Involvement of HbMC1-mediated cell death in tapping panel dryness of rubber tree (Hevea brasiliensis)

Integrated physiology, transcriptome and proteome analyses highlight the potential roles of multiple hormone-mediated signaling pathways involved in tapping panel dryness in rubber tree

Currently, one of the most serious threats to rubber tree is the tapping panel dryness (TPD) that greatly restricts natural rubber production. Over-tapping or excessive ethephon stimulation is regarded as the main cause of TPD occurrence. Although extensive studies have been carried out, the molecular mechanism underlying TPD remains puzzled. An attempt was made to compare the levels of endogenous hormones and the profiles of transcriptome and proteome between healthy and TPD trees. Results showed that most of endogenous hormones such as jasmonic acid (JA), 1-aminocyclopropanecarboxylic acid (ACC), indole-3-acetic acid (IAA), trans-zeatin (tZ) and salicylic acid (SA) in the barks were significantly altered in TPD-affected rubber trees. Accordingly, multiple hormone-mediated signaling pathways were changed. In total, 731 differentially expressed genes (DEGs) and 671 differentially expressed proteins (DEPs) were identified, of which 80 DEGs were identified as putative transcription factors (TFs). Further analysis revealed that 12 DEGs and five DEPs regulated plant hormone synthesis, and that 16 DEGs and six DEPs were involved in plant hormone signal transduction pathway. Nine DEGs and four DEPs participated in rubber biosynthesis and most DEGs and all the four DEPs were repressed in TPD trees. All these results highlight the potential roles of endogenous hormones, signaling pathways mediated by these hormones and rubber biosynthesis pathway in the defense response of rubber trees to TPD. The present study extends our understanding of the nature and mechanism underlying TPD and provides some candidate genes and proteins related to TPD for further research in the future.

Bark transcriptome analyses reveals molecular mechanisms involved in tapping panel dryness occurrence and development in rubber tree (Hevea brasiliensis)

Tapping panel dryness (TPD) has become the mostimportant limiting factor for increasing natural rubber yield, whereas illuminating the molecular mechanisms underlying TPD is the prerequisite for solving the problem of TPD. However, molecular mechanisms underlying TPD are largely unknown. In this study, healthy and different stages of TPD-affected rubber trees were utilized to analyze TPD for the first time. We found that the changing tendencies of key latex physiological parameters were closely related to TPD occurrence and development. To reveal the molecular mechanisms underlying TPD, we sequenced and compared bark transcriptomes among healthy rubber tree, and TPD-affected ones at initial and advanced stages. In total, 8607 genes were identified as TPD-related genes in contrast to healthy rubber tree. According to gene expression profiles, the five samples were divided into three groups including healthy rubber tree, and TPD-affected rubber tree in the initial and advanced stages, which was consistent with the stages of TPD occurrence and development. Interestingly, only asmall proportionof the TPD-related genes were constantly down- or up-regulated with TPD occurrence and development. The TPD-related genes in KEGG pathways significantly enriched were closely associated with protein metabolism, cell division and differentiation, PCD, stress responses, terpene biosynthesis, and various metabolism processes. Moreover, overexpression of HbAPX2 identified as a TPD-related gene enhanced oxidative stress tolerance in S. cerevisiae. The typical symptoms of TPD, partial or complete dry zone (no latex flow) on tapping panel, might attribute to lower IPP available for rubber biosynthesis, and downregulation of the genes in post-IPP steps of rubber biosynthesis and the genes involved in latex flow. Our results not only provide new insights into molecular mechanisms underlying TPD occurrence and development but also contribute to developing effective measures to control TPD in rubber trees.

Construction and analysis of the tapping panel dryness-related lncRNA/circRNA-miRNA-mRNA ceRNA network in latex of Hevea brasiliensis

Tapping panel dryness (TPD) results in a severe reduction in latex yield in Hevea brasiliensis. However, the molecular regulatory mechanisms of TPD occurrence are still largely unclear. In this study, whole-transcriptome sequencing was carried out on latex from TPD and healthy trees. In total, 7078 long noncoding RNAs (lncRNAs), 3077 circular RNAs (circRNAs), 4956 miRNAs, and 25041 mRNAs were identified in latex, among which 435 lncRNAs, 68 circRNAs, 320 miRNAs, and 1574 mRNAs were differentially expressed in the latex of TPD trees. GO and KEGG analyses indicated that plant hormone signal transduction, MAPK signaling pathway, and ubiquitin-mediated proteolysis were the key pathways associated with TPD onset. Phytohormone profiling revealed significant changes in the contents of 28 hormonal compounds, among which ACC, ABA, IAA, GA, and JA contents were increased, while SA content was reduced in TPD latex, suggesting that hormone homeostasis is disrupted in TPD trees. Furthermore, we constructed a TPD-related competitive endogenous RNA (ceRNA) regulatory network of lncRNA/circRNA-miRNA-mRNA with 561 edges and 434 nodes (188 lncRNAs, 5 circRNAs, 191 miRNAs, and 50 mRNAs) and identified two hub lncRNAs (MSTRG.11908.1 and MSTRG.8791.1) and four hub miRNAs (hbr-miR156, miR156-x, miRf10477-y, and novel-m0452-3p). Notably, the lncRNA-miR156/157-SPL module containing three hubs probably plays a crucial role in TPD onset. The expression of network hubs and the lncRNA-miR156/157-SPL module were further validated by qRT-PCR. Our results reveal the TPD-associated ceRNA regulatory network of lncRNA/circRNA-miRNA-mRNA in latex and lay a foundation for further investigation of molecular regulatory mechanisms for TPD onset in H. brasiliensis.

Downregulation of HbFPS1 affects rubber biosynthesis of Hevea brasiliensis suffering from tapping panel dryness

Tapping panel dryness (TPD) is a century-old problem that has plagued the natural rubber production of Hevea brasiliensis. TPD may result from self-protective mechanisms of H. brasiliensis in response to stresses such as excessive hormone stimulation and mechanical wounding (bark tapping). It has been hypothesized that TPD impairs rubber biosynthesis; however, the underlying mechanisms remain poorly understood. In the present study, we firstly verified that TPD-affected rubber trees exhibited lower rubber biosynthesis activity and greater rubber molecular weight compared to healthy rubber trees. We then demonstrated that HbFPS1, a key gene of rubber biosynthesis, and its expression products were downregulated in the latex of TPD-affected rubber trees, as revealed by transcriptome sequencing and iTRAQ-based proteome analysis. We further discovered that the farnesyl diphosphate synthase HbFPS1 could be recruited to small rubber particles by HbSRPP1 through protein-protein interactions to catalyze farnesyl diphosphate (FPP) synthesis and facilitate rubber biosynthesis initiation. FPP content in the latex of TPD-affected rubber trees was significantly decreased with the downregulation of HbFPS1, ultimately resulting in abnormal development of rubber particles, decreased rubber biosynthesis activity, and increased rubber molecular weight. Upstream regulator assays indicated that a novel regulator, MYB2-like, may be an important regulator of downregulation of HbFPS1 in the latex of TPD-affected rubber trees. Our findings not only provide new directions for studying the molecular events involved in rubber biosynthesis and TPD syndrome and contribute to rubber management strategies, but also broaden our knowledge of plant isoprenoid metabolism and its regulatory networks.