Autophagy is a common physiological process in organisms, including higher plants. The ATG8 subfamily, the core member of the autophagy-related gene (ATG) family, plays a key role in plant growth and development and nutrient stress responses. However, the core ATG8 homologs and their roles in stress resistance remain elusive in allotetraploid rapeseed (AACC, Brassica napus L.). In this study, we identified 29 ATG8 subgroup members, consisting of three phylogenetic clades, based on the analysis of genomic annotation and conserved motifs. Differential transcriptional responses of BnaATG8s to salt stress, nitrogen limitation, and other nutrient stresses were investigated, and we identified BnaA8.ATG8F as the core ATG8 member through gene co-expression network analysis. Decreased BnaA8.ATG8F expression repressed the salt tolerance of transgenic rapeseed plants by significantly reducing the root Na+ retention under salt stress. Moreover, downregulation of BnaA8.ATG8F increased nitrogen (N) limitation sensitivity of transgenic rapeseed plants through decreasing N uptake, translocation, and enhancing N remobilization under nitrogen starvation. In summary, we identified the core ATG8 homologs and characterized their physiological and molecular mechanisms underlying salt stress tolerance and nitrogen limitation adaptation. Our results may provide elite genetic resources for the genetic improvement of nutrient stress tolerance in rapeseed.

Wheat plants are ubiquitously simultaneously exposed to salinity and limited iron availability caused by soil saline‐alkalisation. Through this study, we found that both low Fe and NaCl severely inhibited the growth of seminal roots in wheat seedlings; however, sufficient Fe caused greater growth cessation of seminal roots than low Fe under salt stress. Low Fe improved the root meristematic division activity, not altering the mature cell sizes compared with sufficient Fe under salt stress. Foliar Fe spray and split‐root experiments showed that low Fe‐alleviating the salinity‐induced growth cessation of seminal roots was dependent on local low Fe signals in the roots. Ionomics combined with TEM/X‐ray few differences in the root Na+ uptake and vacuolar Na+ sequestration between two Fe levels under salt stress. Phytohormone profiling and metabolomics revealed salinity‐induced overaccumulation of ACC/ethylene and tryptophan/auxin in the roots under sufficient Fe than under low Fe. Differential gene expression, pharmacological inhibitor addition and the root growth performance of transgenic wheat plants revealed that the rootward auxin efflux and was responsible for the low Fe‐mediated amelioration of the salinity‐induced growth cessation of seminal roots. Our findings will provide novel insights into the modulation of crop root growth under salt stress.

The GARP genes are plant-specific transcription factors (TFs) and play key roles in regulating plant development and abiotic stress resistance. However, few systematic analyses of GARPs have been reported in allotetraploid rapeseed (Brassica napus L.) yet. In the present study, a total of 146 BnaGARP members were identified from the rapeseed genome based on the sequence signature. The BnaGARP TFs were divided into five subfamilies: ARR, GLK, NIGT1/HRS1/HHO, KAN, and PHL subfamilies, and the members within the same subfamilies shared similar exon-intron structures and conserved motif configuration. Analyses of the Ka/Ks ratios indicated that the GARP family principally underwent purifying selection. Several cis-acting regulatory elements, essential for plant growth and diverse biotic and abiotic stresses, were identified in the promoter regions of BnaGARPs. Further, 29 putative miRNAs were identified to be targeting BnaGARPs. Differential expression of BnaGARPs under low nitrate, ammonium toxicity, limited phosphate, deficient boron, salt stress, and cadmium toxicity conditions indicated their potential involvement in diverse nutrient stress responses. Notably, BnaA9.HHO1 and BnaA1.HHO5 were simultaneously transcriptionally responsive to these nutrient stresses in both hoots and roots, which indicated that BnaA9.HHO1 and BnaA1.HHO5 might play a core role in regulating rapeseed resistance to nutrient stresses. Therefore, this study would enrich our understanding of molecular characteristics of the rapeseed GARPs and will provide valuable candidate genes for further in-depth study of the GARP-mediated nutrient stress resistance in rapeseed.

Plant roots acquire nitrogen predominantly as two inorganic forms, nitrate (NO3-) and ammonium (NH4+), to which plants respond differentially. Rapeseed (Brassica napus L.) is an important oil-crop species with very low nitrogen-use efficiency (NUE), the regulatory mechanism of which was elusive due to vastness and complexity of the rapeseed genome. In this study, a comparative transcriptomic analysis was performed to investigate the differential signatures of nitrogen-starved rapeseed in responses to NO3- and NH4+ treatments and to identify the key genes regulating rapeseed NUE. The two nitrogen sources differentially affected the shoot and root transcriptome profiles, including those of genome-wide nitrogen transporter and transcription factor (TF)-related genes. Differential expression profiling showed that BnaA6.NRT2;1 and BnaA7.AMT1;3 might be the core transporters responsible for efficient NO3- and NH4+ uptake, respectively; the TF genes responsive to inorganic nitrogen, specifically responding to NO3-, and specifically responsive to NH4+ were also identified. The genes which were commonly and most significantly affected by both NO3- and NH4+ treatments were related to glutamine metabolism. Among the glutamine synthetase (GS) family genes, we found that BnaA2.Gln1;4, significantly responsive to low-nitrogen conditions, showed higher transcription abundance and GS activity in the leaf veins, flower sepals, root cortex and stele, silique petiole, and stem tissues. These characters were significantly different from those of AtGln1;4. The heterologous overexpression of BnaA2.Gln1;4 in Arabidopsis increased plant biomass, NUE, GS activity, and total amino acid concentrations under both sufficient- and low-nitrogen conditions. Overall, this study provided novel information about the genes involved in the adaptation to different nitrogen regimes and identified some promising candidate genes for enhancing NUE in rapeseed.

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