As a highly conserved protein, the Translationally Controlled Tumor Protein (TCTP) carries out vital roles in various life processes. In rubber tree, two TCTP genes, HbTCTP and HbTCTP1, were cloned, but only HbTCTP1 was studied in details. In this study, cis-acting regulatory elements, expression patterns, subcellular localization, interacting proteins and antioxidant activity of HbTCTP were systematically analyzed. Besides the common cis-acting regulatory elements, HbTCTP promoter also harbored various known cis-elements that respond to hormone/stresses. Being consistent with the aforementioned results, HbTCTP was regulated by drought, low temperature, high salt, ethylene (ET), wounding, H2O2, and methyl jasmonate (MeJA) treatments. HbTCTP was expressed throughout different tissues and developmental stages of leaves. In addition, HbTCTP was associated with tapping panel dryness (TPD). HbTCTP was localized in the membrane, cytoplasm and the nucleus, and interacted with four proteins rubber elongation factor (REF), 17.5 kDa heat shock family protein, annexin, and REF-like stress related protein 1. Being similar to HbTCTP1, HbTCTP also indicated antioxidant activity in metal-catalyzed oxidation (MCO) system. Our results are useful for further understanding the molecular characterization and expression profiles of HbTCTP, but also lay a solid foundation for elucidating the function of HbTCTP in rubber tree.

Breeding rubber tree seedling with growth heterosis is vital for natural rubber production. It is the prerequisites for effectively utilizing growth heterosis to elucidate its molecular mechanisms, but the molecular mechanisms remain poorly understood in rubber tree. To elucidate seedling growth heterosis, we conducted comparative transcriptomic analyses between the two hybrids and their parents. By identifying and comparing differently expressed genes (DEGs), we found that the hybrids (BT 3410 and WC 11) show significantly differential expression profiles from their parents (PR 107 and RRIM 600). In BT 3410-parent triad, 1092 (49.95%) and 1094 (50.05%) DEGs indicated clear underdominance or overdominance, respectively. Whereas in WC 11-parent triad, most DEGs (78.2%, 721) showed low- or high-parent dominance; 160 (17.35%) exhibited expression patterns that are not statistically distinguishable from additivity, and 8 (0.87%) and 33 (3.58%) DEGs exhibited underdominance and overdominance, respectively. Furthermore, some biological processes are differentially regulated between two hybrids. Interestingly, the pathway in response to stimulus is significantly downregulated and metabolic pathways are more highly regulated in BT 3410. Taken together, the genotypes, transcriptomes and biological pathways (especially, carbohydrate metabolism) are highly divergent between two hybrids, which may be associated with growth heterosis and weakness. Analyzing gene action models in hybrid-parent triads, we propose that overdominance may play important roles on growth heterosis, whereas dominance on hybrid weakness in rubber tree seedlings. These findings bring new insights into our understanding of growth heterosis of rubber tree seedling.

Melatonin (MT) can enhance plant stress tolerance by activating the internal defense system, but its application in rubber trees has been barely reported up to now. In this study, we found that the relative electrical conductivity (REC), H₂O₂, and malondialdehyde (MDA) contents were significantly higher in the leaves of rubber tree seedlings under drought stress compared to the control (water treatment), whereas chlorophyll contents were obviously lower in the leaves under drought stress compared to the control. MT partly relieves the aforementioned drought-induced adverse effects by dramatically reducing chlorophyll degradation, H₂O₂ accumulation, MDA content, and REC. Comparative transcriptomes among the PEG (P), MT (M), and PEG + MT (PM) treatments against the control showed that 213, 896, and 944 genes were differently expressed in rubber tree seedlings treated with M, P, and PM in contrast to the control. Among the 64 differently expressed genes (DEGs) being common among the three comparisons, the expression profiles of 25 were opposite in MH compared with PH. Intriguingly, all the KEGG pathways of the DEGs mentioned above belonged to metabolism including energy metabolism, carbohydrate metabolism, amino acid metabolism, and the metabolism of cofactors and vitamins. Exogenous application of MT mainly regulated the genes associated with photosynthesis and the anti-oxidative defense system, thereby enhancing the antioxidant protection of rubber tree seedlings under drought stress. These results suggest that exogenous melatonin application can effectively enhance drought tolerance by heightening ROS scavenging to decrease H₂O₂ accumulation in rubber tree seedlings. Our results elucidate the molecular mechanisms of MT’s roles in drought stress, which help to employ exogenous MT to boost drought tolerance in the rubber tree.