New insights into how plants regulate the uptake of organic nitrogen

The widespread use of inorganic nitrogen fertilizers primarily containing ammonium and nitrate has led to elevated nitrogen levels in groundwater in many regions. Organic nitrogen compounds, by contrast, are less soluble making them less prone to leaching. However, research into how plants take up these compounds and how they influence growth has lagged behind and PhD student Laura Tünnermann set out to change that. 

“One advantage of organic nitrogen sources like amino acids is that they provide not only nitrogen but also carbon, giving the plant additional energy,” said Laura Tünnermann. “I first became interested in this topic during my master´s thesis in Torgny Näsholms´ group. Back then, I realized that a master´s project was not enough to answer all my questions. Now, at the end of my PhD, I know that even that is not quite enough - though we know much more than before I started.” 

The researchers observed that plants grown in the presence of an organic nitrogen source, such as the amino acid glutamine, developed larger root systems with more root hairs, resembling nutrient-deficient plants. But since they were not lacking nitrogen, the researchers concluded that this is a glutamine-specific response.

Uncovering the proteins behind organic nitrogen uptake

To further understand the molecular mechanisms behind amino acid uptake, Laura Tünnermann focused on a protein called Lysine Histidine Transporter 1 or LHT1, which is responsible for transporting amino acids from the soil into the model plant Arabidopsis. She and her colleagues wanted to understand how the activity of this transporter is regulated in response to organic nitrogen sources and whether other proteins might interact with LHT1 to regulate it. 

“Almost nothing was known about how amino acid transporters like LHT1 are regulated,” explained Laura Tünnermann. “Transport proteins are often modified by other proteins and this can alter their activity. We identified a protein called Calcium Dependent Kinase 1, short CPK1, and showed that it interacts with LHT1. From there, we wanted to understand how this interaction affects amino acid uptake.” 

To investigate this interaction in isolation, the researchers transferred both plant proteins into yeast cells, a simplified system that removes other plant-specific factors. They also conducted experiments in Arabidopsis plants. Initially, they expected CPK1 to activate LHT1 and enhance amino acid uptake, but toward the end of her PhD, Laura Tünnermann realised that her results told a different story. 

Surprising role of CPK1 in regulating amino acid uptake

It turned out to be the opposite. Plants lacking a functional CPK1 protein absorbed more amino acids than normal plants. The yeast experiments showed a similar pattern, suggesting that CPK1 actually inhibits LHT1 activity and amino acid uptake, contrary to the team’s original hypothesis. 

“We still do not fully understand how CPK1 and LHT1 are activated,” said Laura Tünnermann. “We observed that the abundance of LHT1 decreased as glutamine concentration increased while CPK1 showed the opposite pattern. This suggests their abundance is linked to external amino acid availability although additional factors are likely involved in regulating this important process.” 

Laura Tünnermann believes her findings contribute to a better understanding of how organic nitrogen nutrition affects plant growth and how plants absorb organic nitrogen. She hopes this knowledge will support a shift away from inorganic nitrogen fertilizers and toward more sustainable, organic nitrogen alternatives, ultimately helping to reduce the environmental impact of this type of fertilisation. After defending her PhD thesis, she plans to continue working on the project for a while longer to complete a few remaining experiments and answer some of the questions that still remain.