need to be broken down using expensive and energy-intensive pretreatments. Hay notes, “Our work identifies three laccases that control lignification in a specific cell type. Understanding the genetic control of lignin polymerization across different cell types and plant species may open new frontiers in bioenergy based on cell wall engineering.