Root size and architecture are important crop plant traits, as they determine access to water and soil nutrients. The plant hormone cytokinin is a negative regulator of root growth and branching. Here, we generated transgenic barley () plants with an enlarged root system by enhancing cytokinin degradation in roots to explore the potential of cytokinin modulations in improving root functions. This was achieved through root-specific expression of a gene. Enhanced biomass allocation to roots did not penalize shoot growth or seed yield, indicating that these plants were not source limited. In leaves of transgenic lines, the concentrations of several macroelements and microelements were increased, particularly those with low soil mobility (phosphorus, manganese, and zinc). Importantly, seeds contained up to 44% more zinc, which is beneficial for human nutrition. Transgenic lines also demonstrated dampened stress responses to long-term drought conditions, indicating lower drought sensitivity. Taken together, this work demonstrates that root engineering of cereals is a promising strategy to improve nutrient efficiency, biofortification, and drought tolerance.

SummaryThe recent identification of phage-type RNA polymerases encoded in the nuclear genome of higher plants has provided circumstantial evidence for functioning of these polymerases in the transcription of the mitochondrial and plastid genomes, as demonstrated by sequence analysis and in vitro import experiments. To determine the subcellular localization of the phage-type organellar RNA polymerases in planta, the putative transit peptides of the RNA polymerases RpoT;1 and RpoT;3 from Arabidopsis thaliana and RpoT from Chenopodium album were fused to the coding sequence of a green fluorescent protein (GFP). The constructs were used to stably transform A. thaliana. Transgenic plants were examined for green fluorescence with epifluorescence and confocal laser scanning microscopy. Plants expressing the GFP fusions under control of the CaMV35S promoter exhibited a distinct subcellular localization of the GFP fluorescence for each of the fusion constructs. In plants expressing GFP fusions with the putative transit peptides of ARAth;RpoT;1 and CHEal;RpoT, fluorescence was found exclusively in mitochondria, both in root and leaf cells. In contrast, GFP fluorescence in plants expressing the ARAth;RpoT;3-GFP construct accumulated in chloroplasts of leaf cells and nongreen plastids (leucoplasts) of root cells. By demonstrating targeting in planta, the data add substantial evidence for the phage-type RNA polymerases from C. album and A. thaliana to function in the transcriptional machinery of mitochondria and plastids.