Water shortages around the world put a damper on agricultural production. But Tel Aviv University (TAU) and Weizmann Institute of Science researchers have discovered a mechanism that is responsible for the closing of stomata – thousands of microscopic pores on the surface of plant leaves that are vital for photosynthesis –.to prevent water loss and controls the development of lateral roots to better deal with physical stresses.

 “Understanding plants’ response mechanisms to changing environments will significantly contribute to agriculture during this era of climate change and improve world food supply,” said Dr. Yuqin Zhang and Prof. Eilon Shani of TAU’s School of Plant Sciences and Food Security at the Wise Faculty of Life Sciences who headed the team.

The Israeli researchers, who were assisted by colleagues in Switzerland, Germany, the US, and Denmark, conducted the multi-stage study over six years. The article was published in the journal Science Advances under the title “ABA homeostasis and long-distance translocation are redundantly regulated by ABCG ABA importers.” 

It has long been known that one way that plants respond to a shortage of water is by closing their stomata – small openings surrounded by two doughnut-shaped guard cells on the leaves that enable exchanges with the environment. The circular pore has a hole in the center for gas to enter or leave the plant. When these stomata are open, gas exchange increases, the photosynthesis process is speeded up, and the plant generates energy, grows. and bears fruit. 

But when the plant seeks to hold on to the little water available to it, it closes its stomata and reduces transpiration. This is a particularly sensitive and rapid process through which the plant maintains the proper balance at any given moment. It opens or closes stomata within seconds to minutes in response to any small change in water availability, temperature and the amount of light.”

According to researchers, it was discovered back in the 1960s that a plant hormone called ABA (abscisic acid) – a small signal molecule – leads to stomatal closure. 

When a high level of ABA is present, the stomata tends to close, and, conversely, in its absence, they open. The prevailing belief for years was that ABA is produced in the roots in response to water deficit in the soil and then transported through the stem to the leaves in order to close the stomata. In the current study, researchers examined this hypothesis and found that the reality is far more complex.

“During the course of the study, which took six years, we relied on the plant model Arabidopsis from the Brassicaceae family  (the mustards, crucifers or the cabbage family) and used a broad variety of advanced molecular genetics techniques,” said Shani. |We created mutations through genome editing, cell-type-specific gene activation and physiological characterizations using advanced equipment. Similarly, we used a combination of advanced microscopes and different chemical and genetic methodologies for fluorescent probing in order to find the exact location of ABA molecules in plants as well as the proteins involved.”

 The findings were surprising. The researchers discovered that the ABA signal molecule is stored in an inactive state in the leaves themselves – in cells called mesophylls – that have a central role in photosynthesis. The storage process is actively carried out by two transporter proteins that had been previously unknown, ABCG17 and ABCG18. They transfer the ABA through the cell’s membrane into the mesophyll cells. There, the ABA becomes inactive by binding to a sugar molecule and is stored over time.

To examine the function of these two newly discovered proteins, they created various mutations in the respective genes that produce them and conducted a variety of additional experiments that characterized the proteins’ ABA transport activities with different systems such as frog eggs and yeast cells. They found that changes in the production of these proteins and their activities cause fluctuations in the transport and storage of ABA molecules in the plant and that without these proteins, ABA remains free, reaches the stomata in high concentrations, and promotes their closure. 

This mechanism, they explained, allows plants to respond rapidly to changing environmental conditions. Specifically, when the plant senses drought, the quantity and activity of these two proteins decreases, ABA awakens from its dormancy and the stomata close within a short amount of time.

They also found that the long-distance movement of the ABA hormone in a plant is the opposite of what they had believed up until now. Using the plant vasculature bundles, which is equivalent to the human blood system, ABA travels over great distances – in fact from the leaves to the roots. This movement is also controlled by the transport proteins, ABCG17 and ABCG18. 

A decrease in the activity of these two proteins in a leaf causes a reduction in the storage of dormant ABA in the mesophyll cells, and the free ABA travels in the direction of the root. Accumulation of ABA in the roots controls the development of lateral roots that helps the plant to better respond to abiotic stresses.

“In this study, we have added an important layer in our understanding of the mechanism through which the plant deals with changing conditions such as water shortfalls,” concluded Shani. “For the first time, we discovered a control mechanism through which the plant collects signal molecules in a ‘storeroom’ and releases them under the desired conditions. This discovery will significantly contribute to agriculture during this era of rapid climate change and thus hopefully improve world food supply. In follow-up studies, we are now examining similar mechanisms in two important crops: tomatoes and rice.”