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Tapping panel dryness (TPD) causes a significant reduction in the latex yield of rubber tree (Hevea brasiliensis Muell. Arg.). It is reported that TPD is a typical programmed cell death (PCD) process. Although PCD plays a vital role in TPD occurrence, there is a lack of detailed and systematic study. Metacaspases are key regulators of diverse PCD in plants. Based on our previous result that HbMC1 was associated with TPD, we further elucidate the roles of HbMC1 on rubber tree TPD in this study. HbMC1 was up-regulated by TPD-inducing factors including wounding, ethephon and H2O2. Moreover, the expression level of HbMC1 was increased along with TPD severity in rubber tree, suggesting a positive correlation between HbMC1 expression and TPD severity. To investigate its biological function, HbMC1 was overexpressed in yeast (Saccharomyces cerevisiae) and tobacco (Nicotiana benthamiana). Transgenic yeast and tobacco overexpressing HbMC1 showed growth retardation compared with controls under H2O2-induced oxidative stress. In addition, overexpression of HbMC1 in yeast and tobacco reduced cell survival after high-concentration H2O2 treatment and resulted in enhanced H2O2-induced leaf cell death, respectively. A total of 11 proteins, rbcL, TM9SF2-like, COX3, ATP9, DRP, HbREF/Hevb1, MSSP2-like, SRC2, GATL8, CIPK14-like and STK, were identified and confirmed to interact with HbMC1 by yeast two-hybrid screening and co-transformation in yeast. The 11 proteins mentioned above are associated with many biological processes, including rubber biosynthesis, stress response, autophagy, carbohydrate metabolism, signal transduction, etc. Taken together, our results suggest that HbMC1-mediated PCD plays an important role in rubber tree TPD, and the identified HbMC1-interacting proteins provide valuable information for further understanding the molecular mechanism of HbMC1-mediated TPD in rubber tree., (© The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

Background: 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., Results: 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., Conclusions: 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.

Translationally controlled tumor protein (TCTP) is a highly conserved protein and performs vital roles in regulating general growth and development in eukaryotes. Rubber tree ( Hevea brasiliensis ) harbors two TCTP genes, HbTCTP and HbTCTP1 , but the individual functions of them were unknown in plant. Here, sequence analysis displayed that HbTCTP and HbTCTP1 had the same number of 5′-terminal oligopyrimidine tract (5′-TOP) motifs in the 5′-untranslated region (UTR). However, the number of AUUUA motifs in the 3′-UTR was different between two TCTPs . HbTCTP and HbTCTP1 indicated different expression profiles in phloem and its exudate under ethylene treatment, suggesting that functional divergence likely occurred between two TCTPs in rubber tree. Moreover, HbTCTP and HbTCTP1 had different effect on adventitious shoot differentiation from leaf explants during transformation. Overexpressing HbTCTP or HbTCTP1 in tobacco enhanced the tolerance to H 2 O 2 -mediated oxidative stress and alleviated cell death induced by tunicamycin, suggesting that HbTCTP and HbTCTP1 played overlapping roles in the oxidative stress resistance and anti-apoptosis. Our results provide important insights into function and characteristics of HbTCTP and HbTCTP1 . [ABSTRACT FROM AUTHOR]

• Melatonin improve rubber tree seedlings tolerance to salt stress. • Melatonin enhance salt stress tolerance directly or indirectly by counteracting H 2 O 2 accumulation in rubber tree. • Melatonin enhances salt tolerance by regulating the genes involved in photosynthesis, ROS metabolism, flavonoids and melatonin biosynthesis to counteract the cellular accumulation of H 2 O 2 in rubber tree. As a well-known agent, melatonin plays very important roles in animals. However, its possible function is less clear in plants, especially in rubber tree. In this study, we demonstrate that melatonin acts as a potential agent to enhance salt stress tolerance in rubber tree seedlings. Chlorophyll degradation and H 2 O 2 accumulation in leaves of seedlings under salinity stress were significantly relieved by melatonin treatment. Transcriptome analyses suggested that melatonin was a potential antioxidant, and its exogenous application resulted in enhanced antioxidant protection mainly by regulating the genes involved in photosynthesis, ROS metabolism, flavonoids and melatonin biosynthesis. These results demonstrate that melatonin can enhance salt stress tolerance directly or indirectly by counteracting the cellular accumulation of H 2 O 2 in rubber tree. Our findings are helpful for not only understanding melatonin's roles in salinity stress, but also providing new insights into potential utilization of melatonin against abiotic stresses in rubber tree. [ABSTRACT FROM AUTHOR]