A natural mutation in the promoter of the aconitase gene ZjACO3 influences fruit citric acid content in jujube.

Jujube (Ziziphus jujuba Mill.) is the most economically important fruit tree of the Rhamnaceae and was domesticated from wild or sour jujube (Z. jujuba Mill. var. spinosa Hu). During the process of domestication, there was a substantial reduction in the content of organic acids, particularly malate and citrate, which greatly influence the taste and nutritional value of the fruit. We previously demonstrated that ZjALMT4 is crucial for malate accumulation. However, the mechanism of citrate degradation in jujube remains poorly understood. In the present study, aconitase ZjACO3 was shown to participate in citric acid degradation in the cytoplasm through the GABA pathway. Interestingly, we discovered an E-box mutation in the ZjACO3 promoter (-484A > G; CAAGTG in sour jujube mutated to CAGGTG in cultivated jujube) that was strongly correlated with fruit citrate content; 'A' represented a high-citrate genotype and 'G' represented a low-citrate genotype. We developed and validated an ACO-based Kompetitive allele-specific PCR (KASP) marker for determining citric acid content. Yeast one-hybrid screening, transient dual-luciferase assays, and overexpression analyses showed that the transcription factor ZjbHLH113 protein directly binds to CAGGTG in the promoter of ZjACO3 in cultivated jujube plants, transcriptionally activating ZjACO3 expression, and enhancing citric acid degradation. Conversely, binding ability of the ZjbHLH113 protein to CAAGTG was weakened in sour jujube, thereby promoting citrate accumulation in the fruit. These findings will assist in elucidating the mechanism by which ZjACO3 modulates citrate accumulation in sour jujube and its cultivars.

Low-acidity ALUMINUM-DEPENDENT MALATE TRANSPORTER4 genotype determines malate content in cultivated jujube.

Jujube (Ziziphus jujuba Mill.), the most economically important fruit tree in Rhamnaceae, was domesticated from sour jujube (Z. jujuba Mill. var. spinosa (Bunge) Hu ex H.F.Chow.). During domestication, fruit sweetness increased and acidity decreased. Reduction in organic acid content is crucial for the increase in sweetness of jujube fruit. In this study, the determination of malate content among 46 sour jujube and 35 cultivated jujube accessions revealed that malate content varied widely in sour jujube (0.90-13.31 mg g-1) but to a lesser extent in cultivated jujube (0.33-2.81 mg g-1). Transcriptome sequencing analysis showed that the expression level of Aluminum-Dependent Malate Transporter 4 (ZjALMT4) was substantially higher in sour jujube than in jujube. Correlation analysis of mRNA abundance and fruit malate content and transient gene overexpression showed that ZjALMT4 participates in malate accumulation. Further sequencing analyses revealed that three genotypes of the W-box in the promoter of ZjALMT4 in sour jujube associated with malate content were detected, and the genotype associated with low malate content was fixed in jujube. Yeast one-hybrid screening showed that ZjWRKY7 binds to the W-box region of the high-acidity genotype in sour jujube, whereas the binding ability was weakened in jujube. Transient dual-luciferase and overexpression analyses showed that ZjWRKY7 directly binds to the promoter of ZjALMT4, activating its transcription, and thereby promoting malate accumulation. These findings provide insights into the mechanism by which ZjALMT4 modulates malate accumulation in sour jujube and jujube. The results are of theoretical and practical importance for the exploitation and domestication of germplasm resources.

The regulation of plant lignin biosynthesis under boron deficiency conditions.

Boron (B) is a required micronutrient that is crucial for the growth and development of vascular plants. A deficiency in B is generally regarded as a limiting factor affecting agricultural production in many parts of the world. Boron is involved in the metabolism of plant lignin and additionally, B deficiency can lead to the excessive accumulation of lignin in plant leaves/roots, resulting in corking symptoms and inhibited growth. However, the effect of B on lignin biosynthesis is not as well characterized as the specific function of B in the cell wall. In this article, recent studies on the regulation of lignin biosynthesis in plants under low-B stress conditions are reviewed. Moreover, the following possible mechanisms underlying the lignin synthesis promoted by B deficiency are discussed: (1) the accumulation of phenolic substances during B deficiency directly enhances lignin synthesis; (2) excess H2 O2 has a dual function to the enhancement of lignin under boron deficiency conditions, serving as a substrate and a signaling molecule; and (3) B deficiency regulates lignin synthesis through the expression of genes encoding transcription factors such as MYBs. Finally, future studies regarding physiology, molecules, and transcriptional regulation may reveal the mechanism(s) mediating the relationship between lignin synthesis and B deficiency. This review provides new insights and important references for future research and the enhancement of plant B nutrition.

