Study Shows Plants Regulate Growth-Inhibiting Hormones to Survive

Scientists from Nagoya University have, for the first time, observed one of the natural mechanisms in the regulation of the levels of growth inhibiting hormone in plants. Previously observed in bacteria, its discovery in plants will enable novel ways of increasing crop productivity globally. Using rice plants, the research team discovered that a process called "allosteric regulation" is involved in maintaining the phytohormonal balance in plants. Their research findings could significantly advance the research on plant growth and development, providing a potential solution for food security.

Lead scientist Professor Miyako Ueguchi-Tanaka of Nagoya University explains that they used X-ray crystallography. They found that, as molecules of the enzymes (gibberellin 2-oxidase 3 [GA2ox3], and auxin dioxygenase [DAO]) bind to gibberellin and auxin (respectively), they interact among themselves and form 'multimeric' structures, comprising four and two units, respectively. As the amounts of gibberellin and auxin increase, so does the rate of multimerization of the enzymes. And multimerization enhances the activity of the enzymes, enabling greater degradation of gibberellin and auxin. She adds that synchronous structural changes and activity enhancement are typical of allosteric-regulation events.

The research team further carried out "phylogenetic" analysis of GA2ox3 and DAO, and they discovered that plants independently developed this hormone regulation mechanism at three separate time-points over the course of the evolutionary process.

For more details, read the news article on the Nagoya University website.

Figure: Dimerization of OsGA2ox and OsDAO enhances its enzymatic activity.

Under low substrate concentration (GA₄ or IAA) (left), GA2ox or DAO functions as a monomer with low enzyme activity. At high substrate concentration (right), the interface GA₄ or IAA causes the formation of multimers by bridging two enzyme molecules, resulting in hyper-activation. GA₄ or IAA is retained in a stable interface position, allowing two subunits to enter the active site for the next reaction without a high energy barrier. Lys or Arg is the most important amino acid for retaining GA₄ or IAA and for entering the active site. Furthermore, MD simulation of OsGA2ox3 revealed the presence of a gate, allowing substrate to enter the active site and for product to exit. This gate had a hinge site composed of three amino acids, W106, C186, and V196, and was also stabilized by the interaction between R97 in subunit A and F100 in subunit D. GA₄-dependent dimerization enhanced its enzymatic activity. These mechanisms are conserved in all rice GA2oxs. (Credit: S.Takehara et al. 2020).