The Arabidopsis transcription factors AtPHL1 and AtHB23 act together promoting carbohydrate transport from pedicel-silique nodes to seeds

Carbohydrates are produced in green tissues through photosynthesis and then transported to sink tissues. Carbon partitioning is a strategic process, fine regulated, involving specific sucrose transporters in each connecting tissue. Here we report that a screening of an Arabidopsis transcription factor (TF) library using the homeodomain-leucine zipper I member AtHB23 as bait, allowed identifying the TF AtPHL1 interacting with the former. An independent Y2H assay, and in planta by BiFC, confirmed such interaction. AtHB23 and AtPHL1 coexpressed in the pedicel-silique nodes and the funiculus. Mutant plants (phl1, and amiR23) showed a marked reduction of lipid content in seeds, although lipid composition did not change compared to the wild type. While protein and carbohydrate contents were not significantly different between mutants and control mature seeds, we observed a reduced carbohydrate content in mutant plants young siliques (7 days after pollination). Moreover, using a CFDA probe, we revealed an impaired transport to the seeds, and the gene encoding the carbohydrate transporters SWEET10 and SWEET11, usually expressed in connecting tissues, was repressed in the amiR23 and phl1 mutant plants. Altogether, the results indicated that AtHB23 and AtPHL1 act together, promoting sucrose transport, and the lack of any of them provoked a reduction in seeds lipid content.