Post-transcriptional regulation of the expression of the flowering time gene FT in different light conditions

The shift from a vegetative to a reproductive phase is orchestrated by a number of genes including CONSTANS (CO), FLOWERING LOCUS T (FT), and SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1).Many plants, both perennial and annuals, including A. thaliana, initiate this transition in response to changes in day length. The light signal is perceived in the leaves and transmitted to the apex where it induces flowering and the FT mRNA has been found to be part of this signal. In order to study the regulation of FT, a heat inducible system has been used in this work. In this report I confirm that the flower initiation caused by activation of a Hsp::FT Hsp::GUS transgene requires light. Heat shock induction of GUS and FT suggests that there is no difference in the induction kinetics or the relative induction levels between the two constructs in light and dark, suggesting that the light conditions are not affecting the transcriptional regulation. Instead, my data suggest that FT expression in different light conditions might be controlled by a post-transcriptional regulation possibly including both FT mRNA stability and the efficiency of translation or stability of the FT protein. This regulation might contribute to the reduced efficiency of FT-induced flowering in darkness

The carbon bonus of organic nitrogen enhances nitrogen use efficiency of plants

The importance of organic nitrogen (N) for plant nutrition and productivity is increasingly being recognized. Here we show that it is not only the availability in the soil that matters, but also the effects on plant growth. The chemical form of N taken up, whether inorganic (such as nitrate) or organic (such as amino acids), may significantly influence plant shoot and root growth, and nitrogen use efficiency (NUE). We analysed these effects by synthesizing results from multiple laboratory experiments on small seedlings (Arabidopsis, poplar, pine and spruce) based on a tractable plant growth model. A key point is that the carbon cost of assimilating organic N into proteins is lower than that of inorganic N, mainly because of its carbon content. This carbon bonus makes it more beneficial for plants to take up organic than inorganic N, even when its availability to the roots is much lower - up to 70% lower for Arabidopsis seedlings. At equal growth rate, root:shoot ratio was up to three times higher and nitrogen productivity up to 20% higher for organic than inorganic N, which both are factors that may contribute to higher NUE in crop production

Organic nitrogen enhances nitrogen nutrition and early growth of Pinus sylvestris seedlings

Boreal trees are capable of taking up organic nitrogen (N) as effectively as inorganic N. Depending on the abundance of soil N forms, plants may adjust physiological and morphological traits to optimize N uptake. However, the link between these traits and N uptake in response to soil N sources is poorly understood. We examined Pinus sylvestris L. seedlings' biomass growth and allocation, transpiration and N uptake in response to additions of organic N (the amino acid arginine) or inorganic N (ammonium nitrate). We also monitored in situ soil N fluxes in the pots following an addition of N, using a microdialysis system. Supplying organic N resulted in a stable soil N flux, whereas the inorganic N resulted in a sharp increase of nitrate flux followed by a rapid decline, demonstrating a fluctuating N supply and a risk for loss of nitrate from the growth medium. Seedlings supplied with organic N achieved a greater biomass with a higher N content, thus reaching a higher N recovery compared with those supplied inorganic N. In spite of a higher N concentration in organic N seedlings, root-to-shoot ratio and transpiration per unit leaf area were similar to those of inorganic N seedlings. We conclude that enhanced seedlings' nutrition and growth under the organic N source may be attributed to a stable supply of N, owing to a strong retention rate in the soil medium

Organic nitrogen enhances nitrogen nutrition and early growth of Pinus sylvestris seedlings

Boreal trees are capable of taking up organic nitrogen (N) as effectively as inorganic N. Depending on the abundance of soil N forms, plants may adjust physiological and morphological traits to optimize N uptake. However, the link between these traits and N uptake in response to soil N sources is poorly understood. We examined Pinus sylvestris L. seedlings' biomass growth and allocation, transpiration and N uptake in response to additions of organic N (the amino acid arginine) or inorganic N (ammonium nitrate). We also monitored in situ soil N fluxes in the pots following an addition of N, using a microdialysis system. Supplying organic N resulted in a stable soil N flux, whereas the inorganic N resulted in a sharp increase of nitrate flux followed by a rapid decline, demonstrating a fluctuating N supply and a risk for loss of nitrate from the growth medium. Seedlings supplied with organic N achieved a greater biomass with a higher N content, thus reaching a higher N recovery compared with those supplied inorganic N. In spite of a higher N concentration in organic N seedlings, root-to-shoot ratio and transpiration per unit leaf area were similar to those of inorganic N seedlings. We conclude that enhanced seedlings' nutrition and growth under the organic N source may be attributed to a stable supply of N, owing to a strong retention rate in the soil medium.Peer reviewe

