Trees occupy more than 30% of the land biosphere. They are important from both ecological and environmental standpoints and provide some of the most valuable commodities in the world economy. The perennial nature and size of trees are the critical determinants of their survival in response to biotic and abiotic stresses. The identification of the defense pathways at biochemical and genetic levels in tree pathosystems are beginning to be addressed. The basic physiological and biochemical mechanisms in woody perennials in response to pathogen is homologous to the model annual crop like Arabidopsis, but their secondary metabolic processes and ecological survival strategies are likely to be divergent from their annual counterparts. The limited domestication in tree species makes its molecular mechanisms less comparable to the highly pedigreed crop species. Recent reports have highlighted that the possible difference in genetic programs responding to invasive pathogens between annuals and perennials could be the spatial and temporal pattern of gene regulation. Several reviews on pathogen defense with reference to crop species are available, while similar reports from the tree species are limited to few commercially important species like Populus, Pinus, Picea, Eucalyptus, Castanea, and Pseudotsuga. This paper reviews the present status of pathogenesis-related genes and proteins from tree species with emphasis on the resistant genes and the proteins induced during systemic acquired resistance and highlights the ecological and evolutionary significance of defense-related genes from tree species.

Casuarina equisetifolia has the widest distribution of all Casuarina species and is a nitrogen-fixing tree of considerable social, economic and environmental importance. Trichosporium vesiculosum, a causal agent of blister bark disease, is a serious pathogen of C. equisetifolia. In this study, a cDNA clone encoding class I chitinase (CeChi1) belonging to PR-3 family was cloned and characterized from the needle tissues of C. equisetifolia challenged with the toxic exudate of the fungal pathogen T. vesiculosum. The CeChi1 open reading frame comprised 966 nucleotides that encoded 321 amino acid residues with the molecular mass of mature protein being approximately 34 kDa. Analysis of the predicted amino acid sequence revealed the similarity of CeChi1 protein to class I chitinase from other plant species containing a hydrophobic signal peptide domain and hinge domain. The sequence also harboured a cysteine-rich chitin-binding domain and lysozymelike domain. A hydrophobic C-terminal domain similar to the vacuole targeting sequences of class I chitinases isolated from other plants was also detected. The genomic sequence of CeChi1 indicated that the coding region contained three exons and two introns. In silico analysis of the untranslated regions revealed the presence of several cis-acting regulatory elements associated with hormonal regulation and stress responses. Quantitative real-time PCR analyses at different time points showed upregulation of the transcript during pathogen elicitation and salicylic acid signalling. However, no significant expression of CeChi1 was observed during other abiotic stress condition including wounding, water deficit, salt and heat stress revealing the specific expression of the gene during pathogenesis. This is the first report on isolation of a gene from C. equisetifolia, and the detailed functional analyses of CeChi1 will help in understanding its specific role in defence against pathogens in this tropical tree species.

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.

Genetic transformation of plants offers the possibility of testing hypotheses on the function of individual genes and enables exploration of transgenes for targeted trait improvement. Cloning of the full-length class I chitinase from the Casuarina equisetifolia (CeChi1) was earlier reported by our team. In the present study, tobacco was used as a model system to functionally evaluate the potential of CeChi1 driven by ubiquitin promoter. The pUH-CeChi1 construct was introduced into tobacco by Agrobacterium-mediated transformation and the putative transformants were confirmed for stable gene integration, transgene expression and recombinant protein production using PCR, RT-qPCR, antifungal assays and in planta analysis. The in vitro antifungal bioassay using the total proteins from leaves of transformed plantlets revealed the characteristic lysis of hyphal tips of pathogenic fungi including Trichosporium vesiculosum, Fusarium oxysporum and Rhizoctonia solani. The in planta bioassay of transformed tobacco showed reduced disease symptoms when compared to untransformed wild plants. The study revealed that the class I chitinase isolated from C. equisetifolia can act as a potential gene resource in future transformation programs for incorporating disease tolerance.

Casuarina equisetifolia has the widest distribution of all Casuarina species and is a nitrogen-fixing tree planted in tropical/subtropical littoral zones of Asia, the Pacific and Africa for coastal reclamation, charcoal making, pulp and timber. Trichosporium vesiculosum, the causal agent of blister bark disease, is a serious pathogen of C. equisetifolia. The present study was undertaken to understand the molecular mechanisms involved during pathogen cell wall elicitation in this hardy tree species. Transcript profiling during elicitation induced by cell wall components of T. vesiculosum revealed expression of resistance genes; cytochrome oxidase; trans membrane proteins; genes involved in programmed cell death like 26S proteasome and ubiquitin activating enzyme; early nodulin gene, wound inducible metallocarboxy peptidase inhibitor, glucanase, metal binding protein and signal recognition particle. The fold expression of selected transcripts including glucanase, 26 S proteasome, signal recognition particle, cytochrome oxidase and the metal binding protein using RT-qPCR revealed 12-59 fold increase in expression after 48 hours of elicitor treatment. The expression of these transcripts during abiotic stresses like heat, mechanical wounding, salt (NaCl) and drought (PEG) was also analyzed. Glucanase was up-regulated significantly during wounding and heat stress while proteasome was up-regulated 1-4 fold during NaCl, PEG and wounding stress. The novel transcript CeHMA was up-regulated under all the stress conditions. This is the first report on molecular defense in C. equisetifolia and has provided a pool of candidate genes for detailed molecular dissection to further broaden the knowledge on the response of woody perennials during pathogen cell wall elicitation.