A Rice Ancestral Genetic Resource Conferring Ideal Plant Shapes for Vegetative Growth and Weed Suppression

Abstract

To maximize crop growth, crops need to capture sunlight efficiently. This property is primarily influenced by the shape of the crops such as the angle, area, and arrangement of leaves. We constructed a rice (Oryza sativa L.) inbred line that displayed an ideal transition of plant shapes in terms of sunlight receiving efficiency. During vegetative growth, this line exhibited tiller spreading with increased tiller number, which formed a parabolic antenna-like structure. The architecture probably improved light reception efficiency of individuals compared with the recurrent parent. The line achieved not only acceleration of the vegetative growth, but also significant suppression of weed growth under the canopy. The increased light reception efficiency of the line has consequently reduced the amount of incident light to the ground and supplied significant competitiveness against weeds. The spread tillers became erect from the entry of the reproductive growth phase, adaptively sustaining light reception efficiency in thicker stands. The line carries a small chromosomal segment from Oryza rufipogon Griff., a putative progenitor of Asian cultivated rice. The introduced chromosome segment had little effect on grain yield and quality. Our results shed light on potentials hidden in the wild rice chromosome segment to achieve the valuable traits.

Keywords: Asian cultivated rice (Oryza sativa L); ancestral genetic resource; light reception efficiency; plant shape; tiller angle; vegetative growth; weed control; wild rice (Oryza rufipogon Griff.).

FIGURE 1

Basic properties of GP9-7. (A) A comparative diagram showing the genotyping results of GP9-7, KRIL31, and Koshihikari. The yellow zones indicate the segments derived from Koshihikari and the red zones represent the segments derived from Oryza rufipogon (O. rufipogon). (B,C) Typical plant shapes of the rice plants are used in this study. The photograph (B) shows typical nursling seedlings of Koshihikari and GP9-7 at 10 days after sowing. The photographs (C) indicate typical plant shapes in paddy fields taken from directly above the plants at 33 days after planting (DAP). Scale bars: 10 cm.

FIGURE 2

Basic properties of GP9-7 toward the reproductive growth phase. (A) Developmental transitions of tiller angles from horizontal in Koshihikari (blue circles with broken lines) and GP9-7 (red triangles with solid lines) grown in a test field at Tsukuba, Japan in the summer of 2020 (planting date: May 21, 2020). The tiller angles of the same six plants were continuously measured and the mean values with SE were shown in the graph. The period for heading is indicated by a vertical gray bar. Symbols, ***, **, and * indicate statistically significant differences compared to Koshihikari at P < 0.001, P < 0.01, and P < 0.05, respectively, calculated by the Welch’s t-test. The photographs (B) display typical plant shapes of Koshihikari and GP9-7 during the ripening phase (25 days after heading). Scale bars: 20 cm.

FIGURE 3

Developmental transitions of growth traits of Koshihikari (blue circles with broken lines or blue bars) and GP9-7 (red triangles with solid lines or red bars) from the vegetative to the reproductive growth phase. Transitions of dry weight per plant (A), crop growth rate (CGR) (B), leaf area index (LAI) (C), and net assimilation rate (NAR) (D) of Koshihikari and GP9-7 grown in a test field at Tsukuba, Japan in the summer of 2018 (planting date: May 9, 2018). Mean values of the dry weights and leaf area indexes obtained from seven plants are plotted. The SE values are shown as error bars in the graphs. The period for heading is indicated by a vertical gray bar. Symbols, ***, **, and * indicate statistically significant differences compared to Koshihikari at P < 0.001, P < 0.01, and P < 0.05, respectively, calculated by the Welch’s t-test.

FIGURE 4

Light reception-related properties of GP9-7. (A) Developmental transitions in vegetative coverage of Koshihikari and GP9-7 in the test field at Tsukuba, Japan in the summer of 2018 (planting date: May 28, 2018). The period for heading is indicated by a vertical gray bar. This analysis was performed according to Materials and Methods, using images continuously taken at the same three points in the field, and the mean values with SE were indicated in the graph. (B) Vertical transitions of the relative photosynthetic photon flux density (RPPFD) of Koshihikari and GP9-7 measured on the 52nd DAP that is shown by a vertical gray line in panel (A). Symbols, ***, **, and * indicate statistically significant differences compared to Koshihikari at P < 0.001, P < 0.01, and P < 0.05, respectively, calculated by the Welch’s t-test. Values for Koshihikari are indicated with blue circles with broken lines and values for GP9-7 are represented with red triangles with solid lines.

FIGURE 5

Weed suppression-related properties of GP9-7. (A) Developmental transitions in tiller numbers and the RPPFD at ground level of Koshihikari and the GP9-7 in the test field in Fukuyama, Japan in the summer of 2019 (planting date: June 15, 2019). The periods for heading are indicated by vertical gray bars. (B) Developmental transitions in tiller numbers of the transplanted weed, E. crus-galli var. formosensis under the canopy formed by rice stands. Symbols, ***, **, and * indicate statistically significant differences compared to Koshihikari at P < 0.001, P < 0.01, and P < 0.05, respectively, calculated by the Student’s t-test. Values for Koshihikari are indicated with blue circles with broken lines and values for GP9-7 are represented with red triangles with solid lines.

FIGURE 6

The weed competitiveness of GP9-7 was evaluated by measuring the growth of naturally occurring weeds in the summer of 2019 (planting date: June 15, 2019). The number of emerging weeds (A), their tiller number (B), and the dry matter weight (C) of naturally occurring E. crus-galli var. formosensis under the canopies of Koshihikari or GP9-7 are indicated. Mean values obtained from three plots for Koshihikari and GP9-7 with the SE values are indicated with blue and red bars, respectively. The p-values were calculated by the Student’s t-test and are reported above the bar graphs.

FIGURE 7

Yield-related traits of GP9-7. (A) Heading date (n = 12), (B) panicle number per m² (n = 12), (C) spikelet number per panicle (n = 12), (D) percent ripened grain (n = 5), (E) one-thousand grain weight (n = 3), and (F) weight of unhulled rice (n = 3) of Koshihikari and GP9-7 cultivated in the test field at Tsukuba, Japan in the summer of 2018 (planting date: May 28, 2018) under sparse conditions (interplant space = 36 cm; 7.7 plants m^–2). P-values were calculated by the Welch’s t-test and are reported above the bar graphs. Mean values for Koshihikari and GP9-7 with SE values are indicated with blue and red bars, respectively.

FIGURE 8

Grain quality-related traits of GP9-7 harvested in the Tsukuba field in the fall of 2018. (A) A morphological comparison of typical hulled rice grains of Koshihikari and GP9-7. Scale bar: 2 mm. (B) Summary of rice texture and taste quality tests. Values for Koshihikari are indicated with blue circles with broken lines and values for GP9-7 are represented with red triangles with solid lines.