Overexpression of ATG8 in Arabidopsis Stimulates Autophagic Activity and Increases Nitrogen Remobilization Efficiency and Grain Filling.

Autophagy knock-out mutants in maize and in Arabidopsis are impaired in nitrogen (N) recycling and exhibit reduced levels of N remobilization to their seeds. It is thus impoortant to determine whether higher autophagy activity could, conversely, improve N remobilization efficiency and seed protein content, and under what circumstances. As the autophagy machinery involves many genes amongst which 18 are important for the core machinery, the choice of which AUTOPHAGY (ATG) gene to manipulate to increase autophagy was examined. We choose ATG8 overexpression since it has been shown that this gene could increase autophagosome size and autophagic activity in yeast. The results we report here are original as they show for the first time that increasing ATG8 gene expression in plants increases autophagosome number and promotes autophagy activity. More importantly, our data demonstrate that, when cultivated under full nitrate conditions, known to repress N remobilization due to sufficient N uptake from the soil, N remobilization efficiency can nevertheless be sharply and significantly increased by overexpressing ATG8 genomic sequences under the control of the ubiquitin promoter. We show that overexpressors have improved seed N% and at the same time reduced N waste in their dry remains. In addition, we show that overexpressing ATG8 does not modify vegetative biomass or harvest index, and thus does not affect plant development.

A RNA-Seq Analysis of the Response of Photosynthetic System to Low Nitrogen Supply in Maize Leaf.

Nitrogen is a major limiting factor for crop productivity. The relationship between photosynthesis and nitrogen nutrition has been widely studied. However, the molecular response of leaf photosynthesis to low nitrogen supply in crops is less clear. In this study, RNA sequencing technology (RNA-Seq) was used to investigate the gene expressions related to photosynthesis in maize in response to low nitrogen supply. It was found that low nitrogen supply down-regulated the expression of genes involved in photosystem I (PSI) and photosystem II (PSII). Thus, low nitrogen supply down-regulated the expression of genes related to the antenna system, reduced light absorption, light transport, and electron transport. Correspondingly, the parameters related to chlorophyll fluorescence were very sensitive to nitrogen deficiency. Under low nitrogen supply, leaf chlorophyll content, actual quantum yield of PSII photochemistry, photochemical quenching, and electron transport rate, were reduced. However, the thermal diffusion and chlorophyll fluorescence were increased. RNA-Seq was used to analyze the genes involved in the response of leaf photosynthesis to low nitrogen supply in maize. These results highlight the possibility of utilizing chlorophyll fluorescence parameters, and the related genes, as indicators for plant nitrogen nutrition. This could lead to the development of new tools to make precise nitrogen fertilizer recommendations and select nitrogen-efficient genotypes.

Autophagy and Nutrients Management in Plants.

Nutrient recycling and mobilization from organ to organ all along the plant lifespan is essential for plant survival under changing environments. Nutrient remobilization to the seeds is also essential for good seed production. In this review, we summarize the recent advances made to understand how plants manage nutrient remobilization from senescing organs to sink tissues and what is the contribution of autophagy in this process. Plant engineering manipulating autophagy for better yield and plant tolerance to stresses will be presented.

Dynamic remobilization of leaf nitrogen components in relation to photosynthetic rate during grain filling in maize.

Remobilization of leaf nitrogen (N) contributes greatly to grain N in maize, but leads to low photosynthetic rate (Pn). Pn is determined by various N components involving in light harvest and CO2 reduction. However, it is less clear which N component is the major contributor for the reduction of photosynthesis in modern stay-green maize hybrids. In this study, we analyzed the relationship between remobilization of different N components and Pn during grain filling stage under low N (no N application) and high N (180 kg N ha-1) in a field experiment. The remobilization efficiency of photosynthetic enzymes (PEPc, PPDK and Rubisco) in the leaf was much higher than that of thylakoid N and other N components. Low N supply increased the remobilization efficiency of all the leaf N components. During grain filling stage, the amount of all the N components decreased together with Pn. The ratio of Pn to the N in the PEPc, PPDK and Rubisco kept increase in the whole grain filling stage, while the ratio of Pn to chlorophyll and thylakoid-N decreased. Correlation analysis indicated that Pn was more related to the content of photosynthetic enzymes than to chlorophyll and thylakoid N. It is concluded that photosynthetic enzymes serve as an N storage reservoir at early grain filling stage and their degradation is critical in the reduction of Pn during later grain filling stage. Future breeding targets may be focused on enhancing the efficiency of photosynthetic enzymes during late grain filling stage.

Regulation of nutrient recycling via autophagy.

Autophagy is a universal mechanism in eukaryotes that promotes cell longevity and nutrient recycling through the degradation of unwanted organelles, proteins and damaged cytoplasmic compounds. Autophagy is important in plant resistance to stresses and starvations and in remobilization. Autophagy facilitates bulk and selective degradations, through the delivery of cell material to the vacuole where hydrolases and proteases reside. Large metabolite modifications are observed in autophagy mutants showing the important role of autophagy in cell homeostasis. The control of autophagic activity by nutrients and energy status is supported by several studies in plant and animal. We review how autophagy contributes to nutrient management in plants and how nutrient status control this degradation pathway for adaptation to the environment.

Use of the Stable Nitrogen Isotope to Reveal the Source-Sink Regulation of Nitrogen Uptake and Remobilization during Grain Filling Phase in Maize.

Although the remobilization of vegetative nitrogen (N) and post-silking N both contribute to grain N in maize (Zea mays L.), their regulation by grain sink strength is poorly understood. Here we use 15N labeling to analyze the dynamic behaviors of both pre- and post-silking N in relation to source and sink manipulation in maize plants. The results showed that the remobilization of pre-silking N started immediately after silking and the remobilized pre-silking N had a greater contribution to grain N during early grain filling, with post-silking N importance increasing during the later filling stage. The amount of post-silking N uptake was largely driven by post-silking dry matter accumulation in both grain as well as vegetative organs. Prevention of pollination during silking had less effect on post-silking N uptake, as a consequence of compensatory growth of stems, husk + cob and roots. Also, leaves continuously export N even though grain sink was removed. The remobilization efficiency of N in the leaf and stem increased with increasing grain yield (hence N requirement). It is suggested that the remobilization of N in the leaf is controlled by sink strength but not the leaf per se. Enhancing post-silking N uptake rather than N remobilization is more likely to increase grain N accumulation.

Within-Leaf Nitrogen Allocation in Adaptation to Low Nitrogen Supply in Maize during Grain-Filling Stage.

Nitrogen (N) plays a vital role in photosynthesis and crop productivity. Maize plants may be able to increase physiological N utilization efficiency (NUtE) under low-N stress by increasing photosynthetic rate (P n) per unit leaf N, that is, photosynthetic N-use efficiency (PNUE). In this study, we analyzed the relationship between PNUE and N allocation in maize ear-leaves during the grain-filling stage under low N (no N application) and high N (180 kg N ha(-1)) in a 2-year field experiment. Under low N, grain yield decreased while NUtE increased. Low-N treatment reduced the specific N content of ear leaves by 38% without significant influencing P n, thereby increasing PNUE by 54%. Under low-N stress, maize plants tended to invest relatively more N into bioenergetics to sustain electron transport. In contrast, N allocated to chlorophyll and light-harvesting proteins was reduced to control excess electron production. Soluble proteins were reduced to shrink the N storage reservoir. We conclude that optimization of N allocation within leaves is a key adaptive mechanism to maximize P n and crop productivity when N is limited during the grain-filling stage in maize under low-N conditions.