A mutation associated with phyllotaxy and leaf blade-sheath boundary in rice

Plant architecture of the above-ground part depends mainly on the phyllotaxy and leaf shape. Although the regularity of leaf initiation pattern has interested many plant scientists for a long time, almost nothing is known about the regulatory mechanism of phyllotaxy. The only mutant thus far reported is maize abphyl whose phyllotaxy varies widely; decussate, spiral and distichous (Greyson and Walden 1972; Greyson et a!. 1978). Grass leaves consist of three regions along the proximal-distal axis; leaf sheath, ligule-auricle and leaf blade. This organization of grass leaves provides interesting problems on leaf development. Recently, several dominant mutations which cause the displacement of the blade-sheath boundary have been characterized in maize (Freeling 1992). However, the regulatory mechanism for the establishment of the leaf blade-sheath boundary is still unknown. In this report, we describe a recessive mutation, decussate (dec), affecting both phyllotaxis and leaf blade-sheath boundary in rice.

A dec mutant was identified in a M2 population of Taichung 65 mutagenized with MNU. The embryo of dec was morphologically abnormal, although all embryonic organs and three embryonic leaves were present. After germination, dec shoot produced short and erect leaves rapidly (Fig. IA). Because phyllotactic abnormality was also suggested by the tillenng pattern, we examined the leaf initiation pattern. In the wild type, the phyllotaxis is distichous (1/2 alternate) with a divergence angle of 180°. Both margins of each leaf overlap before the emergence, and each leaf encircles completely the younger leaves (Fig. 1B).

In most dec seedlings, except for the first leaf, two leaves seemed to initiate in pairs. That is, two leaf primordia with the same size and in the same distance from the center of the shoot apical meristem (SAM) were inserted at a divergence angle of 180°, and the margins of these leaves neither overlaped nor encircled each other. Each pair of leaves rotated by 90° relative to the preceding one (Fig. 1C). In addition, the number of leaf primordia produced during five days after germination was four in dec , whereas two in the wild type. These observations indicate that the phyllotaxy of dec seedlings is not distichous but decussate. However, after the production of three or four pairs of leaves in opposite arrangement, dec plants produced short leaves in distichous phyllotaxis.

In dec seedlings, several abnormalities were also detected in leaves. Besides the smallness, the leaf showed defects in the blade-sheath boundary. In the wild type leaves, the blade-sheath boundary is perpendicular against the proximal-distal axis. In some dec leaves, however, the blade-sheath boundary was not perpendicular but oblique, and the ligule was split into several fragments (Fig. 2). This disruption of blade-sheath boundary mainly occurred in the second pair of opposite leaves, and was not observed in the third pair of leaves. Interestingly, the pattern of this disruption of blade-sheath boundary showed a regular manner. In two leaves of the second pair, leaf blade-sheath boundaries invariably inclined downward to the first leaf. Thus, there exists a close relationship between the phyllotactic change and the disruption of leaf blade-sheath boundary.

Next, we observed the SAM (Fig. 3). At five days after germination, the width of dec SAM was about twice as large as that of the wild type, although almost no difference was detected in the SAM height. Consequently, the SAM of dec was flatter in shape and larger in volume than that of the wild type. It is considered that the modification of the SAM shape or size causes the phyllotactic change from 1/2 alternate to decussate in dec mutant.

In conclusion, the dec gene is considered to regulate the phyllotaxis by maintaining the proper SAM structure in the early vegetative phase. In addition, our observations also suggest that the initiation pattern of leaf primordia around the shoot apex is important for the establishment of leaf organization.