In recent years, a considerable number of studies have harnessed the power of genomics to decipher the role of pathogenesis-related (PR) proteins in plant defence against various biotic and abiotic stresses. Chitinases are PR antifungal proteins expressed constitutively at low levels in plants and induced during biotic pressures and are demonstrated to be involved in the plant defence responses. Remarkable induction of chitinase enzymes by various abiotic agents (salicylic acid, jasmonic acid, ethylene and ozone) and biotic components (pathogens, insect pest, fungal cell wall components and oligosaccharides) is well demonstrated in plants. Several reviews on plant chitinase expression during hostpathogen interaction are available for annual species, whilst reports of their expression in tree species are limited to a few woody perennials: Populus, Pinus, Picea, Eucalyptus, Castanea and Pseudotsuga. The aim of this paper is to review the induction of chitinase during various stresses and developmental processes in forest tree species.

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Reference genes are generally used as endogenous normalization factor for relative quantification of target genes in quantitative real-time PCR (qRT-PCR). The present work aimed at identifying suitable reference genes for normalization of qRT-PCR data in tissues of Eucalyptus tereticornis. The expression levels of housekeeping genes like Actin (EtAct2), Isocitrate dehy - drogenase (EtIDH), ribosomal RNA (Et18s rRNA), SAND family protein (EtSAND), Histone protein (EtH2B), α-Tubulin (EtTUB), and eukaryotic initiation factor (EteIF4B) were studied to characterize their normalization stability in different tissues including young leaves, internodes, developing and mature xylem. The expression level of these genes was analyzed using different algorithms like geNorm, NormFinder and Best- Keeper. Among the seven reference genes analyzed, EtAct2 was expressed with less variance and was found to be the most stable reference gene across different tissues using all the three programs, while the least stable gene identified was EtH2B. Further, the normalization efficiency of the reference genes were assessed to predict the expression levels of three primary cell wall specific cellulose synthase transcripts (EtCesAs) in E. tereticornis tissues. The relative expression of EtCesA4, EtCesA5 and EtCesA6 was determined to be 3-19 fold higher in leaf and internode tissues when compared to developing and mature xylem tissues. This study will allow accurate normalization of qRT-PCR experiments across different tissues in E. tereticornis for future genomic research in this tropical eucalypt species.

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Cellulose synthases (CesA) represent a group of β-1, 4 glycosyl transferases involved in cellulose biosynthesis. Recent reports in higher plants have revealed that two groups of CesA gene families exist, which are associated with either primary or secondary cell wall deposition. The present study aimed at identifying developing secondary xylem specific cellulose synthase genes from Eucalyptus tereticornis, a species predominantly used in paper and pulp industries in the tropics. The differential expression analysis of the three EtCesA genes using qRT-PCR revealed 49 to 87 fold relative expression in developing secondary xylem tissues. Three full length gene sequences of EtCesA1, EtCesA2 and EtCesA3 were isolated with the size of 2940, 3114 and 3123 bp, respectively. Phytohormone regulation of all three EtCesA genes were studied by exogenous application of gibberellic acid, naphthalene acetic acid, indole acetic acid and 2, 4-epibrassinolide in internode tissues derived from three-month-old rooted cuttings. All three EtCesA transcripts were upregulated by indole acetic acid and gibberellic acid. This study demonstrates that the increased cellulose deposition in the secondary wood induced by hormones can be attributed to the upregulation of xylem specific CesAs.