Os10g0496900

The rice faded green leaf locus encodes protochlorophyllide oxidoreductase B and is essential for chlorophyll synthesis under high light conditions. OsPORB is essential for maintaining light-dependent Chl synthesis throughout leaf development, especially under HL conditions.

Annotated Information

Function

OsPORB is essential for maintaining light-dependent Chl synthesis throughout leaf development, especially under HL conditions. The rice faded green leaf locus encodes protochlorophyllide oxidoreductase B and is essential for chlorophyll synthesis under high light conditions. NADPH:protochlorophyllide oxidoreductase (POR) catalyzes photoreduction of protochlorophyllide (Pchlide) to chlorophyllide in chlorophyll (Chl) synthesis, and is required for prolamellar body (PLB) formation in etioplasts.

Rice faded green leaf (fgl) mutants develop yellow/white leaf variegation and necrotic lesions during leaf elongation in field-grown plants. Map-based cloning revealed that FGL encodes OsPORB, one of two rice POR isoforms. In fgl, etiolated seedlings contained smaller PLBs in etioplasts, and lower levels of total and photoactive Pchlide. Under constant or high light (HL) conditions, newly emerging green leaves rapidly turned yellow and formed lesions. Increased levels of non-photoactive Pchlide, which acts as a photosensitizer, may cause reactive oxygen accumulation and lesion formation.

fgl harbors a 1-bp deletion in the coding region of OsPORB, resulting in a frameshift mutation and premature translational termination. Based on the expression analysis of two OsPOR genes and phenotypic characterization of the fgl mutant, Yasuhito Sakurabaet al proposed that OsPORB plays important roles in maintaining a threshold Chl level throughout leaf development, especially under HL field conditions.

(a) Genetic mapping of the fgl locus. The fgl locus was initially mapped to within 8.5 cM between two STS markers, S10053 and S10061.5, on chromosome 10.

(b) Physical mapping of the fgl locus with STS markers. Numbers below the thick lines represent the number of recombinants between two flanking markers.

(c) Candidate genes (black boxes) in the 47–kb genomic DNA.

(d) A frameshift mutation of OsPORB in the fgl mutant. Four exons and three introns are indicated by rectangles and thick lines, respectively. The position of the 1-bp deletion (G109) is marked in the second exon.

(e) dCAPS analysis was performed to confirm the 1-bp deletion in OsPORB. AatII digests the genomic PCR products from the fgl mutant (lane 1) but not from the wild type ‘Kinmaze’ (lane 2). Lane 3, a mapping parent ‘Dasanbyeo’; lane 4, F1hybrid (Dasanbyeo/fgl).

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Expression

OsPORB expression is regulated in a circadian rhythm in short-day conditions. OsPORA was expressed at high levels in developing leaves and decreased dramatically in fully mature leaves, whereas OsPORB expression was relatively constant throughout leaf development. However, OsPORB expression is rapidly upregulated by HL treatment.

OsPORB is not required for light-dependent Chl synthesis during greening of etiolated plants

First, Sakuraba et al examined the effect of light on OsPOR expression during greening. When 6–day-old etiolated wild-type seedlings were exposed to light for 24 h, OsPORA expression was rapidly downregulated after illumination, whereas OsPORB expression decreased only slightly (Figure 2a). Consistent with its mRNA levels, OsPORA protein levels were severely reduced after 6 h of light treatment, and were maintained at low levels until 12 h in both wild-type and fgl seedlings (Figure 2b). OsPORB remained relatively constant in the wild type, but OsPORB did not accumulate in fgl because of the frameshift mutation in OsPORB. Next, to examine whether OsPORA contributes to light-dependent Chl synthesis during leaf greening, Sakuraba et al compared the greening speed of wild-type and fgl seedlings. The visible speed of greening (Figure 2c) and the rates of Chl synthesis (Figure 2d) were not significantly altered in fgl, indicating that low levels of OsPORA are sufficient for leaf greening in rice, even in the absence of OsPORB activity in fgl mutants.

Enhanced OsPORB activity is essential for Chl synthesis under HL.

In Arabidopsis, AtPORC expression increases under HL conditions, whereas both AtPORA and AtPORB are downregulated (Masuda et al., 2003), indicating that AtPORC is responsible for Chl synthesis under HL conditions. Thus, Sakuraba et al examined whether OsPORB has a similar function to AtPORC for Chl synthesis in response to HL.

The 14–day-old wild-type and fgl seedlings grown under mild light intensity (200 μmol m−2 s−1) were transferred to HL (1500 μmol m−2 s−1) for 3 days in SD conditions (Figure 3). Compared with the wild type, fgl exhibited a severely photobleached phenotype (Figure 3a,b), possibly as a result of photooxidative damage. In the wild type, OsPORA mRNA levels were lower after 3 h of HL treatment (Figure 3c), whereas OsPORB mRNA levels drastically increased (Figure 3d), indicating that the rice plant requires increased expression of OsPORB in response to HL conditions. The levels of OsPORA and OsPORB were consistent with their mRNA levels (Figure 3e), indicating that rice POR activity is regulated at the transcriptional level.

These results strongly suggest that OsPORB activity is necessary for Chl synthesis under HL conditions in the paddy field, reminiscent of AtPORC function in Arabidopsis (Masuda et al., 2003).

Evolution

There are two rice POR isoforms and three differentially regulated POR isoforms (PORA, PORB and PORC) in Arabidopsis thaliana. Both Arabidopsis and rice PORB genes are constitutively expressed throughout development, but their responses to light intensity were somewhat different. OsPORB expression was rapidly upregulated by HL treatment, similar to the expression of AtPORC. In summary, Sakuraba et al propose that OsPORA has overlapping functions corresponding to AtPORA and AtPORB, but that OsPORB has functions corresponding to AtPORB and AtPORC. In agreement with this hypothesis, a phylogenic tree of Arabidopsis, rice and barley PORs suggests that OsPORA and HvPORA are evolutionarily closer to AtPORA and AtPORB, but that OsPORB and HvPORB are similar to AtPORC.

Labs working on this gene

1. Department of Plant Science, Plant Genomics and Breeding Institute, Research Institute for Agriculture and Life Sciences,Seoul National University, Seoul 151-921, Korea

2. Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8501 Japan. tmasuda@bio.titech.ac.jp

References

1. Yasuhito Sakuraba;Md Lutfor Rahman;Sung-Hwan Cho;Ye-Sol Kim;Hee-Jong Koh;Soo-Cheul Yoo;Nam-Chon Paek

The rice faded green leaf locus encodes protochlorophyllide oxidoreductase B and is essential for chlorophyll synthesis under high light conditions. 
The Plant Journal, 2013, 74(1): 122-133

2. Qiaosong Yang;Han He;Heying Li;Hua Tian;Jianjun Zhang;Liguang Zhai;Jiandong Chen;Hong Wu;Ganjun Yi;Zheng-Hui He;Xinxiang Peng

NOA1 Functions in a Temperature-Dependent Manner to Regulate Chlorophyll Biosynthesis and Rubisco Formation in Rice. PLoS ONE, 2011, 6(5): e20015

3. Masuda, T., Fusada, N., Oosawa, N. et al.

Functional analysis of isoforms of NADPH: protochlorophyllide oxidoreductase (POR), PORB and PORC, in Arabidopsis thaliana. Plant Cell Physiol, 
2003, 44, 963–974.