Patterns of Plant Biomass Partitioning Depend on Nitrogen Source

Nitrogen (N) availability is a strong determinant of plant biomass partitioning, but the role of different N sources in this process is unknown. Plants inhabiting low productivity ecosystems typically partition a large share of total biomass to belowground structures. In these systems, organic N may often dominate plant available N. With increasing productivity, plant biomass partitioning shifts to aboveground structures, along with a shift in available N to inorganic forms of N. We tested the hypothesis that the form of N taken up by plants is an important determinant of plant biomass partitioning by cultivating Arabidopsis thaliana on different N source mixtures. Plants grown on different N mixtures were similar in size, but those supplied with organic N displayed a significantly greater root fraction. 15N labelling suggested that, in this case, a larger share of absorbed organic N was retained in roots and split-root experiments suggested this may depend on a direct incorporation of absorbed amino acid N into roots. These results suggest the form of N acquired affects plant biomass partitioning and adds new information on the interaction between N and biomass partitioning in plants

Nitrogen nutrition and biomass distribution in conifers

The main objectives of the work presented in this thesis were to increase our understanding of how different chemical forms of nitrogen (N) affect the growth and biomass distribution of conifer seedlings and hence their establishment and performance in field. Growth studies of Scots pine (Pinus sylvestris (L.)), Norway spruce (Picea abies (L.) Karst.) and the model plant Arabidopsis (Arabidopsis thaliana, ecotype Col-0) showed that plants can grow at similar (or higher) rates on organic N sources to those on the inorganic N sources ammonium (NH4+) and nitrate (NO3-). Cultivation on arginine also improved the field performance of Norway spruce seedlings by increasing their current-year shoot growth, despite smaller initial shoot length. Moreover, plants supplied with organic N distributed a relatively larger proportion of their biomass to root structures than controls with similar total biomass and N contents grown on inorganic N sources. Detailed studies on Arabidopsis revealed that an increase in the root:shoot ratio coincided with high retention of organic N in the roots, implying that the site of assimilation may be of importance for the short-term distribution of biomass. Further, studies on Scots pine seedlings deprived of carbohydrates suggested that the uptake, reduction and assimilation of NO3- are highly dependent on recent photoassimilates and that use of organic N may have considerable energetic benefits for plants, especially under conditions that limit carbohydrate supplies. The results from the studies underlying this thesis highlight the potential role of organic N in the nutrition of conifer seedlings, the links between seedling nutrition, morphology and field performance, and effects of organic N on biomass distribution. They suggest that organic N may serve as an alternative or complement to inorganic N sources in seedling production, and may help attempts to improve seedling establishment in the field

The carbon bonus of organic nitrogen enhances nitrogen use efficiency of plants

The importance of organic nitrogen (N) for plant nutrition and productivity is increasingly being recognized. Here we show that it is not only the availability in the soil that matters, but also the effects on plant growth. The chemical form of N taken up, whether inorganic (such as nitrate) or organic (such as amino acids), may significantly influence plant shoot and root growth, and nitrogen use efficiency (NUE). We analysed these effects by synthesizing results from multiple laboratory experiments on small seedlings (Arabidopsis, poplar, pine and spruce) based on a tractable plant growth model. A key point is that the carbon cost of assimilating organic N into proteins is lower than that of inorganic N, mainly because of its carbon content. This carbon bonus makes it more beneficial for plants to take up organic than inorganic N, even when its availability to the roots is much lower - up to 70% lower for Arabidopsis seedlings. At equal growth rate, root:shoot ratio was up to three times higher and nitrogen productivity up to 20% higher for organic than inorganic N, which both are factors that may contribute to higher NUE in crop production

Biomass (mg plant⁻¹), N concentrations (mg g⁻¹) and root mass fraction of plants grown in sterile agar culture and with N supplied as either a mixture of 1.5 mM glutamine and 3 mM NO₃⁻ or 3 mM NH₄NO₃.

<p>Average values ± SE, n = 5. Different letters indicate differences at p≤0.05 between N treatments.</p

Biomass (a) and fraction of biomass in roots (b) of <i>Arabidopsis thaliana</i> grown on either NO₃⁻ or on NH₄NO₃ or on different combinations of glutamine and NO₃⁻.

<p>All media had a total N concentration of 6 mM. Plants were grown on sterile agar plates for 21 days. Bars represent average values ± SE, n = 8. Different lower-case letters indicate differences at p≤0.05 between N treatments.</p

Origin of root N, shoot N and plant N, in <i>Arabidopsis thaliana</i> plants grown on 3 mM NH₄NO₃ (a) or a mixture of 1.5 mM glutamine+3 mM NO₃⁻ (b).

<p>Fractions of N derived from individual N sources in the mixtures were calculated from N contents and rates of ¹⁵N abundance in plant parts. Plants were grown on sterile agar plates for 21 days. Bars represent average values ± SE, n = 5. Different lower-case and capital letters indicate differences at p≤0.05 between plant parts and between N sources, respectively.</p