Physiological Responses of Two Wheat Cultivars to Nitrogen Starvation

Plants need to be efficient in nutrient management, especially when they face the temporal nutrient defficiencies. Understanding how crops respond to nitrogen （N） starvation would help in the selection of crop cultivars more tolerant to N deficiency. In the present work, the physiological responses of two wheat cultivars, Yannong 19 （YN） and Qinmai 11 （QM）, to N starvation conditions were investigated. The two cultivars differed in biomass and N rearrangement between shoots and roots during N starvation. QM allocated more N to roots and exhibited higher root/shoot biomass ratio than YN. However, tissue measurement indicated that both cultivars had similar nitrate content in leaves and roots and similar remobilization rate in roots. Microelectrode measurement showed that vacuolar nitrate activity （concentration） in roots of QM was lower than that in roots of YN, especially in epidermal cells. Nitrate remobilization rates from root vacuoles of two cultivars were also identical. Moreover, vacuolar nitrate remobilization rate was proportional to vacuolar nitrate activity. During N starvation, nitrate reductase activity （NRA） was decreased but there were no significant differences between the two cultivars. Nitrate efflux from roots reduced after external N removal and QM seemed to have higher nitrate efflux rate.

HY5-HDA9 Module Transcriptionally Regulates Plant Autophagy in Response to Light-to-Dark Conversion and Nitrogen Starvation

Light is arguably one of the most important environmental factors that determines virtually all aspects of plant growth and development,but the molecular link between light signaling and the autophagy pathway has not been elucidated in plants.In this study,we demonstrate that autophagy is activated during light-to-dark conversion though transcriptional upregulation of autophagy-related genes(ATGs).We showed that depletion of the ELONGATED HYPOCOTYL 5(HY5),a key component of light signaling,leads to enhanced autophagy activity and resistance to extended darkness and nitrogen starvation treatments,contributing to higher expression oiATGs.HY5 interacts with and recruits HISTONE DEACETYLASE 9(HDA9)to ATG5 and ATG8e loci to repress their expression by deacetylation of the Lys9 and Lys27 of histone 3.Furthermore,we found that both darkness and nitrogen depletion induce the degradation of HY5 via 26S proteasome and the concomitant disassociation of HDA9 from ATG5 and ATG8e loci,leading to their depression and thereby activated autophagy.Genetic analysis further confirmed that HY5 and HDA9 act synergistically and function upstream of the autophagy pathway.Collectively,our study unveils a previously unknown transcriptional and epigenetic network that regulates autophagy in response to light-to-dark conversion and nitrogen starvation in plants.

Regulation of nitrogen starvation responses by the alarmone(p)ppGpp in rice

Nitrogen is an essential macronutrient for all living organisms and is critical for crop productivity and quality.In higher plants,inorganic nitrogen is absorbed through roots and then assimilated into amino acids by the highly conserved glutamine synthetase/glutamine:2-oxoglutarate aminotransferase(GS/GOGAT)cycle.How nitrogen metabolism and nitrogen starvation responses of plants are regulated remains largely unknown.Previous studies revealed that mutations in the rice ABNORMAL CYTOKININ RESPONSE1(ABC1)gene encoding Fd-GOGAT cause a typical nitrogen deficiency syndrome.Here,we show that ARE2(for ABC1 REPRESSOR2)is a key regulator of nitrogen starvation responses in rice.The are2 mutations partially rescue the nitrogen-deficient phenotype of abc1 and the are2 mutants show enhanced tolerance to nitrogen deficiency,suggesting that ARE2 genetically interacts with ABC1/Fd-GOGAT.ARE2 encodes a chloroplast-localized Rel A/Spo T homolog protein that catalyzes the hydrolysis of guanosine pentaphosphate or tetraphosphate(p)pp Gpp,an alarmone regulating the stringent response in bacteria under nutritional stress conditions.The are2 mutants accumulate excessive amounts of(p)pp Gpp,which correlate with lower levels of photosynthetic proteins and higher amino acid levels.Collectively,these observations suggest that the alarmone(p)pp Gpp mediates nitrogen stress responses and may constitute a highly conserved mechanism from bacteria to plants.

Nitrogen starvation induces genome-wide activation of transposable elements in Arabidopsis

Nitrogen(N)availability is a major limiting factor for plant growth and agricultural productivity.Although the gene regulation network in response to N starvation has been extensively studied,it remains unknown whether N starvation has an impact on the activity of transposable elements(TEs).Here,we report that TEs can be transcriptionally activated in Arabidopsis under N starvation conditions.Through genetic screening of idm1-14 suppressors,we cloned GLU1,which encodes a glutamate synthase that catalyzes the synthesis of glutamate in the primary N assimilation pathway.We found that glutamate synthase 1(GLU1)and its functional homologs GLU2 and glutamate transport 1(GLT1)are redundantly required for TE silencing,suggesting that N metabolism can regulate TE activity.Transcriptome and methylome analyses revealed that N starvation results in genome-wide TE activation without inducing obvious alteration of DNA methylation.Genetic analysis indicated that N starvationinduced TE activation is also independent of other well-established epigenetic mechanisms,including histone methylation and heterochromatin decondensation.Our results provide new insights into the regulation of TE activity under stressful environments in planta.

