Leaf is a functionally and morphologically specialized organ adapted to the light capture, carbon fixation, and gas exchange for photosynthesis. In most angiosperm leaves, the adaxialabaxial pattern is apparent from the morphological and anatomical characteristics. Recent studies show that several genes are involved in adaxial-abaxial pattern formation (Bowman et al. 2002). In rice, adaxial epidermis differentiates the bulliform cells, a character common to grass leaves. The rice mesophyll tissue shows no remarkable distinction between the adaxial and abaxial sides, although mesophyll tissue differentiates into palisade and spongy tissues along adaxial-abaxial axis in dicots. Thus, it is unclear whether adaxial-abaxial differentiation exists in the mesophyll tissue of rice. To confirm this, we identified a recessive mutant from M₂ population of rice cv. Taichung 65 mutagenized with MNU. This mutant was designated adaxial snowy leaf (ads) because the adaxial surface of the leaf blade appeared whitish, but the abaxial surface appeared normal green (Fig. 1).

The ads plants showed no abnormalities other than leaf color in vegetative phase. Transverse sections of ads fresh leaf blade showed that many adaxial mesophyll cells lacked chloroplasts while all abaxial mesophyll cells were normal green (Fig. 2). Since ads mutation was leaky, albino cells co-existed with the green cells in the adaxial side of mesophyll tissue. The ads phenotype demonstrates that rice mesophyll tissue is divided into adaxial and abaxial regions. Interestingly, no albino cells were observed in the margins, outer to the outermost vascular tissue (data not shown).

The albino cells in ads leaves provide us a good marker for adaxial mesophyll identity. Hayashida et al. (2000) reported that in the adaxialized leaf2 (adl2) mutant, bulliform cells characteristic to adaxial epidermis of leaf blade were also observed in the abaxial epidermis. The epidermis of this mutant seems to be adaxialized. To investigate whether the mesophyll cells as well as epidermal cells were also adaxialized in adl2, we made ads adl2 double mutant. In the double mutant, the albino cells were distributed not only in the adaxial side but also in the abaxial side of mesophyll tissue, indicating that the mesophyll cells n adl2 have adaxial identity (Fig. 2). The double mutant also maintained green cells in the margin of both sides.

The mesophyll cells are the assimilation tissue and have a lot of chloroplasts. It is probable that albino cells generated in the ads leaves had mesophyll cell identity because they retained mesophyll-specific morphology having protuberances inside. ADS gene may play a role in the process of chloroplast differentiation in the adaxial mesophyll cells. This indicates that chloroplast development is regulated b differently etween adaxial and abaxial mesophyll cells.

Another interesting point is that the margin of leaf blade has a different characteristic from the lateral region. All mesophyll cells are normal green in the leaf margin of ads and ads adl2 leaves, suggesting that the adaxial identity is not detected in the leaf margin. These results indicate that leaf margin does not show adaxial-abaxial differentiation. The leaf margin may

have a unique identity independent of the adaxial-abaxial identity.

In conclusion, ADS gene is required for the development of chloroplasts in the adaxial side mesophyll cells. There is no visible distinction between adaxial and abaxial sides of the mesophyll tissue, but the mesophyll cells are genetically differentiated along the adaxial-abaxial axis in rice.

References

Bowman, J. E., Y. Eshed and S. F. Baum, 2002. Establishment of polarity in angiosperm lateral organs. Trends in Genetics 18: 134-141.

Hayashida, E., H. Satoh and Y. Nagato, 2000. ADL genes are required for genetic control of adaxial-abaxial pattern formation in rice leaves. RGN 17: 28-31.