The mechanism analysis of exogenous melatonin in limiting pear fruit aroma decrease under low temperature storage.

Exogenous melatonin (MT) is widely used in fruit preservation, and can increase the storage time and delay the quality deterioration. Firstly, it was found that 150 µM MT was the optimal concentration to treat 'Xinli No.7' under storage at 4 °C for 60 days. MT could significantly improve oxidase activity and inhibit the reduction of physiological indexes, including pulp hardness, weight loss, titratable acid and soluble solid content. MT could also reduce ethylene release and limit the reduction of fruit aroma. The average content of fruit aroma substance increased by 43.53%. A relevant RNA-Seq database was built to further explore the regulation mechanism of MT. A total of 2,761 differentially expressed genes (DEGs) were identified. DEGs were enriched in 64 functional groups and 191 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. DEGs were mainly enriched in alpha-linolenic acid metabolism, fatty acid metabolism and plant hormone signal transduction pathway. The gene pycom09g05270 belonging to long chain acyl-CoA synthetase family and participating in fatty acid metabolism pathway was identified, and its expression level was consistent with fragments per kilobase per million mapped reads (FPKM) values, implying that pycom09g05270 might play a vital role in maintaining quality during the storage process.

Different metabolite profile and metabolic pathway with leaves and roots in response to boron deficiency at the initial stage of citrus rootstock growth.

Boron (B) is a microelement required for higher plants, and B deficiency has serious negative effect on metabolic processes. We concentrated on the changes in metabolite profiles of trifoliate orange leaves and roots as a consequence of B deficiency at the initial stage of growth by gas chromatography-mass spectrometry (GC-MS)-based metabolomics. Enlargement and browning of root tips were observed in B-deficient plants, while any obvious symptom was not recorded in the leaves after 30 days of B deprivation. The distinct patterns of alterations in metabolites observed in leaves and roots due to B deficiency suggest the presence of specific organ responses to B starvation. The accumulation of soluble sugars was occurred in leaves, which may be attributed to down-regulated pentose phosphate pathway (PPP) and amino acid biosynthesis under B deficiency, while the amount of most amino acids in roots was increased, indicating that the effects of B deficiency on amino acids metabolism in trifoliate orange may be a consequence of disruptions in root tissues and decreased protein biosynthesis. Several important products of shikimate pathway were also significantly affected by B deficiency, which may be related to abnormal growth of roots induced by B deficiency. Conclusively, our results revealed a global perspective of the discriminative metabolism responses appearing between B-deprived leaves and roots and provided new insight into the relationship between B deficiency symptom in roots and the altered amino acids profiling and shikimate pathway induced by B deficiency during seedling establishment.

Metabolic profiling reveals altered pattern of central metabolism in navel orange plants as a result of boron deficiency.

We focused on the changes of metabolite profiles in navel orange plants under long-term boron (B) deficiency using a gas chromatography-mass spectrometry (GC-MS) approach. Curling of the leaves and leaf chlorosis were observed only in the upper leaves (present before start of the treatment) of B-deficient plants, while the lower leaves (grown during treatment) did not show any visible symptoms. The metabolites with up-accumulation in B-deficient leaves were mainly proline, l-ornithine, lysine, glucoheptonic acid, fucose, fumarate, oxalate, quinate, myo-inositol and allo-inositol, while the metabolites with down-accumulation in B-deficient leaves were mainly serine, asparagine, saccharic acid, citrate, succinate, shikimate and phytol. The levels of glucose and fructose were increased only in the upper leaves by B deficiency, while starch content was increased in all the leaves and in roots. The increased levels of malate, ribitol, gluconic acid and glyceric acid occurred only in the lower leaves of B-deficient plants. The increased levels of phenols only in the upper leaves indicated that the effects of B on phenol metabolism in citrus plants may be a consequence of disruptions in leaf structure. Metabolites with opposite reactions in upper and lower leaves were mainly glutamine, glycine and pyrrole-2-carboxylic acid. To our knowledge, the phenomena of allo-inositol even higher than myo-inositol occurred characterized for the first time in this species. These results suggested that the altered pattern of central metabolism may be either specific or adaptive responses of navel orange plants to B deficiency.