Source to sink partitioning is altered by the expression of the transcription factor AtHB5

Abstract

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