Before we began our study on rice brassinosteroid (BR)-related mutants, there was no molecular biological or genetical study in this field, even though dwarfism caused by BR deficiency and insensitivity in crops, including rice, is important for breeding (see below).

When we started our project on rice BR-related mutants, some knowledge on BA biosynthesis has been accumulated in Arabidopsis, but not in other plants. This situation gives rise to conditions of limited knowledge on BR. The reason why our paper is highly cited may coincide with this situation, as our paper provides new knowledge of the BR function in rice.

I think so. Some members of cytochrome P450 classified in CYP90, such as CYP90A (CPD) and CYP90B (DWF4), have been recognized as enzymes involved in BR synthesis, whereas another member of CYP90, CYP90C (ROT3), was doubted regarding its involvement in BR biosynthesis, based on the phenotype of the rot3 mutants. Consequently, it was difficult to determine whether CYP90D proteins, which are highly similar to CYP90C/ROT3, are involved in BR biosynthesis.

However, as rice BR-deficient mutant is disrupted in a gene encoding member of CYP90D proteins, we concluded that rice CYP90D is involved in BR biosynthesis, and also suggested that all members of CYP90D and CYP90C may be involved in BR biosynthesis. This provided a unified explanation of the biological function of CYP90 proteins.

The significance of this paper can be summarized by two major points. The first one is that this study has identified the biological function of CYP90D as an enzyme involved in BR biosynthesis, as mentioned above. The second point is that this paper reported the detailed phenotypes of the BR-deficient mutant of rice.

There are more than 60 dwarf mutants of rice, some of which have been used for breeding. We have been studying these dwarf mutants from the viewpoint of the basic science of plant hormones—many dwarfisms are caused by deficiency in gibberellin and BR or by insensitivity to these hormones—and of molecular breeding. Among these rice dwarf mutants, we noticed some dwarf mutants showing a unique characteristic, erect leaves. Mutant cultivars with erect leaves can be planted more densely than their original cultivars, which have bent leaves. Consequently, a greater volume of crop products can be harvested within the same cultivation area.

Thus, we thought that elucidation of the molecular mechanism of the relationship between dwarfism and erect leaves in d2 mutants is important for further molecular breeding in the architectural modification of rice.

Throughout this study, we have succeeded in revealing the molecular mechanism of the relationship between dwarfism and erect leaves in the rice plant. As I wrote above, a phenotype of erect leaves is an important trait in terms of crop production, and this finding opened a new field of molecular breeding. Actually, for example, we succeeded in producing a new rice strain with erect leaves, which shows increased biomass production and grain yield, by modulating the level of BR in leaves (Sakamoto et al., Nature Biotech, 2006).

It’s a famous story that modulation of the synthesis of, or sensitivity to, a plant growth hormone, gibberellin, could produce high-yielding, semi-dwarf cultivars of rice and wheat, and enabled the "green revolution" to occur. Therefore, crop agronomists noticed the importance of gibberellin for improving crop productivity. Our finding revealed that another plant growth hormone, BR, is also an important target for improving crop productivity by genetic manipulation.