24. Petunia NO APICAL MERISTEM ortholog, OsNAM, is expressed in the embryonic SAM and organ boundary in rice

K. HIBARA and Y. NAGATO

Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo, 113-8657 Japan 

 

Elaboration of the body plan in flowering plants depends on the activity of the shoot apical meristem (SAM), which is formed during embryogenesis. Genetic and molecular studies in dicots have revealed that members of the NAC gene family such as NO APICAL MERISTEM (NAM) of Petunia and CUP-SHAPED COTYLEDON (CUC) of Arabidopsis contribute to the establishment of the embryonic SAM (Souer et al. 1996, Aida et al. 1999). For understanding the embryonic SAM formation in rice, we isolated and characterized OsNAM gene that is an ortholog of the NAM/CUC genes.

We found two predicted genes (OSJNBa0021N09.15 and OSJNBa0016N23.129) that were highly related to the NAM/CUC genes from the rice genome database using BLAST search. Phylogenic analyses based on the amino acid sequence of NAC domain revealed that OSJNBa0021N09.15 was orthologous to NAM/CUC1-2 and OSJNBa0016N23.15 was orthologous to CUC3, thus they were designated OsNAM and OsCUC3, respectively (Fig.1). The NAC domain of OsNAM is 76.0%, 80.3% and 83.6% identical to those of CUC1, CUC2 and NAM, respectively. NAM/CUC1-2 class genes in dicots have micro RNA (miR164) recognition sites. OsNAM also have a potential miR164 site.

We investigated the expression of OsNAM by in situ hybridization. OsNAM mRNA was detected at three days after pollination (3 DAP) in the predicted region of embryonic SAM (Fig. 2A). When the SAM was initiated at 4 DAP, however, OsNAM expression disappeared from the SAM, and was detected in the boundary between the SAM and coleoptile (Fig. 2B). These expression patterns during 3-4 DAP embryos were analogous to those of NAM and CUC genes. At 5-6 DAP, the expression was observed also in the boundary between SAM and the first leaf primordium (Fig. 2C, D). In addition, the signal was detected in several adaxial cell layers of coleoptile.

After germination, OsNAM expression was maintained in organ boundaries (Fig. 2E). Interestingly, OsNAM mRNA was also detected in part of the vegetative SAM (Fig. 2E), in contrast to the case of dicots in which NAM/CUC was not expressed in the SAM. This suggests that the function of monocot NAM/CUC gene may be partially different from that in dicots.

In conclusion, OsNAM that is the ortholog of NAM/CUC1-2 would play an important role in SAM initiation and organ separation.   


Fig. 1. The phylogenic tree of NAC family genes deduced from amino acid sequences of NAC domains.

 

Fig. 2. Localization of OsNAM transcripts during embryogenesis and in vegetative shoot. Longitudinal sections of embryo at 3 DAP (A), 4 DAP (B), 6 DAP (C, D) and vegetative shoot at 10 DAG (E). Arrowheads: SAM. Arrows: coleoptile. Asterisks: first leaf primordium. Bar: 50μm.

 

References

Souer, E., A. van Houwelingen, D. Kloos, J. Mol and R. Koes, 1996. The no apical meristem gene of Petunia is required for pattern formation in embryos and flowers and is expressed at meristem and primordia boundaries. Cell 85: 159-170.

Aida, M., T. Ishida and M. Tasaka, 1999. Shoot apical meristem and cotyledon formation during Arabidopsis embryogenesis: interaction among the CUP-SHAPED COTYLEDON and SHOOT MERISTEMLESS genes. Development 126: 1563-1570.