AFB1 and 5 act maternally outside the seed to promote seed growth in Arabidopsis

Abstract

The plant hormone auxin in maternal tissues plays a major role in promoting seed growth. However, whether auxin affects seed growth in maternal tissues in or outside the seed is unclear. In the process of investigating the roles of the SKP1-CULLIN-F- BOX (SCF) ubiquitin ligases in Arabidopsis reproductive development, using protein extracts from inflorescence tissues, we found that, among the six auxin receptors, only AUXIN SIGNALING F-BOX1 (AFB1) and 5 (AFB5) were consistently co- immunoprecipitated with the ARABIDOPSIS SKP1-LIKE1 protein. This result suggests that AFB1 and 5 play major roles in reproductive development. We then found that seeds from afb1 and 5 mutants exhibited reduced sizes (weights). The largest reduction in seed weight occurred in afb5-6; the average weight/1000 dry seeds of afb5-6 was approximately 76% of that of the wild type. The seed weights were further investigated in AFB1 and AFB5 transgenic plants with the transgenes driven by their respective promoters or the ASK1 promoter. The results from the T₂ seeds and homozygous T₃ and T₄ seeds suggest that the wild-type levels of the two transcripts are not limiting for seed growth and farther increasing their levels adversely affects seed growth. Histochemical studies of transgenic plants harboring the AFB1:GUS or AFB5:GUS transgene revealed that AFB1 and AFB5 are not expressed in the seed but in the vascular tissue in the fruit wall and the funiculus. Taken together, these results show that AFB1 and AFB5 promote seed growth in maternal tissues outside the seed.