Boron, though required only in minimal amounts, is vital for plant development. It strengthens cell walls and supports the growth of roots and shoots. Normally, boron, in the form of boric acid, is passively absorbed by plant roots and transported throughout the plant via diffusion. However, boron is often scarce in soil, particularly in arid regions, making passive absorption impossible. To combat this, plants have evolved proteins that actively transport boron from the soil into the plant. In Arabidopsis thaliana, the protein AtNIP5;1—a boric acid channel that helps absorb boron into plant roots—is crucial for active boron absorption. However, to function properly, AtNIP5;1 must be precisely located on the outer surface of root cells—facing the soil—where it can effectively pull boron in. How plants maintain this precise positioning is unclear.
Myosin XI is a plant-specific motor protein that moves along actin filaments and helps in intracellular organelle transport. It also plays a central role in cytoplasmic streaming, a process that keeps the cytoplasm and various intracellular materials flowing through the cell. Previous studies had shown that myosin XI is crucial for delivering proteins and membrane-bound cargo to precise destinations, especially during rapid growth and development. Given its role in intracellular trafficking, could it also be involved in guiding the position of AtNIP5;1 and thus regulating boron transport?
A team of researchers, led by Professor Motoki Tominaga from Waseda University, Japan, has discovered that the motor protein myosin XI plays a crucial role in boron transport in Arabidopsis thaliana. The study, published in Volume 224 of Plant Physiology and Biochemistry and made available online on April 17, 2025, was conducted in collaboration with Haiyang Liu (student) and Riku Chishima (student) of Waseda University, as well as Dr. Keita Muro (researcher) and Professor Junpei Takano of Osaka Metropolitan University.
"Our findings reveal that myosin XI acts like a delivery system, ensuring that the boron channel AtNIP5;1 stays at the correct location on the cell membrane where it can take in boron," said Tominaga. "Without this motor protein, the plant's ability to absorb boron drops significantly, leading to poor growth and development."
To understand the mechanism, the team used genetically modified A. thaliana plants lacking two or more myosin XI proteins. Before modification, there are 13 myosin XI proteins in A. thaliana. The researchers selected three myosin XI proteins, XI-K, XI-2, and XI-1, which are known to be the main driving forces for cytoplasmic streaming, and created double (xi-k, xi-2) and triple (xi-k, xi-1, xi-2) gene knockouts of them.
While the mutant plants grew normally under sufficient boron, they showed severe growth defects under boron-deficient conditions at the seedling stage. Compared to wild-type plants, the myosin XI-deficient lines had significantly shorter roots, smaller leaves, and dramatically lower boron levels in their aerial tissues. The defects became less pronounced as boron concentrations increased, suggesting a direct connection between myosin XI function and boron uptake under stress.
Microscopic analysis revealed that in wild-type plants, the AtNIP5;1 protein maintained a distinct polar localization, concentrated on the outer (soil-facing) membrane of root cells. In contrast, the double and triple myosin XI mutants lost this polarity, with AtNIP5;1 scattered across the cell surface or mislocalized entirely. The team inferred that this misplacement was the main reason why the plant's ability to absorb boron was crippled.
The researchers also found that endocytosis, the cellular process that helps shuffle proteins to and from the cell membrane, was significantly impaired in the myosin XI mutants. Using fluorescent dye markers and confocal microscopy, they showed that myosin XI is necessary for proper endocytic trafficking of membrane proteins in plant roots.
Interestingly, AtBOR1, which localizes to the inside of the cell and transports boron to the center of the root, was less affected by the loss of myosin XI. This suggests that different boron transporters rely on distinct trafficking systems within the plant cell, and AtNIP5;1 is particularly dependent on myosin XI for its function.
To further validate their findings, the researchers treated wild-type plants with chemical inhibitors that block myosin XI or disrupt the actin cytoskeleton along which it moves. In both cases, AtNIP5;1 lost its polar localization, mirroring the results seen in the genetic mutants.
These findings reveal a previously unknown role of myosin XI in boron acquisition in low-nutrient environments. While the study focused on A. thaliana, a model plant, similar mechanisms could exist in crops like rice, wheat, and maize. Future research could explore whether enhancing myosin XI function or stabilizing AtNIP5;1 localization could improve crop yields in boron-deficient soils. As global agriculture faces soil degradation, such insights can be used to develop more resilient crops.
"The long-term goal is to leverage this knowledge to breed or engineer plants that can better tolerate nutrient-poor soils," Tominaga concluded. "Understanding the molecular transport systems inside the cell is the first step toward that goal."
