Yusaku Uga

Rice Applied Genomics Research Unit

［Abstract］

The gene that promotes a deep root system, DEEPER ROOTING 1 (DRO1), was identified in rice. Introducing DRO1 into a shallow-rooting rice cultivar by backcrossing enabled the resulting line to avoid drought by increasing deep rooting and maintaining a high yield performance even under drought conditions as compared to the recipient cultivar.

［Keywords］

drought avoidance, grain yield, gravitropism, quantitative trait locus, root growth angle

［Background］

Drought is the most serious abiotic stress that limits crop production under rainfed conditions. The enhancement of drought resistance in crops is becoming an important strategy to overcome the global food problem. Deep rooting may help plants to avoid drought stress by extracting water from deep soil layers. However, gene for deep rooting has not been identified so far. This study demonstrates that gene for deep rooting improves drought avoidance through the cloning and characterization of DRO1, a rice quantitative trait locus controlling root growth angle.

［Results and Discussion］

1. DRO1 was isolated by using a mapping population derived from a cross between shallow-rooting variety IR64 and deep-rooting variety Kinandang Patong (KP). DRO1 was involved in cell elongation in the root tip that causes asymmetric root growth as the root bends in response to gravity. KP had functional allele of DRO1, whereas IR64 had non-functional allele of DRO1, resulting in less downward bending of the root in response to gravity compared to KP (Fig. 1).
   
2. A near-isogenic line homozygous for the KP allele of DRO1 in the IR64 genetic background (Dro1-NIL) was developed (Fig. 2A). Dro1-NIL increased root growth angle and a maximum root depth more than twice of IR64 under upland conditions (Fig. 2B).
   
3. The grain yields between IR64 and Dro1-NIL under upland fields with no-, moderate-, and severe-drought conditions was analyzed at the International Center for Tropical Agriculture (CIAT). The physiological damage such as leaf wilting and delayed flowering was more prominent in IR64 than in Dro1-NIL under severe drought (Fig. 3A). Moderate drought significantly reduced the grain weight per plant in IR64, whereas Dro1-NIL had almost the same grain weight per plant (Fig. 3B). Under severe drought, the percentage of filled grain in IR64 was nearly zero, whereas that in Dro1-NIL was >30%. In the no-drought plots, both lines had similar yields.
   
4. There was a positive relationship between DRO1 expression and deep rooting, demonstrating that enhancement of DRO1 expression can increase the root growth angle, resulting in deeper rooting.

1. We are evaluating Dro1-NIL under rainfed paddy fields in cooperation with the International Rice Research Institute (IRRI) and CIAT to validate its utilization as breeding material for drought avoidance in the fields.
   
2. Genetic improvement for deep rooting would be expected to increase not only grain yield and logging resistance under irrigated paddy fields but also drought avoidance. The DRO1 homologs in other economically important monocots such as maize, barley, and sorghum may be useful for enhancing drought avoidance in cereal crops.

Fig.1. Effect of DRO1 on root gravitropic curvature. Gravitropic curvature in seminal roots of IR64 and Dro1-NIL at 4 h after rotation from the original vertical axis. θrac, root angle of curvature after rotation. Arrows indicate the direction of gravitational force.	Fig.2. Characterization of the genotype and vertical root distribution of IR64 and Dro1-NIL. (A) Graphical genotype with the homozygous regions from IR64 represented by blue bars and the homozygous region from KP indicated in red. (B) Roots of each line were obtained from an upland field using a monolith sampler (30 cm × 30 cm × 5 cm).

Fig.3. Effect of DRO1 on drought stress. (A) Responses of IR64 and Dro1-NIL plants to 27 days of severe drought stress. (B) Grain weight for IR64 and Dro1-NIL grown under conditions of no-drought, moderate and severe drought stress. Data are shown as mean ± s.d.

 

[Reference]

1. Uga Y, Sugimoto K, Ogawa S, Rane J, Ishitani M, Hara N, Kitomi Y, Inukai Y, Ono K, Kanno N, Inoue H, Takehisa H, Motoyama R, Nagamura Y, Wu J, Matsumoto T, Takai T, Okuno K, Yano M (2013) Control of root system architecture by DEEPER ROOTING 1 increases rice yield under drought conditions. Nature Genetics 45(9): 1097-1102
   
2. International publication #WO2011/078308