Worldwide, citrus is one of the most important fruit crops and is grown in more than 130 countries, predominantly in tropical and subtropical areas. The healthy progress of the citrus industry has been seriously affected by biotic and abiotic stresses. Several diseases, such as canker and huanglongbing, etc., rigorously affect citrus plant growth, fruit quality, and yield. Genetic engineering technologies, such as genetic transformation and genome editing, represent successful and attractive approaches for developing disease-resistant crops. These genetic engineering technologies have been widely used to develop citrus disease-resistant varieties against canker, huanglongbing, and many other fungal and viral diseases. Recently, clustered regularly interspaced short palindromic repeats (CRISPR)-based systems have made genome editing an indispensable genetic manipulation tool that has been applied to many crops, including citrus. The improved CRISPR systems, such as CRISPR/CRISPR-associated protein (Cas)9 and CRISPR/Cpf1 systems, can provide a promising new corridor for generating citrus varieties that are resistant to different pathogens. The advances in biotechnological tools and the complete genome sequence of several citrus species will undoubtedly improve the breeding for citrus disease resistance with a much greater degree of precision. Here, we attempt to summarize the recent successful progress that has been achieved in the effective application of genetic engineering and genome editing technologies to obtain citrus disease-resistant (bacterial, fungal, and virus) crops. Furthermore, we also discuss the opportunities and challenges of genetic engineering and genome editing technologies for citrus disease resistance.

Seed plant ACC synthases widely have C _β -S lyase activity, suggesting that they may be evolved from C _β -S lyases.

Senescence is the final phase of leaf development, characterized by key processes by which resources trapped in deteriorating leaves are degraded and recycled to sustain the growth of newly formed organs. As the gaseous hormone ethylene exerts a profound effect on the progression of leaf senescence, both the optimal timing and amount of its biosynthesis are essential for controlled leaf development. The rate-limiting enzyme that controls ethylene synthesis in higher plants is ACC synthase (ACS). In this study, we evaluated the production of ethylene and revealed an up-regulation of ACS7 during leaf senescence in Arabidopsis. We further showed that the promoter activity of ACS7 was maintained at a relatively high level throughout the whole rosette development process. However, the accumulation level of ACS7 protein was extremely low in the light-grown young seedlings, and it was gradually restored as plants aging. We previously demonstrated that degradation of ACS7 is regulated by its first 14 N-terminal residues, here we compared the phenotypes of transgenic Arabidopsis overexpressing a truncated ACS7 lacking the 14 residues with transgenic plants overexpressing the full-length protein. Results showed that seedlings overexpressing the truncated ACS7 exhibited a senescence phenotype much earlier than their counterparts overexpressing the full-length gene. Fusion of the 14 residues to SSPP, a PP2C-type senescence-suppressed protein phosphatase, effectively rescued the SSPP-induced suppression of rosette growth and development but had no effect on the delayed senescence. This observation further supported that N-terminus-mediated degradation of ACS7 is negatively regulated by leaf senescence signaling. All results of this study therefore suggest that ACS7 is one of the major contributors to the synthesis of ‘senescence ethylene’. And more importantly, the N-terminal 14 residue-mediated degradation of this protein is highly regulated by senescence signaling to enable plants to produce the appropriate levels of ethylene required.

Protected cultivation is a cultivation practice that plays an important role in improving crop quality. Aroma is an important flavour that assesses the quality of yuzu. In this study, C. junos cv. ‘Kitou’ grown in open fields (CJKTF) and plastic greenhouses (CJKTP) were selected as the study material. Significant differences in aroma performance between CJKTF and CJKTP were found by the olfactory senses of the members of this research group and an electronic nose, with CJKTP having a stronger aroma. Regarding VOCs, GC-MS analyses revealed 13 VOCs up-regulated and 28 VOCs down-regulated in CJKTP compared to CJKTF. Transcriptome analysis revealed that 515 genes were up-regulated and 720 genes were down-regulated in CJKTP compared to CJKTF. The differential VOCs nerolidol and γ-cadinene, and the differential genes nerolidol synthase 1 (NES1), nerolidol synthase 1-like (NES1-like), and cadinene synthase (DCS), were in the sesquiterpene synthesis pathway and showed significant correlation. NES1, NES1-like, and DCS encode terpene synthases, which may be involved in the biosynthetic pathway of nerolidol and γ-cadinene. In conclusion, the use of plastic greenhouses for cultivation may improve the quality and aroma intensity of yuzu, as well as alter the expression of related genes, compared to field cultivation. These results suggest that protected cultivation is a suitable cultivation practice to enhance the aroma of yuzu.

Satsuma mandarin (Citrus reticulata “Unshiu”) is a global cultivar with superior fruit characteristics and ranking among the top citrus cultivars in terms of production. It is also a key contributor to citrus breeding. However, the lack of high-quality genome makes the origin of Satsuma mandarin has long been a matter of debate. Here, we assembled a gap-free, high-quality genome of Satsuma mandarin. Meanwhile, we collected and sequenced 15 indigenous citrus varieties in Zhejiang Province, 12 Satsuma mandarins, 21 citrus hybrids related to Satsuma mandarin, 10 modern citrus varieties, and 7 other mandarins. Through high-resolution genome analysis, we inferred that Satsuma mandarin originated from a cross between C. reticulata “Ruju” × C. reticulata “Bendiguang” and proposed that Satsuma mandarin most probably originated in East area in Zhejiang Province of China, where the two parents-like cultivars are still found in a sympatric region to date. These results provide new insights into the origin model of Satsuma mandarin. The spread of mandarin is also discussed, which probably associated with the culture exchange and trade activities between Japan and China from Tang Dynasty and afterwards.