Our paper describes the molecular basis underlying the tropic responses of plants. Plant shoots display both gravitropic and phototropic responses, that is, as they grow upward against gravity and bend to the direction of sunlight, roots grow downward along the pull of gravity. These tropic responses constitute the most fundamental growth habit of plants, which enables them to maximize photosynthetic productivity in terrestrial environments. In spite of this fundamental occurrence, our understanding of tropic responses has long remained obscure (see below). We have demonstrated in this study that one of the auxin- responsive transcription factors, NPH4/ARF7, and one of the auxin-inducible Aux/IAA proteins, MSG2/IAA19, play a central role in both the photo- and gravitropisms of plant stems. This finding unequivocally shows that the Cholodny-Went theory of tropism is, in principle, true. This theory has been popular, but has never actually been demonstrated in the nearly 80 years since its inception. We also believe that our paper is highly cited because of the fundamental nature of our discovery.

Auxin response factors (ARFs), which constitute a gene family of 23 member genes in a small crucifer plant, Arabidopsis, bind to auxin-responsive cis elements and regulate transcription of auxin-inducible genes, including the Aux/IAA genes. The Aux/IAA proteins, which also form a gene family of 29 genes, share a protein-protein interacting domain (the C-terminal domain) with ARFs. The Aux/IAA proteins act as a transcriptional repressor, forming heterodimers with ARFs through their C-terminal domain. Loss-of-function mutant of NPH4/ARF7 and a gain-of-function mutant of MSG2/IAA19 exhibit very similar phenotypes, whose defects are almost restricted to tropic responses. This strongly suggests that NPH4 and MSG2 specifically interact with each other. They may constitute a negative- feedback loop to regulate the tropic responses, because MSG2 expression is controlled by NPH4. This finding suggests that other auxin responses are also controlled by the as-yet-undiscovered pairs of ARFs and Aux/IAAs. Auxin (indole-3-acetic acid) is the growth hormone of plants, and is involved in numerous developmental processes essential for plants. We were not able to explain why so many processes were regulated by a single, simple substance like auxin. Specific combinations between 23 ARFs and 29 Aux/IAAs may be an origin of pleiotropic responses regulated by auxin.

Photo- and gravitropic responses of plants have been one of the major foci of botanical research since the 19th century. In the 1920s, the Cholodny-Went theory was postulated for tropic responses. According to this theory, auxin accumulates in the shaded side of plant stems after exposure to a unilateral irradiation with light, resulting in a promotion of cell elongation in the shaded side. This finally leads to curvature toward the light direction. In the case of gravitropism, auxin level increases in the lower side of stems, producing upward growth. However, people were not convinced of this hypothesis because almost no molecular machineries that worked from the perception of auxin to cell elongation had been disclosed over the 80-plus years which had elapsed since the origin of the Cholodny-Went theory. Our study clearly showed that auxin plays a central role in tropic responses as postulated by said Cholodny-Went theory. Tropic responses are transient because light or gravity stimulus decreases as plant stems exhibit curvature. If stems continue to bend, they would form a 360° bend. Therefore, stems should stop bending soon after they start bending in a response to photo- or gravity stimulus. The negative-feedback loop consisting of NPH4 and MSG2 as described above, thus, should be a central regulatory circuit of tropic responses, and must provide tropic responses with their transient nature.

About 15 years ago, our understanding of signal transduction of auxin was poor in spite of its importance in plant life. At that time, I thought that early-responding genes to auxin may be crucial in the signal transduction of auxin. I carried out differential screening of cDNAs of auxin-responding genes, and isolated a few clones of the Aux/IAA genes. However, differential screening never taught you the physiological role of the isolated genes. Several years later we started screening Arabidopsis mutants that did not display stem curvature even when auxin was applied unilaterally to their stems. The experiments were successful, and we identified two loci involved in tropic responses. One of them encodes an Aux/IAA protein, MSG2/IAA19. It took almost 15 years for me to learn the physiological significance of the Aux/IAA genes.