Submesoscale characteristics and transcription of a fatty acid elongase gene from a freshwater green microalgae, Myrmecia incisa Reisigl

Myrmecia incisa is a green coccoid freshwater microalgae, which is rich in arachidonic acid （ArA, C20： 4ω-6, △5, 8, 11, 14）, a long chain polyunsaturated fatty acid （PUFA）, especially under nitrogen starvation stress. A cDNA library of M. incisa was constructed with λ. phage vectors and a 545 nt expressed sequence tag （EST） was screened from this library as a putative elongase gene due to its 56% and 49% identity to Marchantia polymorpha L. and Ostreococcus tauri Courties et Chretiennot-Dinet, respectively. Based upon this EST sequence, an elongase gene designated MiFAE was isolated from M. incisa via 5＇/3＇ rapid amplification of cDNA ends （RACE）. The cDNA sequence was 1 331 bp long and included a 33 bp 5＇-untranslated region （UTR） and a 431 bp 3＇-UTR with a typical poly-A tail. The 867 bp ORF encoded a predicted protein of 288 amino acids. This protein was characterized by a conserved histidine-rich box and a MYxYY motif that was present in other members of the elongase family. The genomic DNA sequence of MiFAE was found to be interrupted by three introns with splicing sites of Introns I （81 bp）, II （81 bp）, and III （67 bp） that conformed to the GT-AG rule. Quantitative real-time PCR showed that the transcription level of MiFAE in this microalga under nitrogen starvation was higher than that under normal condition. Prior to the ArA content accumulation, the transcription of MiFAE was enhanced, suggesting that it was possibly responsible for the ArA accumulation in this microalga cultured under nitrogen starvation conditions.

Melatonin activates the OsbZIP79–OsABI5 module that orchestrates nitrogen and ROS homeostasis to alleviate nitrogen-limitation stress in rice

Melatonin(Mel)has previously been reported to effectively alleviate nitrogen-limitation(N-L)stress and thus increase nitrogen-use efficiency(NUE)in several plants,but the underlying mechanism remains obscure.Here,we revealed that OsbZIP79(BASIC LEUCINE ZIPPER 79)is transcriptionally activated under N-L conditions,and its expression is further enhanced by exogenous Mel.By the combined use of omics,genetics,and biological techniques,we revealed that the OsbZIP79–OsABI5(ABSCISIC ACID INSENSITIVE 5)module stimulated regulation of reactive oxygen species(ROS)homeostasis and the uptake and metabolismof nitrogen under conditions of indoor nitrogen limitation(1/16 normal level).OsbZIP79 activated the transcription of OsABI5,and OsABI5 then bound to the promoters of target genes,including genes involved in ROS homeostasis and nitrogen metabolism,activating their transcription.This module was also indispensable for upregulation of several other genes involved in abscisic acid catabolism,nitrogen uptake,and assimilation under N-L and Mel treatment,although these genes were not directly transactivated by OsABI5.Field experiments demonstrated that Mel significantly improved rice growth under low nitrogen(L-N,half the normal level)by the same mechanism revealed in the nitrogen-limitation study.Mel application produced a 28.6%yield increase under L-N and thus similar increases in NUE.Also,two OsbZIP79-overexpression lines grown in L-N field plots had significantly higher NUE(+13.7%and+21.2%)than their wild types.Together,our data show that an OsbZIP79–OsABI5 module regulates the rice response to N insufficiency(N limitation or low N),which is important for increasing NUE in rice production.

Sodium Acetate Stimulates PHB Biosynthesis in Synechocystis sp. PCC 6803

Synechocystis sp. PCC 6803 cell growth and poly β hydroxybutyrate (PHB) biosynthesis were studied in the presence of sodium acetate (NaAc). For nitrogen sufficient conditions, 15 mmol/L NaAc improved the PHB content up to 9.9% (w/w) while for nitrogen starved conditions, the PHB content was up to 15.2% (w/w). NaAc at levels below 20 mmol/L promoted cell growth in the first six days, but the growth slowed on the seventh day when the NaAc concentration exceeded 15 mmol/L. The PHB content in the final biomass reached 11.0% of the dry cellular weight in the presence of 20 mmol/L NaAc. Two adjacent open reading frames (ORFs) in the genome of Synechocystis sp. PCC 6803, slr1993 and slr1994, were assigned to phbA and phbB, respectively, while the phbC gene was found to be far from these genes. This may account for the low expression of PHB in cyanobacteria.