Source-to-sink partitioning is altered by changes in the expression of the transcription factor AtHB5 in Arabidopsis.

Carbohydrates are transported from source to sink tissues. The efficiency of this transport determines plant growth and development. The process is finely regulated and transcription factors are crucial in its modulation. AtHB5 is a homeodomain-leucine zipper I transcription factor that is repressed during stem maturation. However, its function in this developmental event is unknown. Here, we investigated the expression pattern and role of AtHB5. AtHB5 was expressed in roots, hypocotyls, stems, petioles, pedicels, and central leaf veins. athb5 mutant plants exhibited wider and more lignified stems than controls, whereas AtHB5 overexpressors showed the opposite phenotype. Cross sections of athb5 mutant stems showed enlarged vascular bundle, xylem, phloem, and petiole areas, whereas AtHB5 overexpressors had callose deposits. Several genes involved in starch biosynthesis and degradation had altered transcript levels in athb5 mutants and AtHB5 overexpressors. Rosette and stem biomass was enhanced in athb5 mutants, positively impacting seed yield, protein, and lipid content. Moreover, these effects were more evident in debranched plants. Finally, transport to roots was significantly slowed in AtHB5 overexpressors. Altogether, the results indicated that AtHB5 is a negative modulator of carbon partitioning and sucrose transport from source to sink tissues, and its overexpression diminished plant biomass and seed yield.