Breeding Science 67: 181–190 (2017) doi:10.1270/jsbbs.16185
Research Paper
Genetic variation of root angle distribution in rice (Oryza sativa L.) seedlings Asami Tomita1), Tadashi Sato2), Yusaku Uga3), Mitsuhiro Obara4) and Yoshimichi Fukuta*5) 3) 4) 5) 1) 2)
Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan Graduate School of Agricultural Science, Tohoku University, Sendai, Miyagi 981-8555, Japan Institute of Crop Science, National Agriculture and Food Research Organization, Tsukuba, Ibaraki 305-8602, Japan Biological Resources Division, Japan International Research Center for Agricultural Sciences, Tsukuba, Ibaraki 305-8686, Japan Tropical Agriculture Research Front, Japan International Research Center for Agricultural Sciences, Ishigaki, Okinawa 907-0002, Japan
We developed a new method of using seedling trays to evaluate root angle distribution in rice (Oryza sativa. L), and found a wide genetic variation among cultivars. The seedling tray method can be used to evaluate in detail the growth angles of rice crown roots at the seedling stage by allocating nine scores (10° to 90°). Unlike basket methods, it can handle large plant populations over a short growth period (only 14 days). By using the method, we characterized the root angle distributions of 97 accessions into two cluster groups: A and B. The numbers of accessions in group A were limited, and these were categorized as shallow rooting types including soil-surface root. Group B included from shallow to deep rooting types; both included Indica and Japonica Group cultivars, lowland and upland cultivars, and landraces and improved types. No relationship between variation in root vertical angle and total root number was found. The variation in root angle distribution was not related to differentiation between the Japonica and Indica Groups, among ecosystems used for rice cultivation, or among degrees of genetic improvement. The new evaluation method and associated information on genetic variation of rice accessions will be useful in root architecture breeding of rice. Key Words: evaluation method, genetic variation, rice (Oryza sativa L.), root angle distribution, seedling tray.
Introduction
Root angle distribution, which is regulated by the growth angle of the crown roots, is one of the root morphological traits used to determine the area of soil over which root capture water and nutrients (Uga et al. 2015a). For example, deep rooting is useful for extracting more water from the soil and minimizing drought stress under upland and rainfed lowland conditions (Fukai and Cooper 1995, Uga et al. 2013a, Yoshida and Hasegawa 1982). In contrast, under waterlogged conditions, shallow rooting can help plants to absorb oxygen on the soil surface and helps avoid hypoxic conditions (Hanzawa et al. 2013, Ueno and Sato 1989). It is also useful for absorption of nutrients such as phosphorus that accumulate at the soil surface in untilled culture systems (Hanzawa et al. 2013, Uga et al. 2012). However, few studies have demonstrated genetic variations in root angle distribution and the adaptation of root distribution in rice plants in response to soil problems and different rice cultivation systems. The relationship between genetic variation in rice root angle distribution and diversity of ecosystems for rice cultivations will needs to be clarified if we establish breeding strategies for suitable cultivars adapted to various field conditions. Several studies have investigated genetic variations in
An Indica Group rice (Oryza sativa L.) cultivar, IR64, was developed at the International Rice Research Institute (IRRI) by introducing a semi-dwarf gene, sd-1, from the Chinese dwarf cultivar Dee Geo Woo Gen, and it has been cultivated widely in tropical and subtropical countries (Khush 1987). To improve yield potential in rice, ideotype traits such as shoot type (including plant height and panicle number) have been the main focus of study or modification in breeding programs (Khush 2013, Peng and Khush 2003, Peng et al. 1999, 2008). Recently, breeders have considered genetic improvement of root morphological traits to be an important challenge in crop yield production (de Dorlodot et al. 2007). The genetic and physiological mechanisms behind root morphological traits have been identified, but the effectiveness of these traits in rice breeding programs is unclear (Wu and Cheng 2014). Communicated by Toshio Yamamoto Received November 28, 2016. Accepted December 29, 2016. First Published Online in J-STAGE on May 10, 2017. *Corresponding author (e-mail: [email protected])
