Genome-Wide Identification and Functional Analysis of the AP2/ERF Transcription Factor Family in Citrus Rootstock under Waterlogging Stress.

Citrus plants are sensitive to waterlogging, and the roots are the first plant organ affected by hypoxic stress. The AP2/ERF (APETALA2/ethylene-responsive element binding factors) can modulate plant growth and development. However, the information on AP2/ERF genes in citrus rootstock and their involvement in waterlogging conditions is limited. Previously, a rootstock cultivar, Citrus junos cv. Pujiang Xiangcheng was found to be highly tolerant to waterlogging stress. In this study, a total of 119 AP2/ERF members were identified in the C. junos genome. Conserved motif and gene structure analyses indicated the evolutionary conservation of PjAP2/ERFs. Syntenic gene analysis revealed 22 collinearity pairs among the 119 PjAP2/ERFs. The expression profiles under waterlogging stress showed differential expression of PjAP2/ERFs, of which, PjERF13 was highly expressed in both root and leaf. Furthermore, the heterologous expression of PjERF13 significantly enhanced the tolerance of transgenic tobacco to waterlogging stress. The overexpression of PjERF13 decreased the oxidative damage in the transgenic plants by reducing the H2O2 and MDA contents and increasing the antioxidant enzyme activities in the root and leaf. Overall, the current study provided basic information on the AP2/ERF family in the citrus rootstock and uncovered their potential function in positively regulating the waterlogging stress response.

Comparative Transcriptomic Analysis of Inarching Invigorating Rootstock onto Incompatible Grafts in Citrus.

Grafting is a technique that is widely used in citrus production. Graft incompatibility often occurs in the orchard. Inarching can effectively improve the vigor of incompatible grafts, but its mechanism remains poorly understood. Our previous studies investigated the scion-rootstock interaction of citrus and highlighted the role of hormonal balance and genes in abscisic acid biosynthesis regulation. To further elucidate the mechanism of inarched grafts rejuvenation, Hm/Pt combination (Citrus maxima (Burm.) Merrill cv. 'Hongmian miyou' grafted onto Poncirus trifoliata) were inarched with 'Pujiang Xiangcheng' (a novel citrus rootstock cultivar recently selected from wild Citrus junos populations), and comprehensive analysis was performed to compare the inarched grafts and controls. Compared with incompatible grafts, the results revealed that inarching could recover the leaf metabolism balance, including reducing starch content, increasing chlorophyll content and restoring the cell structure. Additionally, our results corroborated that hormonal balance and hormone-related genes played a central role in inarching compatibility. Furthermore, the roles of Hsf4, ERF1, NCED3 and PYL were highlighted, and a model for explaining inarched grafts recovery invigoration was proposed. This study shed light on the mechanism of inarching regulation tree vigor and offered deep insights into the scion-rootstock interaction in citrus.

The Effects of a New Citrus Rootstock Citrus junos cv. Shuzhen No. 1 on Performances of Ten Hybrid Citrus Cultivars.

The importance of rootstock in citrus production lies in its crucial role in determining tree growth, environmental stress tolerance, and fruit quality. Citrus junos Siebold ex Tanaka cv. Shuzhen No. 1, a recently developed rootstock, demonstrates excellent graft compatibility and abiotic stress tolerance. The objective of this study was to assess ten hybrid citrus cultivars grafted onto two C. junos rootstock selections, with the aim of determining the potential for industrial utilization of the new citrus rootstock. All graft junctions are mature and well established. Vigorous growth characterized all ten citrus cultivars on Shuzhen No. 1, with the largest tree's height reaching 280.33 cm (Wogan scion) and the widest scion's diameter being 67.52 cm (Chunjian scion). However, the scion-to-rootstock diameter ratio was the lowest at 0.62 (Chunxiang scion). C. junos rootstock selections significantly affected fruit weight (five of ten scions) and fruit color (seven of ten scions) but had negligible impact on peel thickness (nine of ten scions). Furthermore, rootstock type had a significant influence on fruit quality. In conclusion, our findings indicate strong graft compatibility between all scions and C. junos rootstocks, which can impact overall size and fruit quality. Based on these results, Shuzhen No. 1 is recommended as a valuable citrus rootstock.

Transcriptome sequencing analyses uncover mechanisms of citrus rootstock seedlings under waterlogging stress.

Citrus plants are sensitive to waterlogging, which can cause yield reduction. Their production heavily depends on the rootstock being used for grafting of scion cultivars, and the rootstock is the first organ to be affected by waterlogging stress. However, the underlying molecular mechanisms of waterlogging stress tolerance remain elusive. In this study we investigated the stress response of two waterlogging-tolerant citrus varieties (Citrus junos Sieb ex Tanaka cv. Pujiang Xiangcheng and Ziyang Xiangcheng), and one waterlogging-sensitive variety (red tangerine) at the morphological, physiological, and genetic levels in leaf and root tissues of partially submerged plants. The results showed that waterlogging stress significantly decreased the SPAD value and root length but did not obviously affect the stem length and new root numbers. The malondialdehyde (MDA) content and the enzyme activities of superoxide dismutase (SOD), guaiacol peroxidase (POD), and catalase (CAT) were enhanced in the roots. The RNA-seq analysis revealed that the differentially expressed genes (DEGs) were mainly linked to 'cutin, suberine, and wax biosynthesis', 'diterpenoid biosynthesis', and 'glycerophospholipid metabolism' in the leaves, whereas were linked to 'flavonoid biosynthesis', 'biosynthesis of secondary metabolites and metabolic pathways' in the roots. Finally, we developed a working model based on our results to elucidate the molecular basis of waterlogging-responsive in citrus. Therefore, our data obtained in this study provided valuable genetic resources that will facilitate the breeding of citrus varieties with improved waterlogging tolerance.