Genome-wide identification and expression analysis of AUX/LAX family genes in Chinese hickory (Carya cathayensis Sarg.) Under various abiotic stresses and grafting.

Auxin is essential for regulating plant growth and development as well as the response of plants to abiotic stresses. AUX/LAX proteins are auxin influx transporters belonging to the amino acid permease family of proton-driven transporters, and are involved in the transport of indole-3-acetic acid (IAA). However, how AUX/LAX genes respond to abiotic stresses in Chinese hickory is less studied. For the first time identification, structural characteristics as well as gene expression analysis of the AUX/LAX gene family in Chinese hickory were conducted by using techniques of gene cloning and real-time fluorescent quantitative PCR. Eight CcAUX/LAXs were identified in Chinese hickory, all of which had the conserved structural characteristics of AUX/LAXs. CcAUX/LAXs were most closely related to their homologous proteins in Populus trichocarpa , which was in consistence with their common taxonomic character of woody trees. CcAUX/LAXs exhibited different expression profiles in different tissues, indicating their varying roles during growth and development. A number of light-, hormone-, and abiotic stress responsive cis-acting regulatory elements were detected on the promoters of CcAUX/LAX genes. CcAUX/LAX genes responded differently to drought and salt stress treatments to varying degrees. Furthermore, CcAUX/LAX genes exhibited complex expression changes during Chinese hickory grafting. These findings not only provide a valuable resource for further functional validation of CcAUX/LAXs, but also contribute to a better understanding of their potential regulatory functions during grafting and abiotic stress treatments in Chinese hickory.

Identification and expression analysis of auxin-responsive GH3 family genes in Chinese hickory (Carya cathayensis) during grafting.

The GH3 genes play vital roles in auxin homeostasis by conjugating excess auxin to amino acids. However, how GH3 genes function during grafting in Chinese hickory (Carya cathayensis) is largely unknown. Here, based on the transcriptome database, a comprehensive identification and expression profiling analysis of 12 GH3 genes in Chinese hickory were performed. Phylogenetic analysis indicated that CcGH3-x exists in a specific subfamily. To understand the roles of CcGH3 genes, tissue-specific expression and the response to different phytohormones were determined. Expression profiles of GH3 genes of Chinese hickory during grafting were analysed. The data suggested that 10 CcGH3 genes were down-regulated at an early stage of grafting, indicating that auxin homeostasis regulated by the CcGH3 family might be inhibited at initial stages. At the completion of grafting, expression levels of members of the CcGH3 family were restored to normal levels. Endogenous auxin levels were also measured, and the data showed that free auxin decreased to the lowest level at an early stage of grafting, and then increased during grafting. Auxin amino acid conjugation increased at an early stage of grafting in rootstock, and then decreased with progression of the graft union. Our results demonstrate that the reduced expression of CcGH3 family genes during grafting might contribute to the release of free auxin, making an important contribution to the recovery of auxin levels after grafting.

Melatonin regulates the functional components of photosynthesis, antioxidant system, gene expression, and metabolic pathways to induce drought resistance in grafted Carya cathayensis plants.

The Chinese hickory (Carya cathayensis) is an economically important tree species popular for its nuts. However, the tree requires a long time to reach the nut-producing phase. To overcome this problem, grafting is widely used to reduce the time from the vegetative to the reproductive phase. This tree species also faces many environmental challenges due to climate change; drought is an important factor affecting growth and development. Here, we designed an experiment to assess the protective efficiency of melatonin in grafted Chinese hickory plants under drought stress. The results revealed that exogenously applied melatonin successfully recovered the growth of grafted Chinese hickory plants and improved photosynthetic efficiency. Exogenously applied melatonin also boosted the antioxidative defense system of the plants under drought stress, resulting in enhanced reactive oxygen species (ROS) scavenging. The accumulation of compatible solutes such as total soluble sugars and proline was also triggered by melatonin. Moreover, the analyses using metabolomics revealed that drought-stressed plants treated with melatonin regulated key metabolic pathways such as phenylpropanoid, chlorophyll and carotenoid biosynthesis, carbon fixation, and sugar metabolism. To further validate the physiological, biochemical, and metabolomic factors, we studied the molecular mechanisms by analyzing the expression of key genes involved in chlorophyll metabolism (chlorophyllase, CHLASE), antioxidative defense (superoxide dismutase, SOD; catalase, CAT; ascorbate peroxidase, APX; peroxidase, POD), and phenylalanine ammonia-lyase (PAL). Exogenously applied melatonin significantly regulated the transcript levels of key genes involved in the biological processes mentioned above. Melatonin also showed crosstalk with other hormones (zeatin, gibberellin A14, 24-epibrassinolide, jasmonic acid, and abscisic acid) to regulate the physiological processes. The results of this study show that melatonin regulates biological processes at the metabolic and molecular levels to resist drought stress.

Identification and expression profiling of the Aux/IAA gene family in Chinese hickory (Carya cathayensis Sarg.) during the grafting process.

Auxin is an essential regulator in various aspects of organism growth and development. Members of the Aux/IAA family of genes encode short-lived nuclear proteins and mediate the responses of auxin-regulated gene expression. Here, the first identification and characterization of 22 cDNAs encoding the open reading frame of the Aux/IAA family in Chinese hickory (named as CcIAA) has been performed. The proteins encoded by these genes contain four whole or partially conserved domains of the Aux/IAA family. Phylogenetic analysis indicated that CcIAAs were unevenly distributed among eight different subgroups. The spatio-specific expression profiles showed that most of the CcIAAs preferentially expressed in specific tissues. Three CcIAA genes, including CcIAA11, CcIAA27a2 and CcIAAx, were predominantly expressed in stem. The predominant expression of CcIAA genes in stems might play important roles in vascular reconnection during the graft process. Furthermore, expression profiles of Aux/IAA genes during the grafting process of Chinese hickory have been analysed. Our data suggested that 19 CcIAAs were down-regulated and 3 CcIAAs (including CcIAA28, CcIAA8a and CcIAA27b) were induced, indicating their specializations during the grafting process. The involvement of CcIAA genes at the early stage after grafting gives us an opportunity to understand the role of auxin signalling in the grafting process.