Exogenous Paclobutrazol Promotes Tiller Initiation in Rice Seedlings by Enhancing Sucrose Translocation

The inhibition of low-position tillering in machine-transplanted seedlings affects rice yields. Paclobutrazol (PBZ) is a plant growth regulator that can improve seedling quality and promote low-position tillering in machine-transplanted seedlings. However, the physiological mechanisms underlying the promotion of tiller bud formation induced by exogenous PBZ via sucrose transport remain unclear. Thus, rice cultivar ‘Yongyou 12’ was used to analyze the effects of different seeding rates and the application of exogenous PBZ, gibberellin (GA3), and water (control) on sucrose transport and metabolism as well as tiller bud development. Exogenous PBZ application combined with a low seeding rate significantly increased the number of tillers as well as seedling fullness (by 42.35%). Increases were also detected for the seedling cytokinin content, chlorophyll content (by 10.55%), and sucrose transport from leaves to the stem base. These changes were associated with the upregulated expression of sucrose transporter genes in leaves and the stem base, as well as increased activities of key sucrose-metabolizing enzymes in the stem base. Notably, the opposite trend was observed after exogenous GA3 was applied or a high seeding rate was used. Hence, a low seeding rate combined with exogenous PBZ application is useful for controlling seedling height, promoting the formation of low-position tillering, facilitate sucrose translocation from leaves to the stem base, and increasing sucrose metabolism in the basal part of rice plants. These findings provide a theoretical basis for optimizing low-position tillering in machine-transplanted seedlings.