Plants are the principal source of iron in most diets, yet iron availability often limits plant growth. In response to iron deficiency, Arabidopsis roots induce the expression of the divalent cation transporter IRT1. Here, we present genetic evidence that IRT1 is essential for the uptake of iron from the soil. An Arabidopsis knockout mutant in IRT1 is chlorotic and has a severe growth defect in soil, leading to death. This defect is rescued by the exogenous application of iron. The mutant plants do not take up iron and fail to accumulate other divalent cations in low-iron conditions. IRT1-green fluorescent protein fusion, transiently expressed in culture cells, localized to the plasma membrane. We also show, through promoter:: ␤ -glucuronidase analysis and in situ hybridization, that IRT1 is expressed in the external cell layers of the root, specifically in response to iron starvation. These results clearly demonstrate that IRT1 is the major transporter responsible for high-affinity metal uptake under iron deficiency.

Source-to-sink transport of sugar is one of the major determinants of plant growth and relies on the efficient and controlled distribution of sucrose (and some other sugars such as raffinose and polyols) across plant organs through the phloem. However, sugar transport through the phloem can be affected by many environmental factors that alter source/sink relationships. In this paper, we summarize current knowledge about the phloem transport mechanisms and review the effects of several abiotic (water and salt stress, mineral deficiency, CO2, light, temperature, air, and soil pollutants) and biotic (mutualistic and pathogenic microbes, viruses, aphids, and parasitic plants) factors. Concerning abiotic constraints, alteration of the distribution of sugar among sinks is often reported, with some sinks as roots favored in case of mineral deficiency. Many of these constraints impair the transport function of the phloem but the exact mechanisms are far from being completely known. Phloem integrity can be disrupted (e.g., by callose deposition) and under certain conditions, phloem transport is affected, earlier than photosynthesis. Photosynthesis inhibition could result from the increase in sugar concentration due to phloem transport decrease. Biotic interactions (aphids, fungi, viruses…) also affect crop plant productivity. Recent breakthroughs have identified some of the sugar transporters involved in these interactions on the host and pathogen sides. The different data are discussed in relation to the phloem transport pathways. When possible, the link with current knowledge on the pathways at the molecular level will be highlighted.

The transition from a green, hard, and acidic pericarp to a sweet, soft, coloured, and sugar-rich ripe fruit occurs in many unrelated fruit species. High throughput identification of differentially expressed genes in grape berry has been achieved by the use of 50-mers oligoarrays bearing a set of 3,200 Unigenes from Vitis vinifera to compare berry transcriptome at nine developmental stages. Analysis of transcript profiles revealed that most activations were triggered simultaneously with softening, occurring within only 24 h for an individual berry, just before any change in colouration or water, sugar, and acid content can be detected. Although most dramatically induced genes belong to unknown functional categories, numerous changes occur in the expression of isogenes involved in primary and secondary metabolism during ripening. Focusing on isogenes potentially significant in development regulation (hormonal control of transcription factor) revealed a possible role for several hormones (cytokinin, gibberellin, or jasmonic acid). Transcription factor analysis revealed the induction of RAP2 and WRKY genes at véraison, suggesting increasing biotic and abiotic stress conditions during ripening. This observation was strengthened by an increased expression of multiple transcripts involved in sugar metabolism and also described as induced in other plant organs during stress conditions. This approach permitted the identification of new isogenes as possible control points: a glutathione S-transferase exhibits the same expression profile as anthocyanin accumulation and a new putative sugar transporter is induced in parallel with sugar import.

Plant growth and development has to be continuously adjusted to the available resources. Their optimization requires the integration of signals conveying the plant metabolic status, its hormonal balance, and its developmental stage. Many investigations have recently been conducted to provide insights into sugar signaling and its interplay with hormones and nitrogen in the fine-tuning of plant growth, development, and survival. The present review emphasizes the diversity of sugar signaling integrators, the main molecular and biochemical mechanisms related to the sugar-signaling dependent regulations, and to the regulatory hubs acting in the interplay of the sugar-hormone and sugar-nitrogen networks. It also contributes to compiling evidence likely to fill a few knowledge gaps, and raises new questions for the future.