181
BS
Breeding Science Vol. 67 No. 3
Tomita, Sato, Uga, Obara and Fukuta
the root angle distribution of rice cultivars. Ueno and Sato (1989) evaluated the number of soil-surface roots in 56 rice cultivars grown under three conditions (submerged, submerged and aerated, and control) in paper pots. They found that the Indonesian Japonica Group landrace ecotype Bulu (ecotype classified by Ueno et al. 1990) formed soil-surface roots under all three conditions. Ueno and Sato (1992) examined the growth angles of four crown roots at 5 days after sowing on solidified agar in 130 rice cultivars by using a five-score scale [–90° (opposite direction to gravity), –45°, 0°, 45° and 90° (direction of gravity)]. They found a wide variation in the growth angles of crown roots in Indica Group cultivars, including Indian (Aus, Aman, and Boro) and Indonesian (Bulu and Tjereh) ecotypes and Japanese lowland and upland cultivars. Oyanagi et al. (1993) developed a method of measuring the growth angles of wheat cultivar roots emerging from a meshed hemispherical basket buried in the soil. Kato et al. (2006) introduced the basket method to investigate for the root distribution of rice cultivars and found genetic variation in the frequencies of higher root growth angle (>50°), defined as deeper roots, in 12 rice cultivars (9 Japonica cultivars, 2 Indica, and 1 Aus). On the basis of this information, Uga et al. (2009) investigated the ratios of deeper roots in 59 rice cultivars by using the basket method. They found no significant difference in deep rooting among one Japonica and two Indica Groups, as classified by using DNA polymorphism data. These studies focused on the frequencies of soil-surface or deeper roots in rice plants, and did not consider the whole distribution of roots in detail. Moreover, the relationship between genetic variation in root angle distribution and differentiation among Japonica and Indica Groups, and among ecosystems for rice cultivation (such as irrigated and rainfed lowland, upland, swampy, and deep water), have not been fully clarified. Hanzawa et al. (2013) used the basket method and classified the growth angles of crown roots on a four-score scale [
Research Paper
Genetic variation of root angle distribution in rice (Oryza sativa L.) seedlings Asami Tomita1), Tadashi Sato2), Yusaku Uga3), Mitsuhiro Obara4) and Yoshimichi Fukuta*5) 3) 4) 5) 1) 2)
Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan Graduate School of Agricultural Science, Tohoku University, Sendai, Miyagi 981-8555, Japan Institute of Crop Science, National Agriculture and Food Research Organization, Tsukuba, Ibaraki 305-8602, Japan Biological Resources Division, Japan International Research Center for Agricultural Sciences, Tsukuba, Ibaraki 305-8686, Japan Tropical Agriculture Research Front, Japan International Research Center for Agricultural Sciences, Ishigaki, Okinawa 907-0002, Japan
We developed a new method of using seedling trays to evaluate root angle distribution in rice (Oryza sativa. L), and found a wide genetic variation among cultivars. The seedling tray method can be used to evaluate in detail the growth angles of rice crown roots at the seedling stage by allocating nine scores (10° to 90°). Unlike basket methods, it can handle large plant populations over a short growth period (only 14 days). By using the method, we characterized the root angle distributions of 97 accessions into two cluster groups: A and B. The numbers of accessions in group A were limited, and these were categorized as shallow rooting types including soil-surface root. Group B included from shallow to deep rooting types; both included Indica and Japonica Group cultivars, lowland and upland cultivars, and landraces and improved types. No relationship between variation in root vertical angle and total root number was found. The variation in root angle distribution was not related to differentiation between the Japonica and Indica Groups, among ecosystems used for rice cultivation, or among degrees of genetic improvement. The new evaluation method and associated information on genetic variation of rice accessions will be useful in root architecture breeding of rice. Key Words: evaluation method, genetic variation, rice (Oryza sativa L.), root angle distribution, seedling tray.
