Learning the role of plants’ molecular ‘gatekeepers’

Everything a plant needs to grow must pass through its cell membranes. To do that, it must pass through a sieve of microscopic pores called aquaporins.

Now, researchers at the Australian National University have uncovered new information on how this network of “gatekeepers” works.

The knowledge could be key to developing food crops with improved crop productivity and bigger yields.

An aquaporin is basically a water channel. They form pores in the membranes of cells and selectively conduct water molecules through, while blocking other small molecules. These ancient channels are vital for many plant processes from water transport, development, stress response, root nutrient update and photosynthesis.

Researchers identified all the aquaporins in tobacco, a plant species closely related to major crops such as tomato, potato, eggplant and capsicum.

“There are varying numbers of aquaporins across different plant species,” said former PhD student Annamaria De Rosa with the ARC Centre of Excellence for Translational Photosynthesis.

De Rosa said aquaporins can be grouped in certain sub-groups or subtypes and, within those groups, there can be variations in gene numbers. “Each sub-group can give us some information on where the aquaporin can be found within the cell and their potential function.”

For example, she said, aquaporin sub-groups can be found in the periphery of the plant cell in the plasma membrane, controlling the passage of dissolvable substances, or solutes, into the cell. Other sub-groups can be found in internal compartments within a cell.

De Rosa said that plants require highly selective and efficient uptake and transport of solutes from roots to stems, leaves, flowers and developing seeds. Aquaporins have evolved to accommodate this necessity with subtle variations in gene and protein sequences that result in specific aquaporins having diverse roles within plants.

Because each aquaporin has unique variations in their gene sequences, they have slightly different protein channel compositions, said De Rosa.

“The building blocks that make up the protein channel (the amino acids) provide the aquaporin’s solute specificity,” she said. “As scientists, we need to improve our understanding about exactly which amino acid variations determine what solutes can pass through specific aquaporin pores.”

With aquaporins found everywhere in the plant, they are capable of transporting very different molecules to every location at a rate of 100 million molecules per second.

“The physiological significance of this in plants is that they can sense changing external conditions and adapt through a rapid uptake and/or redistribution of solutes. We would expect each aquaporin gene to have a specific target location within the cell, to have a particular selectivity to certain solutes, and to function in one or several plant tissues.”

As much as aquaporins soak up solutes for the benefit of plant growth, they are just as capable at alerting plants to harmful molecules or pathogens.

“Aquaporins could improve a plant’s ability to survive in soils with toxic levels of heavy metal concentration in the soil. They could also be involved in the plant-stress signalling in response to attack by pathogens.”

De Rosa said she does not fully know how the aquaporin protein function is affected by climate change, but several plant processes in which aquaporins are involved will be negatively affected with increasingly variable climatic conditions.

“For example, water uptake from soils and transport throughout tissues would be impacted with increased drought and warmer temperatures.”

De Rosa said that by improving the understanding of aquaporins functions, it could be possible to engineer crops with higher yield or that are more resilient to pests.

Michael Groszmann of the Research School of Biology at ANU, said in a news release that potential applications for crop improvement from this research ranges from improved salt tolerance, more efficient fertilizer use, improved drought tolerance, and more effective response to disease infection.

“They (aquaporins) are currently being used in water filtration systems and our results could help to expand these applications. We believe they are a key player in energy conversion through photosynthesis, and also control how a plant uses water. That is why we think we can use aquaporins to enhance plant performance and crop resilience to environmental changes.”

The research was published in the Journal BMC Plant Biology.

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