A stiff polymer called lignin (stained red) is deposited in a precise pattern in the cell walls of exploding seed pods. Researchers identified three laccase enzymes required to form this lignin. No lignin forms in the cell wall (stained blue) when all three genes are knocked out by CRISPR/Cas9 gene editing.

Copper explodes seed pods

Researchers identify the genes that control the mechanical structure of exploding seed pods

Plants have developed numerous strategies for widely dispersing their seeds. Some scatter their seeds in the wind, while others tempt animals and birds to eat their seed-filled fruits. And a few rare plants – such as the popping cress Cardamine hirsuta have evolved exploding seed pods that propagate 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 investigate which genes regulate the mechanical structure of these exploding seed pods. Their findings show that an important micronutrient buyer is essential for capturing a precise pattern of lignin in the seed pods. Lignin is an abundant plant polymer found in lignocellulose, the main structural material in plants. It is present in the walls of plant cells and is responsible for making wood stiff.

A rigid polymer called lignin (colored red) is deposited in a precise pattern in the cell walls of exploding seed pods. Researchers identified three laccase enzymes needed to form this lignin. No lignin forms in the cell wall (stained blue) when all three genes are knocked out by CRISPR/Cas9 gene editing.

© Miguel Perez Anton

A rigid polymer called lignin (colored red) is deposited in a precise pattern in the cell walls of exploding seed pods. Researchers identified three laccase enzymes needed to form this lignin. No lignin forms in the cell wall (stained blue) when all three genes are knocked out by CRISPR/Cas9 gene editing.

© Miguel Perez Anton

C. hirsuta seed pods consist of two long valves. When the seeds are ready to be dispersed, these valves quickly separate and roll back, ejecting the seeds over a wide area. The secret to the explosive nature of these pods is their unique mechanical design, which consists of three rigid rods of lignin connected by hinges. These hinges are critical to the explosive release of potential energy stored in the pod. To create these hinged structures, lignin is deposited in a precise pattern in a single layer of seedpod cells called endocarpb.

Hay explains: “The mechanical design that allows these pods to explode depends on lignin being deposited in a precise pattern in this single layer of cells. We know little about what controls this pattern of lignin deposition, so we went looking for the genes that control this process We found three genes that are needed to lignify the cell wall in exploding seed pods These genes code for enzymes called laccases that polymerize lignin When C. hirsuta plants all three laccases genes, they also lack lignin in this particular cell type.”

The research team also discovered another gene, called SPL7, which is required for the lignification of C. hirsuta seed pods. This gene codes for a protein that regulates copper levels in plants. The researchers discovered SPL7 in a mutated screen. Mutant plants lacking this gene also lack lignin in endocarpb cell walls. Without lignin, they would not be able to spread 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 exploding seed pods, especially when copper levels are low.

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

Interestingly, laccases are copper-binding proteins that depend on copper for their function. “The connection between these two findings is copper,” says Hay. “Plants need SPL7 to cope with low copper levels in the soil, and laccases need to bind copper for their enzymatic activity. Since lignin is critical to the mechanics of exploding seed pods, and copper-requiring laccases regulate this lignification, it makes seed dispersal dependent on the control of copper levels by SPL7.

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

Copper deficiency in the soil affects plants and trees in many different ways and is addressed through the use of copper fertilizers. It is particularly a problem for forestry, as low copper content can weaken trees due to poor lignification. “Our work makes a molecular link between copper and lignin via SPL7 and laccases. These insights can inspire new approaches to sustainable forest management,” explains Hay.

These findings may also be important for the more sustainable production of biofuels in the future. Lung cell walls pose a challenge for biofuel production because they are resistant to degradation and thus must be broken down with expensive and energy-intensive pre-treatments. 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.”

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