Physiological and molecular effects of gibberellic acid and paclobutrazol on young clonal palm growth and development

Gibberellin (GA) and paclobutrazol (PBZ) are both plant hormones for stimulating and retardant of plant growth and development, respectively. The role of both hormones in oil palm growth and development is not thoroughly investigated. Therefore, this study focused on the physiological effects of gibberellic acid (GA3) or PBZ application on young clonal oil palm and its associated molecular responses. It also involved optimization of a tissue preparation procedure for cryosectioning in future transcriptome profiling of inflorescences. Sucrose treatment with snap freezing of fresh oil palm inflorescences provided cryosections with good cellular morphology and higher RNA yields and quality. On phytohormone application, GA3 and PBZ treatments were separately conducted on young clonal palms for 56 weeks. The GA3-treated palms were taller with longer fronds than the control but produced lower biomass. These physiological changes were associated with a higher expression of cell cycle-related genes such as MYB3R-1, E2FB, CK1d, CDK5RAP3 and SRO1 and cellulose synthase CSLC5 (five-fold) and CCR1 (three-fold) for lignin synthesis. In contrast, PBZ application likely blocked GA biosynthesis by suppressing expression of GA biosynthesis genes GA20ox and GA3ox. PBZ-treated palms were shorter in height and frond length with lower biomass. Higher expression of the growth suppressor RCC and two- to eight-fold lower expression of cytoskeletal motor genes actin 3 and kinesin 3, cellulose synthase CESA2, pectin activator WAK3 and pectin lyase-like suggested that PBZ may suppressed cell expansion in leaves by enhancing secondary cell wall rigidity in oil palm. Besides that, lower SPAD values together with two- to five-fold lower expression of chlorophyll synthesis genes UROD, ChlD, LIL3, PORA and CAO suggested that GA3 suppressed chlorophyll synthesis in GA3-treated palms. Conversely, three fold higher GGR expression in leaflets of PBZ-treated palms associating with higher SPAD values suggested that accumulated geranylgeranyl pyrophosphate (GGPP) due to suppression of GA synthesis might be channeled towards chlorophyll synthesis. On reproductive development, late flowering in PBZ-treated palms correlated with suppression of floral activator SOC1 while late flowering GA3-treated palms displayed elevated expression of flowering repressors LSD1 (three-fold) and GATA21 (two- to five-fold) and reduced expression of the flowering accelerator COL5 (three-fold). However, sex of inflorescences was not affected by these treatments. In conclusion, GA3 treatment of oil palm promoted height increment and frond elongation by promoting cell division and elongation but suppressed biomass production. On the contrary, growth retardation following PBZ treatment might be attributed to suppression of cell expansion. Both GA3 and PBZ treatments appeared to delay flowering but did not affect sex determination in young clonal oil palms.

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