Mechanism of gravity sensing in plants revealed by NIBB: Future crop improvements expected

Plants grow properly by sensing the direction of gravity, extending their roots downward and spreading their shoots upward (gravitropism). Plant organs that undergo gravitropism include intracellular organelles called statoliths (dense amyloplasts) that accumulate starch grains and have a high specific gravity. The "starch-statolith hypothesis", which states that plants sense gravity by the means of the sinking of these starch grains, was proposed more than 100 years ago. However, it was not known how the physical phenomenon of amyloplast sedimentation is translated into and transmitted to other signals within the cell.

A research group led by Assistant Professor Takeshi Nishimura, Assistant Professor Hiromasa Shikata, Technical Staff Shogo Mori, and Professor Miyo Morita (Terao) of the National Institute for Basic Biology, in collaboration with Professor Masatsugu Toyota, former graduate student Yoshinori Abe, graduate student Takuma Hagihara of Saitama University, together with Professor Hiroshi Yoshikawa of Osaka University as well as Professor Takumi Higaki of Kumamoto University, has clarified how plant roots detect the direction of gravity. The LAZY1-LIKE (LZY) protein in amyloplasts senses the direction of gravity by translocating to the plasma membrane, where it causes an auxin concentration gradient that induces gravitropism. Through this, the missing link in gravitropism has been found. The group's results have been published in Science.

Using a vertical stage confocal microscope of their own design, Morita and his colleagues observed cells in plants growing normally under a maintained direction of gravity. Then, by rotating the stage, they were able to observe cellular responses to changes in the direction of gravity.

Previously, the LZY protein was shown to be involved in the process of signal transduction in gravity-sensitive cells. Notably, LZY is biased to moving toward the side of the plasma membrane that faces gravity, and its localization site on the plasma membrane changes in response to changes in the direction of gravity. Additionally, the regulator of chromosome condensation 1-like domain (RLD) protein, which is involved in the regulation of auxin transport, was discovered to interact with the LZY protein. Notably, it was shown that auxin transport in the direction of gravity is promoted by LZY and RLD.

Morita said, "This is a microscope that really should not be laid on its side, but I acquired it from a professor in my laboratory more than 20 years ago, and with the cooperation of Olympus and a local factory, we have repeatedly modified it to enable a variety of observations."

Using this microscope system, the researchers observed the subcellular localization of fluorescently labeled LZY protein molecules in columella cells, the gravity-sensitive cells of the root. The LZY protein was found to localize to both the amyloplast and plasma membrane.

Mutant proteins were generated through substitution and partial deletion of the amino acid sequence of the LZY protein, and their subcellular localization and activity were investigated. Notably, the LZY protein localizes to the plasma membrane through its positively charged amino acid-rich region, which then undergoes electrostatic interaction with the negatively charged phospholipids that make up the plasma membrane. The researchers also found that LZY can be quantitatively regulated by proteasome-dependent degradation. Furthermore, the N-terminal portion of LZY is important for its localization to the amyloplast.

When observations were made while maintaining the direction of gravity, LZY proteins on the plasma membrane were observed only in the vicinity of the amyloplasts. Then, when the stage was rotated to change the direction of gravity, the amyloplasts in the columella cells sank toward the new direction of gravity. In this case, the LZY molecules located on the plasma membrane in the original direction of gravity rapidly disappeared, and simultaneously, LZY molecules rapidly accumulated on the plasma membrane in the vicinity of the new amyloplasts (the plasma membrane on the side of the new direction of gravity).

The changes in the localization of LZY protein in response to changes in the direction of gravity were observed immediately after sedimentation of the amyloplasts, indicating that the auxin transport regulator RLD is recruited to the new direction of gravity. Furthermore, it was shown that these events occurred prior to the formation of the auxin concentration gradient and bending in the root, and the changes in the localization of the LZY protein on the plasma membrane were an important factor in the transduction of gravitational information directly related to the control of auxin transport.

The next step was to study how the LZY proteins move to the plasma membrane on the new side of the direction of gravity. A light-converting fluorescent protein was added to the LZY molecule and observed in columella cells. When UV light was applied only to the LZY protein localized on the amyloplast (using a light stimulator equipped with a microscope), the labeled LZY protein migrated rapidly from the amyloplast to the adjacent plasma membrane.

It was shown that in mutants in which amyloplasts do not settle and are randomly present, the localization of LZY protein on the cell membrane is no longer biased. Furthermore, when amyloplasts were brought close to the cell membrane using optical tweezers regardless of the direction of gravity, LZY protein was observed to rapidly accumulate on the nearby cell membrane.

These results indicate that LZY protein acts as a molecular informant to transmit the positional information of the amyloplast (i.e., the direction of gravity) to the plasma membrane by moving from the amyloplast to the neighboring plasma membrane. This is the crux of the gravity-sensing mechanism.

Morita stated, "In rice, the angle of elongation of leaves and roots under the influence of gravity has been found to be related to crop yield. The mechanism we have discovered may also be applicable to these crops and may help to improve them in the future."

Journal Information
Publication: Science
Title: Cell polarity linked to gravity sensing is generated by LZY translocation from statoliths to the plasma membrane
DOI: 10.1126/science.adh9978