Genome-Wide Characterization, Expression Profile Analysis of WRKY Family Genes in Santalum album and Functional Identification of Their Role in Abiotic Stress.

WRKY proteins are a large superfamily of transcription factors that are involved in diverse biological processes including development, as well as biotic and abiotic stress responses in plants. WRKY family proteins have been extensively characterized and analyzed in many plant species, including Arabidopsis, rice, and poplar. However, knowledge on WRKY transcription factors in Santalum album is scarce. Based on S. album genome and transcriptome data, 64 SaWRKY genes were identified in this study. A phylogenetic analysis based on the structures of WRKY protein sequences divided these genes into three major groups (I, II, III) together with WRKY protein sequences from Arabidopsis. Tissue-specific expression patterns showed that 37 SaWRKY genes were expressed in at least one of five tissues (leaves, roots, heartwood, sapwood, or the transition zone), while the remaining four genes weakly expressed in all of these tissues. Analysis of the expression profiles of the 42 SaWRKY genes after callus was initiated by salicylic acid (SA) and methyl jasmonate (MeJA) revealed that 25 and 24 SaWRKY genes, respectively, were significantly induced. The function of SaWRKY1, which was significantly up-regulated by SA and MeJA, was analyzed. SaWRKY1 was localized in the nucleus and its overexpression improved salt tolerance in transgenic Arabidopsis. Our study provides important information to further identify the functions of SaWRKY genes and to understand the roles of SaWRKY family genes involved in the development and in SA- and MeJA-mediated stress responses.

Identification and functional characterization of three new terpene synthase genes involved in chemical defense and abiotic stresses in Santalum album.

BACKGROUND: It is well known that aromatic essential oils extracted from the heartwood of Santalum album L. have wide economic value. However, little is known about the role of terpenoids in response to various adverse environmental stresses as other plants do in the form of signals during plant-environment interactions. RESULTS: In this study, trace amounts of volatiles consisting of α-santalene, epi-ß-santalene, ß-santalene, α-santalol, ß-santalol, (E)-α-bergamotene, (E)-ß-farnesene and ß-bisabolene were found in the leaves of mature S. album trees. We identified more than 40 candidate terpene synthase (TPS) unigenes by mining publicly-available RNA-seq data and characterized the enzymes encoded by three cDNAs: one mono-TPS catalyzes the formation of mostly α-terpineol, and two multifunctional sesqui-TPSs, one of which produces (E)-α-bergamotene and sesquisabinene as major products and another which catalyzes the formation of (E)-ß-farnesene, (E)-nerolidol and (E,E)-farnesol as main products. Metabolite signatures and gene expression studies confirmed that santalol content is closely related with santalene synthase (SaSSY) transcripts in heartwood, which is key enzyme responsible for santalol biosynthesis. However, the expression of three new SaTPS genes differed significantly from SaSSY in the essential oil-producing heartwood. Increased activities of antioxidant enzymes, superoxide dismutase, catalase, peroxidase and ascorbate peroxidase, were detected in different tissues of S. album plants after applying 1 mM methyl jasmonate (MeJA) and 1 mM salicylic acid (SA), or exposure to 4°C, 38°C and high light intensity. MeJA and SA dramatically induced the expression of SaTPS1 and SaTPS2 in leaves. SaTPS1 to 3 transcripts were differentially activated among different tissues under adverse temperature and light stresses. In contrast, almost all SaSSY transcripts decreased in response to these environmental stresses, unlike SaTPS1 to 3. CONCLUSIONS: Multifunctional enzymes were biochemically characterized, including one chloroplastic mono-TPS and two cytosolic sesqui-TPSs in sandalwood. Our results suggest the ecological importance of these three new SaTPS genes in defensive response to biotic attack and abiotic stresses in S. album.

Sandalwood: basic biology, tissue culture, and genetic transformation.

