Phytochrome-Mediated Inhibition of Coleoptile Growth in Rice: Age-dependency and Action Spectra†

Abstract

Phytochrome has been shown to be the major photoreceptor involved in the photo-inhibition of coleoptile growth in Japonica-type rice (Oryza sativa L.). We have characterized this typical photomorphogenetic response of rice using mutants deficient in phytochrome A (phyA) and phytochrome B (phyB) and with respect to age-dependency and action spectra. Seedlings were irradiated with a pulse of light 40 h or 80 h after germination (i.e. at an early or late developmental stage) and the final coleoptile length of these seedlings was determined. A saturating pulse of red light (R) had a stronger effect when it was given in the late stage than in the early stage. It was found that the photoinhibition is mediated by both the phyA and the phyB in the late stage but predominantly by phyB in the early stage. Consistent with many other reported responses, the photo-inhibition in the phyA mutant, which was observed in the early and late developmental stages and is thought to be mediated mainly by phyB, occurred in the low-fluence range (10(1)-10(3) micromol m(-2)) of R and was far-red-light (FR)-reversible; the photo-inhibition in the phyB mutant, which was observed in the late developmental stage and is thought to be mediated mainly by phyA, occurred in the very-low-fluence range (10(-2)-10(0) micromol m(-2)) and was FR-irreversible. The action spectra (350-800 nm at 50 nm intervals) obtained at the two developmental stages using phyA and phyB mutants indicated that both the phyB-mediated low-fluence response and the phyA-mediated very-low-fluence response have a major peak at 650 nm and a minor peak at 400 nm.

Similarities and differences between phytochrome-mediated growth inhibition of coleoptiles and seminal roots in rice seedlings

“…At the early stage of development [40 h after inducing germination (AIG)], photoinhibition was predominantly due to the phyB-mediated low-fluence response (LFR), but at the late developmental stage (80 h AIG), it consisted of the phyA-mediated very lowfluence response (VLFR) as well as the phyB-mediated LFR. 3,4 In the case of root growth, the sensitivity of photoinhibition also depended on age, and was most sensitive in the period of 48-96 h AIG when seedlings were irradiated for 24 h. Using rice phytochrome mutants, 5 we found that far-red light for root growth inhibition was perceived exclusively by phyA, that red light was perceived by both phyA and phyB, and that phyC had little or no role in growth inhibition. Furthermore, the fluence rate required for phyB-mediated inhibition was more than 10,000-fold greater than that required for phyA-mediated inhibition.…”

“…3 In this study, we examined photoinhibition of seminal root growth, and found similarities and differences between light-induced growth inhibition of the two organs in rice seedlings. Although coleoptile growth was inhibited by pulses of light, growth inhibition of seminal roots required light irradiation longer than 6 h. The Bunsen-Roscoe law of reciprocity was not observed in the growth inhibition of seminal root.…”

“…These characteristics of photoinhibition in seminal roots are similar to those found in coleoptiles at the late stage of development. 3 In seminal roots, photoinhibition appeared to be mediated by photoreceptors in the root itself.…”

Light-modulated seminal wavy roots in rice mediated by nitric oxide-dependent signaling

“…Several photoreceptor genes have been identified in rice plants that included phytochromes (OsPHYA, OsPHYB, OsPHYC [Takano et al 2001), cryptochromes (OsCRY1a, OsCRY1b, OsCRY2 [Hirose et al 2006;Zhang et al 2006]), and phototropins (OsPHOT1a, OsPHOT1b, OsPHOT2 [Jain et al 2007;Goh et al 2009]). PHYA and PHYB were involved in the photoinhibition of rice coleoptile elongation (Xie et al 2007), and they also participated in the signal transduction of light-induced root coiling in high-nitrogen conditions (Shimizu et al 2009). PHYA, CRY1a, and CRY1b have been found to be involved in the regulation of jasmonate biosynthetic enzymes such as allene oxide synthase (AOS) in rice plants (Haga and Iino 2004;Hirose et al 2006).…”

Overexpression of a phytochrome-regulated tandem zinc finger protein gene, OsTZF1, confers hypersensitivity to ABA and hyposensitivity to red light and far-red light in rice seedlings

“…The recent generation and characterization of phytochrome single, double, and triple mutants have revealed that phyB plays important roles in seedling de-etiolation, flowering, and fertility in rice (Takano et al 2005;Xie et al 2007;Takano et al 2009). In addition to its roles in photomorphogenesis, we have recently observed that phyB-mediated light signal affects resistance to Magnaporthe grisea and drought tolerance in rice (Xie et al 2011;Liu et al 2012).…”