Polyploid breeding is an effective approach to improve plant biomass and quality. Both fast growth and dwarf types of in vitro or ex vitro plants are produced after polyploidization. However, little is known regarding the dwarf type mechanism in
polyploids grown in vitro. In this study, the morphological and cytological characteristics were measured in
tetraploid and diploid hybrid sweetgum (Liquidambar styraciflua × L. formosana) with the same genetic background.
RNA sequencing (
RNA-Seq) was used to analyse shoot and root variations between
tetraploid and diploid plants; important metabolites were validated. The results showed that the shoot and root lengths were significantly shorter in
tetraploids than in diploids after 25 d of culture. Most
tetraploid root cells were wider and more irregular, and the length of the meristematic zone was shorter, while
tetraploid cells were significantly larger than diploid cells. Differentially expressed genes (DEGs) were significantly enriched in the plant growth and organ elongation pathways, such as
plant hormone biosynthesis and signal transduction,
sugar and
starch metabolism, and cell cycles.
Hormone biosynthesis and signal transduction genes, such as YUCCA, TAA1, GH3, SAUR, CPS, KO, KAO, GA20ox, GA3ox, BAS1 and CYCD3, which help to regulate organ elongation, were generally downregulated. The
auxin,
gibberellin, and
brassinolide (BL) contents in roots and stems were significantly lower in
tetraploids than in diploids, which may greatly contribute to slow growth in the roots and stems of
tetraploid regenerated plants. Exogenous
gibberellic acid (GA3) and
indole-3-acetic acid (IAA), which induced plant cell elongation, could significantly promote growth in the stems and roots of
tetraploids. In summary, comparative transcriptomics and metabolite analysis showed that the slow growth of regenerated
tetraploid hybrid sweetgum was strongly related to
auxin and
gibberellin deficiency. Our findings provide insights into the molecular mechanisms that underlie
dwarfism in allopolyploid hybrid sweetgum.