MAIN CONCLUSION: Sustainable resource preservation of Santalum species that yield commercially important forest products is needed. This review provides an understanding of their basic biology, propagation, hemi-parasitic nature, reproductive biology, and biotechnology. Many species of the genus Santalum (Santalaceae) have been exploited unremittingly for centuries, resulting in the extinction of one and the threatened status of three other species. This reduction in biodiversity of sandalwood has resulted from the commercial exploitation of its oil-rich fragrant heartwood. In a bid to conserve the remaining germplasm, biotechnology provides a feasible, and effective, means of propagating members of this genus. This review provides a detailed understanding of the biological mechanisms underlying the success or failure of traditional propagation, including a synopsis of the process of hemi-parasitism in S. album, and of the suitability of host plants to sustain the growth of seedlings and plants under forestry production. For the mass production of economically important metabolites, and to improve uniformity of essential oils, the use of clonal material of similar genetic background for cultivation is important. This review summarizes traditional methods of sandalwood production with complementary and more advanced in vitro technologies to provide a basis for researchers, conservationists and industry to implement sustainable programs of research and development for this revered genus.

Host-species-dependent physiological characteristics of hemiparasite Santalum album in association with N2-fixing and non-N2-fixing hosts native to southern China.

Understanding the interactions between the hemiparasite Santalum album L. and its hosts has theoretical and practical significance in sandalwood plantations. In a pot study, we tested the effects of two non-N2-fixing (Bischofia polycarpa (Levl.) Airy Shaw and Dracontomelon duperreranum Pierre) and two N2-fixing hosts (Acacia confusa Merr. and Dalbergia odorifera T. Chen) on the growth characteristics and nitrogen (N) nutrition of S. album. Biomass production of shoot, root and haustoria, N and total amino acid were significantly greater in S. album grown with the two N2-fixing hosts. Foliage and root δ(15)N values of S. album were significantly lower when grown with N2-fixing than with non-N2-fixing hosts. Significantly higher photosynthetic rates and ABA (abscisic acid) concentrations were seen in S. album grown with D. odorifera. Similarity in the proportional amounts of amino acid of root xylem sap between S. album and its host D. odorifera was also evident, suggesting major access to nitrogenous solutes from D. odorifera to S. album. Irrespective of host species, S. album clearly appeared to optimize xylem sap extraction from its hosts by higher transpiration and lower water-use efficiency than its host. The growth of two non-N2-fixing hosts parasitized by S. album was significantly greater than the equivalent values for unparasitized treatments, and lower growth and photosynthesis were observed for parasitized A. confusa, and significant decreases in root N, photosynthesis and transpiration for parasitized D. odorifera compared with unparasitized treatments. Furthermore, foliage ABA concentrations were significantly higher in all hosts parasitized by S. album than in their unparasitized counterparts. Our study is probably the first to report on host dependence and preference in the hemiparasite S. album, and the generated results may have important implications for understanding of the physiological interactions between host species and parasitic plants, and for successfully mixing plantations of S. album with D. odorifera.

Ontogenesis of the collapsed layer during haustorium development in the root hemi-parasite Santalum album Linn.

The structure and development of collapsed layers of the haustorium were studied in Santalum album Linn. Through light and transmission electron microscopy, it was shown that the collapsed layers originated from starch-containing cells when the haustorium developed an internal gland, thickened gradually and ultimately developed into the mantle, which, combined with the sucker, buckled the host root. We report on the presence of inter-collapsed layers for the first time. These layers develop after penetration into the host and are located between the intrusive tissues and the vascular meristematic region, gradually linking the collapsed layers and remains around the sucker. The proliferation of cells in the meristematic region and the 'host tropism' of cortical layers contribute to pressure within the haustorium and result in development of the collapsed layers. Besides, starch-containing cells that turn into collapsed layers are vulnerable to pressure as they lack a large vacuole, have uneven cell wall thickness and a loose cell arrangement. We proposed that the functions of collapsed layers are to efficiently assure that cell inclusion and energy concentrate at the inner meristematic region and are recycled to affect penetration, reinforce the physical connection between the sandalwood haustorium and host root, and supply space for haustorial development.