Impaired induction of the jasmonate pathway in the rice mutant hebiba

Abstract

The elongation of rice (Oryza sativa) coleoptiles is inhibited by light, and this photoinhibition was used to screen for mutants with impaired light response. In one of the isolated mutants, hebiba, coleoptile elongation was stimulated in the presence of red light, but inhibited in the dark. Light responses of endogenous indolyl-3-acetic acid and abscisic acid were identical between the wild type and the mutant. In contrast, the wild type showed a dramatic increase of jasmonate heralded by corresponding increases in the content of its precursor o-phytodienoic acid, whereas both compounds were not detectable in the mutant. The jasmonate response to wounding was also blocked in the mutant. The mutant phenotype was rescued by addition of exogenous methyl jasmonate and o-phytodienoic acid. Moreover, the expression of O. sativa 12-oxophytodienoic acid reductase, an early gene of jasmonic acid-synthesis, is induced by red light in the wild type, but not in the mutant. This evidence suggests a novel role for jasmonates in the light response of growth, and we discuss a cross-talk between jasmonate and auxin signaling. In addition, hebiba represents the first rice mutant in which the induction of the jasmonate pathway is impaired providing a valuable tool to study the role of jasmonates in Graminean development.

Figure 1.

Phenotype of the hebiba mutant. A, Seedlings of wild type (WT) and hebiba (hb) that have been raised for 6 d either in complete darkness (D) or under the red light used for the screen (RL). White arrows indicate the position of the node separating coleoptile and mesocotyl, white arrowheads the coleoptile tip pierced by the primary leaves. Size bar = 10 mm. B, Appearance of adult plants of wild type and hebiba. C, Leaf base of adult wild-type and hebiba plants. Note the light green color of the leaves in the mutant.

Figure 2.

A, Length increment of coleoptiles that had been precultivated for 6 d and had been either kept in ongoing darkness (black symbols) or transferred to red light (white symbols). The dashed lines give the mean values over the whole population. B, Two individual coleoptiles that had been kept in ongoing darkness (black symbols) or transferred to red light (white symbols). Note the discontinuous growth of the dark-grown coleoptiles and the complete inhibition of de-etiolated coleoptiles after a lag phase that can vary among different individuals (data not shown).

Figure 3.

A, Response of IAA to red light in coleoptiles from wild type (WT) and hebiba (hb) that had been precultivated for 6 d and then irradiated with red light for 0, 30, 60, and 120 min, respectively. The data represent averages from at least 14 independent experimental series comprising 700 to 1,300 individual coleoptiles. B, Dose response curve of elongation growth in response to exogenous IAA. Segments of decapitated coleoptiles from wild type (WT) and hebiba (hb) were predepleted from endogenous auxin for 1 h and then incubated in solutions of IAA of different concentrations for 1 h. Mean values for 29 to 62 individual segments from at least two independent experimental series are shown. C, Response of ABA to red light, determined in the same coleoptiles as those in A. FW, Fresh weight.

Figure 4.

Response of JA (A) and OPDA (B) to red light in coleoptiles from wild type (WT) and hebiba (hb). For details, refer to the legend of Figure 3. The insets show basal levels of both, JA (A) and OPDA (B) present in etiolated coleoptiles. The data represent averages from at least five independent experimental series comprising 500 to 600 individual coleoptiles for A and at least 17 independent experimental series comprising 850 to 1,300 individual coleoptiles for B. FW, Fresh weight.

Figure 5.

Response of JA (A) and OPDA (B) to wounding in coleoptiles from wild type (WT) and hebiba (hb). Data from 300 to 450 (A) and 300 to 600 (B) individual seedlings from at least six independent experimental series, respectively.

Figure 6.

Rescue of the hebiba phenotype by exogenous application of methyl jasmonate (MeJA) and OPDA. Rice seedlings were grown for 6 d in darkness (A) or darkness interrupted by 24 h of red-light irradiation from d 4 to 5 (B) in water or solutions of 100 nm MeJA or 10 μm OPDA. Black arrows indicate the position of the node separating coleoptile and mesocotyl, black arrowheads the coleoptile tip pierced by the primary leaves. Note that it was not possible to reach the complete inhibition of growth that was observed in the wild type for red-light-irradiated seedlings grown in 10 μm OPDA.

Figure 7.

Expression of OsOPR transcripts in response to red light. Seedlings of wild type and mutant were irradiated for the indicated time intervals with red light, and subsequently the RNA was isolated and examined by northern blotting for the abundance of OsOPR mRNA. The transcripts were induced in the wild type upon irradiation, whereas they were not induced in the mutant. Note the low level of OsOPR expression in hebiba even before irradiation.