Studies on mechanisms of tip specific blue-light perception in maize coleoptiles; in relation to first positive phototropism
Year: 2015
Type: dissertation
Abstract: Auxins are the class of plant hormones mediating many plant physiologies, growth, development, and environmental responses. Phototropism is one of these auxin mediated phenomena. It is induced by the perception of unilateral blue-light by phototropin (phot)s, and the signal triggers lateral transport of a natural auxin, indole-3-acetic acid (IAA), at the tip region of monocot coleoptiles. In Part I, I searched PHOT genes from the maize genome database, and obtained two genes of phototropins, a putative ZmPHOT2 gene sequence as well as ZmPHOT1 which is already known. To characterize both genes, I analyzed their expression patterns by the quantitative real-time PCR analysis. ZmPHOT1 was abundantly expressed in etiolated coleoptiles, while lower expressions of both ZmPHOT1 and ZmPHOT2 were observed in young leaves. These genes were not specifically expressed in the tip of coleoptile where is a key region for photo-perception in phototropism. A pulsed low-fluence blue-light (LBL) (0.33 μmol m^<-2> s^<-1> X 8 sec) and continuous high-fluence blue-light (HBL) (10 μmol m^<-2> s^<-1>) rapidly decreased the expression level of ZmPHOT1 gene in coleoptiles, whereas the expression level of ZmPHOT2 did not significantly change after the treatments. In contrast, no drastic transcriptional changes were observed in either ZmPHOT1 or ZmPHOT2 by LBL or HBL irradiation in young leaves. These results indicated that Zmphot1 might work as a main blue-light receptor for LBL-induced phototropism in coleoptiles. Thus, protein modification of Zmphot1 was investigated by western blot analysis using two specific antibodies against Osphot1. Zmphot1 was detected in microsomal fractions, and accumulated at coleoptiles than leaves as similar as its mRNA expressions. HBL caused rapid phosphorylation of the protein and reduced the amount of protein similar to other plant phot1. Interestingly, no Zmphot1 phosphorylation was detected by LBL in this study. As previously reported, the tip specific ZmNPH3 like and ZmPGP like gene expressions, I confirmed that ZmNPH3-like and ZmPGP-like proteins distributed similarly to mRNA expression patterns. Particle bombardment analysis showed that Zmphot1 and ZmNPH3-like proteins could interact in the cells and were co-localized on plasma membrane. I will discuss the possible involvement of these proteins in tip-specific blue-light perception and IAA re-distribution in maize coleoptiles. The LBL irradiation to the topmost part of coleoptiles is sufficient to induce the phototropism. The tip of coleoptile is also the region where the asymmetric indole-3-acetic acid (IAA) distribution occurs after the LBL perception, and IAA is actually synthesized at this specific region. Previous report showed that treatment of NPA (1-N-naphthylphthalamic acid), a potent inhibitor of IAA efflux, inhibited LBL-induced phototropic responses. On the other hand, little is known about the effects of the IAA influx inhibitors on phototropism in maize coleoptiles. 1-naphtoxyacetic acid (1-NOA), an analog of the synthetic auxin, 1-N-naphtalene acetic acid (NAA), is the one of IAA influx inhibitors. Although 1-NOA inhibits auxin efflux, it also has an auxin activity at a relatively high concentration possibly due to its structural similarity to NAA. In the previous studies at our laboratory, several candidates of novel IAA influx inhibitors were screened from 10,000 chemical library. One of them, 7-B3, ethyl 2-[(2-chloro-4-nitrophenyl)thio]acetate, showed similar structure to a synthetic auxin, 2,4-D (2,4-diclorophenoxyacetic acid),. Therefore, in Part II, I conducted characterization of 7-B3 as a new IAA influx inhibitor using Arabidopsis seedlings. The results indicated that 7-B3 did not exhibit auxin activity at high concentration, though 1-NOA at the same concentration induced IAA response such as DR5::GUS expression. It is reported that 1-NOA inhibits apical hock formation of etiolated Arabidopsis seedling. I found 7-B3 inhibited the apical hock formation more effectively than 1-NOA. This effect seemed solely through inhibition of IAA influx by 7-B3. These results suggested that 7-B3 is a potential inhibitor of IAA influx without auxin activity. Recent studies have indicated that the IAA influx activity is important to create IAA maxima at the local and specific tissue, and mediate physiological responses, such as formation of apical hock, root initiation, and phototropism. Thus, I propose that 7-B3 will be an effective tool to analyze the local asymmetric IAA distribution after LBL irradiation.
Authors: スズキ, ヒロミ, 鈴木, 洋弥, Suzuki, Hiromi
Language: English
Topic: Light effects on plants
Subfield: Plant Science
Domain: Life Sciences
SDG: Zero hunger
Open Access status: green
OpenAlex ID: W7146307079