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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.

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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, 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) but to a lesser extent in cultivated jujube (0.33–2.81 mg/g). 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.

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 H2O2 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.