apical displacement 1 affects the shoot position and the number of radicles in rice embryo

7. apical displacement 1 affects the shoot position and the number of radicles in rice embryo

During rice embryogenesis, embryo differentiates several organs such as shoot, radicle, scutellum and epiblast in a fixed number and fixed positions under strict genetic control. Shoot and radicle that are essential for establishing plant body plan are produced in the apical and basal region of the embryo, respectively. In higher plants, however, available information is quite limited on the mechanism by which the number and position of embryonic organ are regulated. We previously reported mr mutant that causes the reduction of apical region and the enlargement of basal region (Kinae et a!. 1998). This mutation results in the underdeveloped shoot and two or three radicles in embryo. We have identified another embryo mutant, apical displacement 1 (apdl), exhibiting one shoot at the apical end and one or two radicles in the basal region. In this paper, we describe the developmental course of apdl embryo.

III.Genetics of Morphological Traits 7. apical displacement 1 affects the shoot position and the number of radicles in rice embryo T. KINAE�, M. Ma tsuoka2 and Y. NAGATO1 1) Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo, 113-8657 Japan 2) BioScience Center, Nagoya University, Nagoya, 464-8601 Japan During rice embryogenesis, embryo differentiates several organs such as shoot, radicle, scutellum and epiblast in a fixed number and fixed positions under strict genetic control. Shoot and radicle that are essential for establishing plant body plan are produced in the apical and basal region of the embryo, respectively. In higher plants, however, available information is quite limited on the mechanism by which the number and position of embryonic organ are regulated. We previously reported mr mutant that causes the reduction of apical region and the enlargement of basal region (Kinae et a!. 1998). This mutation results in the underdeveloped shoot and two or three radicles in embryo. We have identified another embryo mutant, apical displacement 1 (apdl), exhibiting one shoot at the apical end and one or two radicles in the basal region. In this paper, we describe the developmental course of apdl embryo. The apdl mutant was derived from cv. Taichung 65 mutagenized with MNU. In the mature embryo, apdl had a reduced shoot at the apex and two radicles in the middle and basal regions (Fig. 1D). Furthermore, scutellum and epiblast were reduced. The first and second leaves were malformed, and the third leaf was not formed in the embryo. Due to the underdevelopment of scutellum, mature embryo was small, nearly 70 % of the wild type in length. Embryonic development was examined using paraffin sections. At four days after pollination (4 DAP), wild type embryo differentiated coleoptile on the ventral side just above the shoot apical meristem, and at 5 DAP the first leaf primordium was formed on thebasal flank of the shoot meristem. In apdl, the early embryo development before 4 DAP did not differ from that of the wild type. The shoot apical meristem initiated at a normal position, but the coleoptile protrusion was not prominent and the scutellum was also underdeveloped (Fig. 1A, B). Since the scutellum and coleoptile did not develop but the basal region was enlarged, the shoot position gradually moved toward the apex (Fig. 1B,C). Thus, in apdl embryo, the early embryonic regionalization is normal. Final shoot position in embryo apex is due to the underdevelopment of apical region (scutellum and coleoptile) and the enlargement of the basal region. In about 70% of apdl embryos, two radicles were produced. However, at 8 DAP, only one radicle was detected in a normal position, indicating that ectopic radicle would initiate in the basal region at a later stage. Embryonic regionalization of apdl was explored by in situ hybridization using a rice homeobox gene OSHI as a probe. OSHI is known to be expressed in the shoot apical meristem and the adjacent tissues (epiblast, circumstance of radicle etc.) of wild-type embryo. The expression pattern of OSH1 in apdl embryo was almost normal through 5 DAP (Fig. 1E). However, concomitant with the apical displacement of shoot and the enlargement of basal region, the expression domain of OSHI was extended gradually to the basal region. In addition to the expression in the shoot menstem, the hybridization signals were detected in duplicate in the basal region, as if surrounding two expected radicles (Fig. 1F,G). Nearly half of apdl seeds germinated. In the early vegetative phase, apdl produced several narrow and white leaves (Fig. 2A). Thereafter, green but yet narrow leaves emerged (Fig. 2B). Mature apdl plants were slender and dwarf. The panicle was small with only 10-20 flowers (Fig. 2C), and the seed fertility was very low. It is hypothesized that apdl causes the reduction of apical region of embryo that results in the enlargement of basal region after the morphogenetic events occur. Thus the wild type APD1 gene must be associated with the apical-basal patterning of embryo through regulating the relative size of apical/basal region. Reference Kinac, T., H. Satoh, H. Kitano andY. Nagato, 1998. A multiple radiclel mutation affects the number of radicles and the vegetative growth in rice. RGN 15: 93-96.

Embryonic development was examined using paraffin sections. At four days after pollination (4 DAP), wild type embryo differentiated coleoptile on the ventral side just above the shoot apical meristem, and at 5 DAP the first leaf primordium was formed on thebasal flank of the shoot meristem. In apdl, the early embryo development before 4 DAP did not differ from that of the wild type. The shoot apical meristem initiated at a normal position, but the coleoptile protrusion was not prominent and the scutellum was also underdeveloped (Fig. 1A, B). Since the scutellum and coleoptile did not develop but the basal region was enlarged, the shoot position gradually moved toward the apex (Fig. 1B,C). Thus, in apdl embryo, the early embryonic regionalization is normal. Final shoot position in embryo apex is due to the underdevelopment of apical region (scutellum and coleoptile) and the enlargement of the basal region. In about 70% of apdl embryos, two radicles were produced. However, at 8 DAP, only one radicle was detected in a normal position, indicating that ectopic radicle would initiate in the basal region at a later stage.

Embryonic regionalization of apdl was explored by in situ hybridization using a rice homeobox gene OSHI as a probe. OSHI is known to be expressed in the shoot apical meristem and the adjacent tissues (epiblast, circumstance of radicle etc.) of wild-type embryo. The expression pattern of OSH1 in apdl embryo was almost normal through 5 DAP (Fig. 1E). However, concomitant with the apical displacement of shoot and the enlargement of basal region, the expression domain of OSHI was extended gradually to the basal region. In addition to the expression in the shoot menstem, the hybridization signals were detected in duplicate in the basal region, as if surrounding two expected radicles (Fig. 1F,G).

Nearly half of apdl seeds germinated. In the early vegetative phase, apdl produced several narrow and white leaves (Fig. 2A). Thereafter, green but yet narrow leaves emerged (Fig. 2B). Mature apdl plants were slender and dwarf. The panicle was small with only 10-20 flowers (Fig. 2C), and the seed fertility was very low.

It is hypothesized that apdl causes the reduction of apical region of embryo that results in the enlargement of basal region after the morphogenetic events occur. Thus the wild type APD1 gene must be associated with the apical-basal patterning of embryo through regulating the relative size of apical/basal region.

Reference

Kinac, T., H. Satoh, H. Kitano andY. Nagato, 1998. A multiple radiclel mutation affects the number of radicles and the vegetative growth in rice. RGN 15: 93-96.