Researchers Unlock Key Potassium Channel That Controls Plant Stalk Elongation

Potassium (K) is one of the three essential nutrients for plants. It plays a critical role in a wide range of processes, including cell growth, photosynthesis, and regulation of water balance. Plants take up K from the soil and circulate it throughout their bodies via molecules called K+ channels.

The function of one such K+ channel (called AKT5) in the model plant Arabidopsis thaliana has remained a mystery, despite over 30 years of study. Understanding the gateways for what is essentially the lifeforce of plants is crucial for finding new ways to boost agricultural productivity.

To solve this mystery, the Uozumi laboratory at Tohoku University have identified the K+ channel activity of AKT5 from the Arabidopsis plant. This research represents the very first reveal of AKT5's actual function. Physiologically, AKT5 promotes leaf stalk growth, offering new insights into developing efficient high-density cropping systems.

The leaf stalk is the edible part of many vegetables, such as celery and Swiss chard. By regulating AKT5 activity in these plants, it may be possible to tailor the size and texture of these stalks to meet consumer needs."

Nobuyuki Uozumi, Tohoku University

The research group attempted to detect the K+ transport activity of AKT5 using animal cell expression systems. Initially, no activity was detected. However, when a specific enzyme that adds phosphate groups to proteins was introduced together with AKT5, K+ transport activity was detected for the first time. It turns out that AKT5 is activated when a phosphate group attaches to a specific amino acid in the protein, which is why there was no activity when K+ alone was added.

To better understand how AKT5 is activated, they used cryo-electron microscopy to determine its three-dimensional structure in the pre-open and closed states. AKT5 adopts a shape very similar to other known K+ channels, yet it can transform. In fact, when a single amino acid in AKT5 was changed (aspartate at position 403 was replaced with alanine), the structure of the molecule changed dramatically, and AKT5 gained K+ transport activity even without phosphorylation. These findings suggest that AKT5 normally adopts an inactive shape, and upon phosphorylation, undergoes a structural change centered around the 403rd amino acid, switching to an active shape that can transport K+.

To investigate the role of AKT5 in plants, the researchers examined where AKT5 is located. They found that in plants, AKT5 is concentrated in the leaf stalks - the slender structures that connect leaves to the stem. When plants lacking AKT5 were analyzed, their leaf stalks were found to be shorter than normal. Leaf stalks are important for positioning leaves to capture sunlight efficiently, which becomes especially critical in crowded growing conditions. When AKT5-deficient plants were grown under crowded conditions, their growth was reduced and they became smaller compared to normal plants. These results suggest that AKT5 promotes cell elongation in leaf stalks by taking up K+, increasing internal pressure within the cells and thereby stretching the leaf stalks.

AKT5 is a key molecule that helps plants compete in high-density growing environments. Gaining a deeper understanding of how AKT5 functions in plants and learning how to control its activity could enable us to grow crops at high density without reducing productivity. These findings are expected to provide an important foundation for addressing global challenges such as population growth and the shortage of farmland.