copy
Plant architecture is defined as the three-dimensional organization of the plant. For the aerial part, this includes plant height, branching/tillering pattern, foliar arrangement and morphology, and reproductive organ structure. Plant architecture is a trait of major agronomic importance as it has a strong effect on harvest index and grain yield potential (
Reinhardt and Kuhlemeier, 2002). In the field, crops are generally grown at high planting density and with high nitrogen input, two factors that promote stem elongation and lodging. To ensure high yield and avoid lodging under these conditions, cereal crops with semidwarf and/or erect leaf phenotypes are highly desired (
Van Camp, 2005). Semidwarf varieties of rice (
Oryza sativa) and wheat (
Triticum aestivum) with enhanced yield and resistance to lodging are at the basis of the green revolution (
Athwal, 1971). Green biomass is another important trait, especially in energy crops (
Xie and Peng, 2011).
BR-deficient and BR-insensitive mutants of
Arabidopsis are generally dwarfed with shorter petioles and hypocotyls. By contrast, BR catabolic mutants and transgenic plants overexpressing the BR biosynthetic genes or positive regulators of BR signaling generally display increased growth and elongated organ phenotypes, including larger rosettes/taller plants with elongated leaves and longer petioles (
Figure 1; see
Supplemental Table 1 online). BR-deficient and BR-signaling mutants of other dicotyledonous (dicot) plant species, such as pea (
Pisum sativum) and tomato (
Solanum lycopersicum; formerly
Lycopersicon esculentum) mutants of the
BRASSINOSTEROID INSENSITIVE1 (
BRI1) gene encoding the BR receptor, also present a dwarf phenotype (
Bishop and Koncz, 2002) (
Figure 1; see
Supplemental Table 2 online). Similar phenotypes are observed in monocotyledonous (monocot) species. The shortened hypocotyl and leaf petiole of BR-deficient or BR-insensitive
Arabidopsis mutants are mirrored in the respective rice mutants by shortened internodes and more erect leaves due to a reduced lamina joint inclination (e.g.,
Arabidopsis bri1 compared with
O. sativa bri1 [
Osbri1/d61] mutants) (
Bishop and Koncz, 2002;
Nakamura et al., 2006). Conversely, the elongated organ phenotype of BR mutant/transgenic plants in
Arabidopsis translates in mutant/transgenic rice plants with increased leaf bending (e.g., the dominant
Arabidopsis suppressor of phyb-4 7 [
sob7]-D versus the rice
bending lamina2 [
bla2] mutant phenotype) (
Turk et al., 2005;
Park et al., 2006). Leaf angle is an important trait in cereal crops because it allows higher density planting and therefore can have a major impact on biomass and grain yield per hectare (
Sakamoto et al., 2006). The molecular and cellular mechanisms by which BRs regulate lamina joint inclination remain unclear. Reduced leaf angle in the rice BR-deficient and BR-insensitive mutants is caused by an elongation failure in the abaxial lamina joint cells. This feature has been used as a marker in screens for BR-related mutants with mild phenotypes (
Hong et al., 2004). Differential expression of one or several component(s) of the BR pathways may explain why some tissues are more sensitive than others to changes in BR levels and responses.