The expression of seed storage proteins is under tight developmental regulation and represents a powerful model system to study the regulation of gene expression during plant development. In this study, we show that three homologous B3 type transcription factors regulate the model storage protein gene, At2S3, via two distinct mechanisms: FUSCA3 (FUS3) and LEAFY COTYLEDON2 (LEC2) activate the At2S3 promoter in yeast suggesting that they regulate At2S3 by directly binding its promoter; ABSCISIC ACID INSENSITIVE3 (ABI3), however, appears to act more indirectly on At2S3, possibly as a cofactor in an activation complex. In accordance with this, FUS3 and LEC2 were found to act in a partially redundant manner and differently from ABI3 in planta: At2S3expression is reduced to variable and sometimes only moderate extent in fus3 and lec2 single mutants but is completely abolished in the lec2 fus3 double mutant. In addition, we found that FUS3and LEC2 expression patterns, together with an unsuspected regulation of FUS3 by LEC2, enable us to explain the intriguing expression pattern of At2S3 in lec2 or fus3 single mutants. Based on these results, we present a model of At2S3regulation and discuss its implications for other aspects of seed maturation.

In Arabidopsis thaliana, four major regulators (ABSCISIC ACID INSENSITIVE3 [ABI3], FUSCA3 [FUS3], LEAFY COTYLEDON1[LEC1], and LEC2) control most aspects of seed maturation, such as accumulation of storage compounds, cotyledon identity, acquisition of desiccation tolerance, and dormancy. The molecular basis for complex genetic interactions among these regulators is poorly understood. By analyzing ABI3 and FUS3 expression in various single, double, and triple maturation mutants, we have identified multiple regulatory links among all four genes. We found that one of the major roles of LEC2 was to upregulate FUS3 and ABI3. The lec2 mutation is responsible for a dramatic decrease in ABI3 and FUS3 expression, and most lec2 phenotypes can be rescued by ABI3 or FUS3 constitutive expression. In addition, ABI3 and FUS3 positively regulate themselves and each other, thereby forming feedback loops essential for their sustained and uniform expression in the embryo. Finally, LEC1 also positively regulates ABI3 and FUS3 in the cotyledons. Most of the genetic controls discovered were found to be local and redundant, explaining why they had previously been overlooked. This works establishes a genetic framework for seed maturation, organizing the key regulators of this process into a hierarchical network. In addition, it offers a molecular explanation for the puzzling variable features of lec2 mutant embryos.

Iron is an essential element for plant metabolism because of its redox properties. Its long distance and intracellular trafficking require specialized proteins and low molecular mass chelates because of its insolubility and toxicity in presence of oxygen. Iron deficiency induces various morphological and biochemical changes. They include root hair morphogenesis, differentiation of rhizoder-ma1 cells into transfer cells, yellowing of leaves and ultrastructural disorganisation of chloroplasts and mitochondria, as well as increased synthesis of organic acids and phenolics, and activation of root systems responsible for an enhanced iron uptake capacity. Upon iron resupply, these alterations disappeared within few days and a transient accumulation of the iron storage protein ferritin in the plastids is one of the early events in this process. Iron excess can also occur in plants where it elicits an oxidative stress leading to necrotic spots in the leaves. Induction of ferritin synthesis is again an early event of the plant response to this iron toxicity. Plant hormones such as auxin, abscisic acid and ethylene, as well as reactive oxygen intermediates play an important role in the transduction pathways, allowing plants to respond to these iron-deficiency and excess stresses. Similarities and differences among the various mechanisms responsible for iron uptake and storage in mammals, higher plants and yeast are outlined. Relationships between iron and copper metabolism are also indicated. plant / root / chloroplast / iron / ferritinIron traffk in non-stressed plants

The PII signal transducing protein is involved in carbon/nitrogen (C/N) sensing in bacteria and cyanobacteria. In higher plants the function of the PII homolog GLB1 is not known. GLB1 transcripts were found in all plant organs tested, while in Arabidopsis leaves GLB1 expression and PII protein levels were not significantly affected by either the day/night cycle or N-nutrition. Its putative regulatory role in plants has been studied by analysing Arabidopsis thaliana T-DNA insertion lines in the GLB1 gene. These PII mutants showed an 80% (PIIV1 mutant) and 100% (PIIS2 mutant) reduced AtGLB1 transcript level and no detectable PII protein. They did not display an altered growth or developmental phenotype when grown under non-limiting conditions suggesting that the PII protein does not play a crucial role in plants. However, in vitro grown PII mutants did show a higher sensitivity to nitrite (NO (2) (-) ) compared to the wild-type plants. This observation is reminiscent of the role of PII in the regulation of NO (2) (-) metabolism in cyanobacteria. Furthermore, when grown hydroponically, the PII mutants displayed a slight increase in carbohydrate (starch and sugars) levels in response to N starvation and a slight decrease in the levels of ammonium (NH (4) (+) ) and amino acids (mainly Gln) in response to NH (4) (+) resupply. Although the phenotypic changes are rather small in the mutant lines, these data support the hypothesis of a subtle involvement of the PII protein in the regulation of some steps of primary C and N metabolism.

Two pathways have been implicated in the regulation of maize ferritin synthesis in response to iron. One of them involves the plant hormone abscisic acid (ABA) and controls the expression of ZmFer2 gene(s). Another pathway, ABA-independent, has been characterized in a de-rooted maize plantlet system and involves an oxidative step. The ZmFer1 maize ferritin gene is not regulated by ABA, and it is shown in this paper that the corresponding mRNA accumulates in de-rooted maize plantlets and BMS (Black Mexican Sweet) maize cell suspension cultures in response to iron via the oxidative pathway described previously. To investigate ZmFer1 gene regulation further, the BMS cell system has been used to develop a transient expression assay using a ZmFer1-␤-glucuronidase fusion. Both iron induction and antioxidant inhibition of ZmFer1 gene expression were observed in this system. Using Northern blot analysis and transient expression experiments, it was shown that both okadaic acid and calyculin A, two serine/ threonine phosphatase inhibitors, specifically inhibit ZmFer1 gene expression. These data indicate that an okadaic acid-sensitive protein phosphatase activity is involved in the regulation of the ZmFer1 ferritin gene in maize cells, and this activity is required for iron-induced expression of this gene.