The Roles of Strigoalctones in the Control of Rice Tillering

Strigolactones (SLs) are a group of terpenoid lactones that were first known for their function in rhizosphere symbiotic and parasitic interactions. Recently, SLs have been identified as plant hormones to inhibit axillary bud outgrowth in diverse plant species. Deficiencies in SL production and perception lead to excessive outgrowth of axillary buds arising from leaf axils. Studies with branching mutants, more axillary growth (max) in Arabidopsis, dwarf and tillering (d mutants) in rice, ramosus (rms) in pea and decreased apical dominance (dad) in petunia, have identified key players in SL biosynthesis and signaling, indicating the largely conserved mechanisms underlying the SL action in regulating axillary bud outgrowth.

Rice tillers are specialized branches bearing grains. As a result, tiller number and tiller angle are both key agronomic traits for achieving ideal plant architecture and increasing grain yield. We have identified a series of tillering mutants, which are defective in tiller number and tiller angle, and investigated the molecular basis of rice tillering. We found that SLs are not only involved in regulating tiller number, but also involved in tiller angle. Through screening for suppressors of lazy1 (la1), a classic rice mutant exhibiting large tiller angle and defective shoot gravitropism, we identified multiple suppressors of LA1 that are components involved in the SL biosynthetic or signaling pathway. Our study revealed that SLs can inhibit auxin biosynthesis and attenuate rice shoot gravitropism mainly through decreasing the local IAA content. We also verified that the SL-mediated shoot gravitropic mechanism is also conserved between dicot and monocot plants.