Genome-Wide Identification and Expression Profile Analysis of the Phenylalanine Ammonia-Lyase Gene Family in Hevea brasiliensis.

The majority of the world's natural rubber comes from the rubber tree (Hevea brasiliensis). As a key enzyme for synthesizing phenylpropanoid compounds, phenylalanine ammonia-lyase (PAL) has a critical role in plant satisfactory growth and environmental adaptation. To clarify the characteristics of rubber tree PAL family genes, a genome-wide characterization of rubber tree PALs was conducted in this study. Eight PAL genes (HbPAL1-HbPAL8), which spread over chromosomes 3, 7, 8, 10, 12, 13, 14, 16, and 18, were found to be present in the genome of H. brasiliensis. Phylogenetic analysis classified HbPALs into groups I and II, and the group I HbPALs (HbPAL1-HbPAL6) displayed similar conserved motif compositions and gene architectures. Tissue expression patterns of HbPALs quantified by quantitative real-time PCR (qPCR) proved that distinct HbPALs exhibited varying tissue expression patterns. The HbPAL promoters contained a plethora of cis-acting elements that responded to hormones and stress, and the qPCR analysis demonstrated that abiotic stressors like cold, drought, salt, and H2O2-induced oxidative stress, as well as hormones like salicylic acid, abscisic acid, ethylene, and methyl jasmonate, controlled the expression of HbPALs. The majority of HbPALs were also regulated by powdery mildew, anthracnose, and Corynespora leaf fall disease infection. In addition, HbPAL1, HbPAL4, and HbPAL7 were significantly up-regulated in the bark of tapping panel dryness rubber trees relative to that of healthy trees. Our results provide a thorough comprehension of the characteristics of HbPAL genes and set the groundwork for further investigation of the biological functions of HbPALs in rubber trees.

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.

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.

Genome-wide identification of lncRNAs, miRNAs, mRNAs and their regulatory networks involved in tapping panel dryness in rubber tree (Hevea brasiliensis).

Noncoding RNAs (ncRNAs) play pivotal roles in various biological processes in plants. However, the role of ncRNAs in tapping panel dryness (TPD) of rubber tree (Hevea brasiliensis Muell. Arg.) is largely unknown. Here, the whole transcriptome analyses of bark tissues from healthy and TPD trees were performed to identify differentially expressed long ncRNAs (DELs), microRNAs/miRNAs (DEMs), genes (DEGs) and their regulatory networks involved in TPD. A total of 263 DELs, 174 DEMs and 1574 DEGs were identified in the bark of TPD tree compared with that of healthy tree. Kyoto Encyclopedia of Genes and Genomes analysis revealed that most of the DEGs and targets of DELs and DEMs were mainly enriched in metabolic pathways, biosynthesis of secondary metabolites and plant hormone signal transduction. Additionally, the majority of DEGs and DELs related to rubber biosynthesis were downregulated in TPD trees. Furthermore, 98 DEGs and 44 DELs were targeted by 54 DEMs, 190 DEGs were identified as putative targets of 56 DELs, and 2 and 44 DELs were predicted as precursors and endogenous target mimics of 2 and 6 DEMs, respectively. Based on these, the DEL-DEM-DEG regulatory network involved in TPD was constructed, and 13 hub DELs, 3 hub DEMs and 2 hub DEGs were identified. The results provide novel insights into the regulatory roles of ncRNAs underlying TPD and lay a foundation for future functional characterization of long ncRNAs, miRNAs and genes involved in TPD in rubber tree.