Characterization of the rubber tree metallothionein family reveals a role in mitigating the effects of reactive oxygen species associated with physiological stress

Abstract

Metallothioneins (MTs) as reactive oxygen species (ROS) scavengers play important roles in stress response and heavy metal homeostasis. In Hevea brasiliensis (the para rubber tree that is the source of commercial natural rubber) and in other trees, the functions of MTs are not well understood. Latex exudes when the rubber tree is tapped. The flow of latex and its regeneration can be enhanced by tapping, wounding and ethylene treatment, all of which produce ROS as a by-product. Here, we show the presence of four MT genes in H. brasiliensis, comprising three Type 2 (HbMT2, -2a and -2b) and one Type 3 (HbMT3L) isoforms, representing one of the smallest MT gene families among angiosperms. The four HbMTs exhibited distinct tissue expression patterns: HbMT2 and HbMT3L mainly in leaves, HbMT2a specifically in flowers and HbMT2b in diverse tissues. The expression of HbMT2b, an isoform present in latex, decreased significantly in the latex following the stress-inducing treatments of tapping, wounding and ethephon (an ethylene generator). The expressions of the leaf-abundant isoforms, HbMT2 and -3L were up-regulated following pathogenic fungus infection and high-temperature stress, but down-regulated by low-temperature stress. These reactions were consistent with multiple defense- and hormone-responsive cis-acting elements in the HbMT promoters. Nine transcription factors were shown to implicate in the high-temperature responsiveness of HbMT2 and -3L in leaves. Overexpression of HbMT2 in Escherichia coli enhanced the bacterium's tolerance to heavy metals and ROS, consistent with its predicted role as an ROS scavenger. Taken together, our results, along with other relevant studies, suggest an important role of HbMTs in latex regeneration as well as species adaptation via the regulation of ROS homeostasis.

Identification and evaluation of suitable reference genes for gene expression analysis in rubber tree leaf

“…In this study we focused on two reported transcript profiles for screening new candidate reference genes from rubber tree leaf at developmental stages (bronze, color change, pale-green and bright green) as wells as under temperature stress (high and low temperature) [ 32,33]. Genes were selected as candidate reference genes if they showed reads per kilobase per million mapped reads (RPKM) fold change <1.5.…”

“…In this study, we used two previously reported [ 32,33] transcript profiles of rubber tree in order to select new candidate reference genes from leaf, including developmental stages (bronze, color change, pale-green and bright green) and abiotic stress (high and low temperature). When the gene expression data was analyzed for those genes where the change in expression less than 1.5 fold, we observed that there were 3,673 and 1,987 transcripts that could be selected from the two experiments, respectively.…”

“…In this study, two transcriptome sequencing profiles of leaves were screened for appropriate reference genes [ 32,33]. The expression of selected genes was verified by qRT-PCR under four different experimental conditions.…”

Genome-Wide Identification of Aluminum-Activated Malate Transporter (ALMT) Gene Family in Rubber Trees (Hevea brasiliensis) Highlights Their Involvement in Aluminum Detoxification

“…The rubber tree (Hevea brasiliensis) is one of the most important cash trees widely cultivated in tropical regions. It is native to the Amazon forest and is the sole commercially cultivated source of natural rubber (cis-1, 4-polyisoprene) which originates from the milky latex flowing out once rubber trees are tapped [25]. Rubber trees are cultivated in about 95,000 hectares in 45 countries [26].…”

HbWRKY82, a novel IIc WRKY transcription factor from Hevea brasiliensis associated with abiotic stress tolerance and leaf senescence in Arabidopsis

“…In natural rubber production, ET is a widely used stimulant in commercial latex production which can significantly increase natural latex yield of the rubber trees through prolonging the latex flow and latex regeneration (Zhu and Zhang, 2009). ROS is a by-product of ET stimulation on latex production in the rubber trees after applied in the bark, while excessive ROS in laticifers would cause TPD which can halt the latex flow (Zhang et al, 2017(Zhang et al, , 2019Huang et al, 2018). Our study found that HbWRKY82 is an ET-responsive transcription factor and can regulate the expression of the key transcription factor (EIN3) in ethylene signaling pathway.…”