Expression of non-symbiotic hemoglobin 1 and 2 genes in rice (Oryza sativa) embryonic organs

Abstract

Rice (Oryza sativa) contains five copies of the non-symbiotic hemoglobin (hb) gene, namely hb1 to hb5. Previous analysis by RT-PCR revealed that rice hb1 expresses in roots and leaves and hb2 expresses in leaves. However, it is not known whether or not hb1 and hb2 express in rice embryonic organs. Here, we report the expression of hb1 and hb2 genes in rice embryonic organs using RT-PCR and specific oligos for Hb1 and Hb2. Our results indicate that hb1 and hb2 genes express in embryonic organs in rice growing under normal conditions. Specifically, hb1 expresses in rice embryos and seminal roots, and hb2 expresses in embryos, coleoptiles and seminal roots. These observations suggest that Hb1 and Hb2 coexist and function in rice embryonic organs.

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Non-symbiotic hemoglobins (nsHbs) are widespread in land plants. Nucleotide sequences coding for these proteins have been identified in plants ranging from primitive bryophytes to evolved angiosperms, including monocots and dicots.1–3 In monocots, rice (Oryza sativa) contains five copies of the nshb gene, namely hb1 to hb5.4,5 Western blot analysis and immunolocalization by confocal microscopy using polyclonal anti-rice Hb1 antibodies revealed that nsHbs are localized in specific tissues of rice embryonic organs, such as the seed aleurone and scutellum, and of vegetative organs, such as the leaf schlerenchyma and root cap.6,7 However, these analyses did not differentiate among individual nsHbs because detected signals could result from any one of the rice nsHbs (i.e., Hb1, Hb2, Hb3, Hb4 or Hb5), or from a combination of them. Previous analysis by RT-PCR using specific oligos revealed that rice hb1 expresses in roots and leaves, hb2 expresses in leaves, and hb5 expresses in embryonic and vegetative organs.5,8 Also, the analysis of an OsNSHB2 promoter fused to the gus reporter gene revealed tissue-specific expression of the rice hb2 gene in roots, vasculature of young leaves, flowers and the pedicel/stem junction of transgenic Arabidopsis.9 These observations suggest that Hb1, Hb2 and Hb5 coexist in rice leaves and roots. However, it is still not known whether or not hb1 and hb2 express in rice embryonic organs.

We analyzed the expression of hb1 and hb2 genes in rice embryonic organs using RT-PCR and specific oligos for Hb1 and Hb2. Seed germination, isolation of poly(A⁺) RNA, reverse transcription and PCR amplification were performed as described by Arredondo-Peter et al.8 The Hb1 and Hb2 transcripts are each approximately 550 bp in length. Figure 1 shows that PCR fragments of the expected size for Hb1 were amplified from embryos and seminal roots, and for Hb2 were amplified from embryos, coleoptiles and seminal roots. These PCR fragments were cloned and sequenced, and the resulting sequences were identical to those of the rice Hb1 and Hb2 transcripts (Genbank accession number U76030.1 and U76031.1, respectively). This result indicates that hb1 and hb2 genes express in embryonic organs in rice growing under normal conditions. It was previously reported that hb2 apparently does not express in rice roots,8 however in this work Hb2 transcripts were detected in rice seminal roots (Fig. 1). This observation suggests that expression of hb2 is downregulated during root development.

The above observations together with those reported by Arredondo-Peter et al.8 and Garrocho-Villegas et al.5 suggest that Hb1, Hb2 and Hb5 coexist in rice embryonic and vegetative organs. Specifically, it is likely that Hb1, Hb2 and Hb5 coexist and function in rice embryos, seminal roots and leaves; Hb2 and Hb5 coexist and function in coleoptiles; and Hb1 and Hb5 coexist and function in roots (Table 1). Results reported here complement our knowledge of the expression of nshb genes in rice organs.

Abbreviations

Hb	=	hemoglobin
nsHb	=	non-symbiotic hemoglobin
RT-PCR	=	reverse transcriptase-polymerase chain reaction

Figures and Tables

Figure 1 Expression analysis of hb1 and hb2 genes in rice embryonic organs. Amplification of the rice Hb1 and Hb2 transcripts was performed by RT-PCR using specific oligos for Hb1 and Hb2 and conditions described by Arredondo-Peter et al.8

Table 1 Expression of nshb genes in rice organs

Expression* in rice
nshb gene		Embryonic organs		Vegetative organs		References
	Embryos	Coleoptiles	Seminal roots	Leaves	Roots
hb1	+	ND	+	+	+	This work; Arredondo-Peter et al.8
hb2	+	+	+	+	ND	This work; Arredondo-Peter et al.8
hb3	NA	NA	NA	NA	NA
hb4	NA	NA	NA	NA	NA
hb5	+	+	+	+	+	Garrocho-Villegas et al.5

* ND , not detected; NA, not analyzed.

Acknowledgements

Authors wish to express their gratitude to Miss Gillian Klucas and Dr. Gautam Sarath for English corrections and useful comments to improve the contents of this addendum.

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