Leaf development in shoot organization2 mutant of rice

The lateral dimension of the leaf is divided into central, lateral and marginal domains based on the anatomical and mutant phenotypes (Freeling 1992). The central domain, where the midrib is differentiated, is the first domain to initiate from the shoot apical meristem (SAM). The lateral domain is flat and forms a major part of the leaf. The marginal domain occupies the edge of the leaf. Especially in the sheath, it is membranous. Although a mutant defective in leaf margin has been reported in maize (Scanlon et al. 1996), the mechanism of
patterning these domains is yet poorly understood. In this report, we describe a recessive mutation, shoot organization2 (sho2) jn rice, which exhibits a variety of leaf phenotypes. sho2 plant shows pleiotropic phenotypes in embryogenesis, early vegetative phase and reproductive phase. In the early vegetative phase, sho2 meristem rapidly produces thread-like leaves with aberrant phyllotaxis (Fig. IA). However, in the late vegetative phase, relatively normal plants are recovered (Fig. lB) except leaf phenotypes such as thread-like leaves with normal dorsoventrality (Fig. 1C). Another phenotype of the sho2 leaves is the partial loss of the leaf domains. In some cases, the marginal domain is deleted from the leaf. Less severe phenotypes show the bifurcation or trifurcation of the leaf blade (Fig. ID). In some leaves, the midrib is separated from the other tissues at the middle position of the abaxial leaf blade (Fig. 1D).

Next, we observed the developmental course of leaf primordia in the wild type and sho2 using scanning electron microscopy (SEM). In the wild type, leaf primordia initiate on the flank of the SAM, resulting in a �crescent-structure�. The central domain of the primordium where the midrib will develop elongates more rapidly than the other two domains. Following the elongation of the central domain, the lateral and marginal domains of the primordium gradually enlarge and encircle the shoot apex, forming the �hood-structure� (Fig. 2A).

The developmental pattern of sho2 leaf primordia varies widely with the developmental stages. During one week after germination, leaf primordia initiate at random positions and do not encircle the shoot apex. These leaf primordia often develop into thread-like leaves. At two to three weeks after germination, although distichous phyllotaxy is recovered, the leaf morphology is still thread-like and we never observe hood-like structure (Fig. 2B). In these leaves, only central domain is elongated. At one month after gennination, a pair of bulges is observed on both side of the thread-like primordium (Fig. 2C). As the progress of leaf development, the pair of bulges encircles the shoot apex independently of thread-like structure (Fig. 2D). Therefore, the pair of bulges is regarded as the lateral and marginal domains of the leaf primorthum.

The above observation indicates that the wide variation of the sho2 leaf phenotypes is caused by the lesion of one or two leaf domains or by the non-synchronous growth of the three domains. Moreover, SEM observation shows that the central domain invariably develop earlier than the other two domains. In sho2 leaves, synchronous initiation and growth of the three domains are severely disrupted. Thus SHO2 gene is considered to be required for the proper induction of leaf domains in the SAM.

References

Freeling, M., 1992. A conceptual framework for maize leaf development. Dev. Biol. 153:44-58.

Scanlon, Mi., R.G. Schneeberger and M. Freeling, 1996. The maize mutant narrow sheath fails to establish leaf margin identity in a menstematic domain. Development 122: 1683-1691.