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.

The AtHB1 Transcription Factor Controls the miR164-CUC2 Regulatory Node to Modulate Leaf Development.

The presence of small tooth-like indentations, or serrations, characterizes leaf margins of Arabidopsis thaliana plants. The NAC family member CUP-SHAPED COTYLEDON 2 (CUC2), which undergoes post-transcriptional gene silencing by three micro-RNA genes (MIR164A, B and C), controls the extension of leaf serration. Here, we analyzed the role of AtHB1, a transcription factor (TF) belonging to the homeodomain-leucine zipper subfamily I, in shaping leaf margins. Using mutants with an impaired silencing pathway as background, we obtained transgenic plants expressing AtHB1 over 100 times compared to controls. These plants presented an atypical developmental phenotype characterized by leaves with deep serration. Transcript measurements revealed that CUC2 expression was induced in plants overexpressing AtHB1 and repressed in athb1 mutants, indicating a positive regulation exerted by this TF. Moreover, molecular analyses of AtHB1 overexpressing and mutant plants revealed that AtHB1 represses MIR164 transcription. We found that overexpression of MIR164B was able to reverse the serration phenotype of plants overexpressing AtHB1. Finally, chromatin immunoprecipitation assays revealed that AtHB1 was able to bind in vivo the promoter regions of all three MIR164 encoding loci. Altogether, our results indicate that AtHB1 directly represses MIR164 expression to enhance leaf serration by increasing CUC2 levels.

A matter of quantity: Common features in the drought response of transgenic plants overexpressing HD-Zip I transcription factors.

Plant responses to water deficit involve complex molecular mechanisms in which transcription factors have key roles. Previous reports ectopically overexpressed a few members of the homeodomain-leucine zipper I (HD-Zip I) family of transcription factors from different species, and the obtained transgenic plants exhibited drought tolerance which extent depended on the level of overexpression, triggering diverse molecular and physiological pathways. Here we show that most HD-Zip I genes are regulated by drought in the vegetative and/or reproductive stages. Moreover, uncharacterized members of this family were expressed as transgenes both in Col-0 and rdr6-12 backgrounds and were able to enhance drought tolerance in host plants. The extent of such tolerance depended on the expression level of the transgene and was significantly higher in transgenic rdr6-12 than in Col-0. Comparative transcriptome analyses of Arabidopsis thaliana plants overexpressing HD-Zip I proteins indicated that many members have common targets. Moreover, the water deficit tolerance exhibited by these plants is likely due to the induction and repression of certain of these common HD-Zip I-regulated genes. However, each HD-Zip I member regulates other pathways, which, in some cases, generate differential and potentially undesirable traits in addition to drought tolerance. In conclusion, only a few members of this family could become valuable tools to improve drought-tolerance.