Introduction
Root angle distribution, which is regulated by the growth angle of the crown roots, is one of the root morphological traits used to determine the area of soil over which root capture water and nutrients (Uga et al. 2015a). For example, deep rooting is useful for extracting more water from the soil and minimizing drought stress under upland and rainfed lowland conditions (Fukai and Cooper 1995, Uga et al. 2013a, Yoshida and Hasegawa 1982). In contrast, under waterlogged conditions, shallow rooting can help plants to absorb oxygen on the soil surface and helps avoid hypoxic conditions (Hanzawa et al. 2013, Ueno and Sato 1989). It is also useful for absorption of nutrients such as phosphorus that accumulate at the soil surface in untilled culture systems (Hanzawa et al. 2013, Uga et al. 2012). However, few studies have demonstrated genetic variations in root angle distribution and the adaptation of root distribution in rice plants in response to soil problems and different rice cultivation systems. The relationship between genetic variation in rice root angle distribution and diversity of ecosystems for rice cultivations will needs to be clarified if we establish breeding strategies for suitable cultivars adapted to various field conditions. Several studies have investigated genetic variations in
An Indica Group rice (Oryza sativa L.) cultivar, IR64, was developed at the International Rice Research Institute (IRRI) by introducing a semi-dwarf gene, sd-1, from the Chinese dwarf cultivar Dee Geo Woo Gen, and it has been cultivated widely in tropical and subtropical countries (Khush 1987). To improve yield potential in rice, ideotype traits such as shoot type (including plant height and panicle number) have been the main focus of study or modification in breeding programs (Khush 2013, Peng and Khush 2003, Peng et al. 1999, 2008). Recently, breeders have considered genetic improvement of root morphological traits to be an important challenge in crop yield production (de Dorlodot et al. 2007). The genetic and physiological mechanisms behind root morphological traits have been identified, but the effectiveness of these traits in rice breeding programs is unclear (Wu and Cheng 2014). Communicated by Toshio Yamamoto Received November 28, 2016. Accepted December 29, 2016. First Published Online in J-STAGE on May 10, 2017. *Corresponding author (e-mail: [email protected])
181
BS
Breeding Science Vol. 67 No. 3
Tomita, Sato, Uga, Obara and Fukuta
the root angle distribution of rice cultivars. Ueno and Sato (1989) evaluated the number of soil-surface roots in 56 rice cultivars grown under three conditions (submerged, submerged and aerated, and control) in paper pots. They found that the Indonesian Japonica Group landrace ecotype Bulu (ecotype classified by Ueno et al. 1990) formed soil-surface roots under all three conditions. Ueno and Sato (1992) examined the growth angles of four crown roots at 5 days after sowing on solidified agar in 130 rice cultivars by using a five-score scale [–90° (opposite direction to gravity), –45°, 0°, 45° and 90° (direction of gravity)]. They found a wide variation in the growth angles of crown roots in Indica Group cultivars, including Indian (Aus, Aman, and Boro) and Indonesian (Bulu and Tjereh) ecotypes and Japanese lowland and upland cultivars. Oyanagi et al. (1993) developed a method of measuring the growth angles of wheat cultivar roots emerging from a meshed hemispherical basket buried in the soil. Kato et al. (2006) introduced the basket method to investigate for the root distribution of rice cultivars and found genetic variation in the frequencies of higher root growth angle (>50°), defined as deeper roots, in 12 rice cultivars (9 Japonica cultivars, 2 Indica, and 1 Aus). On the basis of this information, Uga et al. (2009) investigated the ratios of deeper roots in 59 rice cultivars by using the basket method. They found no significant difference in deep rooting among one Japonica and two Indica Groups, as classified by using DNA polymorphism data. These studies focused on the frequencies of soil-surface or deeper roots in rice plants, and did not consider the whole distribution of roots in detail. Moreover, the relationship between genetic variation in root angle distribution and differentiation among Japonica and Indica Groups, and among ecosystems for rice cultivation (such as irrigated and rainfed lowland, upland, swampy, and deep water), have not been fully clarified. Hanzawa et al. (2013) used the basket method and classified the growth angles of crown roots on a four-score scale [
