Molecular mechanism of differential development and lipid accumulation between two components in the seed of tung tree (Vernicia fordii Hemsl.)

Lipid accumulation is regulated during seed development. However, the molecular link between the development of seed components and lipid accumulation remains largely unknown, especially in tung tree seeds. Tung tree (Vernicia fordii Hemsl.) is an oil-bearing woody plant that produces seeds rich in α-eleostearic acid. In this study, it was found that the tissue outside the cotyledon in the kernel of tung tree can be further divided into the inner and outer parts. The inner part was gradually enlarged to form the mature endosperm with a large lipid volume. The outer part turned into a dry, thin layer with little oil accumulation. Transcriptome data revealed that the inner part enlargement could be due to the rapid cell proliferation and the activation of the lipid biosynthesis pathway during the oil accumulation period. The outer part should function as the delivery of maternal nutrition to support the inner part expansion during seed development. Furthermore, a set of candidate transcription factors were identified that regulated different fates of the inner and outer parts during the oil accumulation period. The transcript repressor, VfVAL3 was highly expressed in the outer part but low level in the inner part. Conversely, VfVAL3 potential target genes in the LEC1/B3 network had higher expressions in the inner part. The Sph/RY motif recognized by the VAL protein was highly enriched in the promoters or introns of the LEC1/B3 network genes. However, the yeast one-hybrid assay showed that VAL3 didn’t bind to the promoter or intron sequences of the LEC1/B3 network genes. Overexpression of VfVAL3 reduced oil content in transgenic Arabidopsis. Therefore, VfVAL3 may block the development and lipid accumulation of the outer part by suppressing the LEC1/B3 network. This study provides essential information on understanding the molecular connection of different seed parts and lipid accumulation during seed development in tung tree.