Performance and genetic assessment of rubber tree clones in Southern Thailand

Thailand is the world leader in the production of latex extracted from the rubber tree (Hevea brasiliensis). However, the most cultivated clone RRIM 600, is highly susceptible to diseases, and there is economic incentive to develop new rubber tree clones. Four rubber tree clones (T2, SK1, NK1 and SK3) that have high latex yield potential from plantations in Southern Thailand were selected for this study. Yield performance, latex biochemical parameters and anatomical characteristics of bark were monitored for two years, using RRIM 600 clones in the same fields as paired controls. The average yields of the clones SK1, NK1 and SK3 were 129.3, 74.2 and 53.9 g per tree per tapping, respectively, surpassing the paired RRIM 600 controls (94.3, 49.9 and 43.9 g per tree per tapping in matching order). There was a difference in girth increment of SK1, SK3 and T2 clones when compared with RRIM 600, whereas the clones SK1 and T2 had higher renewed bark thickness than the paired RRIM 600. The anatomical measurements showed that the diameter of the latex vessels and density of latex vessels mm−2 were the highest in clone NK1, which also had the best latex biochemical parameters. This indicates NK1 is superior, and supports its use in Hevea breeding programs to improve latex yield. Our genetic characterization and assessment of the four clones selected used Random Amplified Polymorphic DNA (RAPD) and Simple Sequence Repeats (SSR). Seventeen recommended rubber clones were included as references. The clones SK3 and SK1 were closely related to RRIM 600 with similarity coefficients of 0.891 and 0.809, while NK1 and T2 were closely related to RRIT 250 (0.836) and RRIC 110 (0.864), respectively

Performance and genetic assessment of rubber tree clones in Southern Thailand

Thailand is the world leader in the production of latex extracted from the rubber tree (Hevea brasiliensis). However, the most cultivated clone RRIM 600, is highly susceptible to diseases, and there is economic incentive to develop new rubber tree clones. Four rubber tree clones (T2, SK1, NK1 and SK3) that have high latex yield potential from plantations in Southern Thailand were selected for this study. Yield performance, latex biochemical parameters and anatomical characteristics of bark were monitored for two years, using RRIM 600 clones in the same fields as paired controls. The average yields of the clones SK1, NK1 and SK3 were 129.3, 74.2 and 53.9 g per tree per tapping, respectively, surpassing the paired RRIM 600 controls (94.3, 49.9 and 43.9 g per tree per tapping in matching order). There was a difference in girth increment of SK1, SK3 and T2 clones when compared with RRIM 600, whereas the clones SK1 and T2 had higher renewed bark thickness than the paired RRIM 600. The anatomical measurements showed that the diameter of the latex vessels and density of latex vessels mm−2 were the highest in clone NK1, which also had the best latex biochemical parameters. This indicates NK1 is superior, and supports its use in Hevea breeding programs to improve latex yield. Our genetic characterization and assessment of the four clones selected used Random Amplified Polymorphic DNA (RAPD) and Simple Sequence Repeats (SSR). Seventeen recommended rubber clones were included as references. The clones SK3 and SK1 were closely related to RRIM 600 with similarity coefficients of 0.891 and 0.809, while NK1 and T2 were closely related to RRIT 250 (0.836) and RRIC 110 (0.864), respectively

Characterization of the phenylalanine ammonia lyase gene from the rubber tree (Hevea brasiliensis Müll. Arg.) and differential response during Rigidoporus microporus infection

Phenylalanine ammonia lyase (PAL) is a specific branch point enzyme of primary and secondary metabolism. It plays a key role in plant development and defense mechanisms. Phenylalanine ammonia lyase from Hevea brasiliensis (HbPAL) presented a complete open reading frame (ORF) of 2,145 bp with 721 encoded amino acids. The sequence alignment indicated that the amino acid sequence of HbPAL shared a high identity with PAL genes found in other plants. Phylogenetic tree analysis indicated that HbPAL was more closely related to PALs in Manihot esculenta and Jatropha curcas than to those from other plants. Transcription pattern analysis indicated that HbPAL was constitutively expressed in all tissues examined, most highly in young leaves. The HbPAL gene was evaluated by quantitative real-time PCR (qRT-PCR) after infection with Rigidoporus microporus at 0, 12, 24, 48, 72 and 96 hours post inoculation. The expression patterns of the PAL gene differed among the three rubber clones used in the study. The transcription level of the white root rot disease tolerant clone, PB5/51 increased sharply during the latter stages of infection, while it was relatively subdued in the white root rot disease susceptible clones, RRIM600 and BPM24. These results suggest that the HbPAL gene may play a role in the molecular defense response of H. brasiliensis to pathogen attack and could be used as a selection criterion for disease tolerance

Carotenoid Accumulation and Carotenogenic Gene Expression during Fruit Development in Novel Interspecific Inbred Squash Lines and Their Parents

Carotenoid levels and composition during squash fruit development were compared in Cucurbita moschata, Cucurbita maxima, and two lines of their interspecific inbred lines, namely, Maxchata1 and Maxchata2. Eight genes associated with carotenoid biosynthesis were analyzed by quantitative RT-PCR. The two squash species and their interspecific inbred lines exhibited different qualitative and quantitative carotenoid profiles and regulatory mechanisms. <i>C. moschata</i> had the lowest total carotenoid content and mainly accumulated α-carotene and β-carotene, as expected in a fruit with pale-orange flesh. Low carotenoid content in this species was probably due to the comparatively low expression of all genes investigated, especially <i>PSY1</i> gene, compared to the other squashes. The predominant carotenoids in <i>C. maxima</i> were violaxanthin and lutein, which produced a corresponding yellow flesh color in mature fruit. The relationship between the expression of the <i>CHYB</i> and <i>ZEP</i> genes may result in almost equal concentrations of violaxanthin and lutein in <i>C. maxima</i> at fruit ripening. In contrast, their interspecific inbred lines principally accumulated lutein and β-carotene, leading to orange flesh color. The <i>PSY1</i> gene exhibited higher expression levels at earlier stages of fruit development in the Maxchata lines, potentially triggering the increased carotenoid accumulation seen in these fruits. Likewise, the higher transcription level of <i>CHYB</i> gene observed in the two interspecific inbred lines might be correlated with high lutein in these hybrids. However, this study could not explain the observed β-carotene accumulation on the basis of gene expression

Antioxidant Enzymatic Activities and Gene Expression Associated with Heat Tolerance in the Stems and Roots of Two Cucurbit Species (“Cucurbita maxima” and “Cucurbita moschata”) and Their Interspecific Inbred Line “Maxchata”

The elucidation of heat tolerance mechanisms is required to combat the challenges of global warming. This study aimed to determine the antioxidant enzyme responses to heat stress, at the enzymatic activity and gene expression levels, and to investigate the antioxidative alterations associated with heat tolerance in the stems and roots of squashes using three genotypes differing in heat tolerance. Plants of heat-tolerant “C. moschata”, thermolabile “C. maxima” and moderately heat-tolerant interspecific inbred line “Maxchata” genotypes were exposed to moderate (37 °C) and severe (42 °C) heat shocks. “C. moschata” exhibited comparatively little oxidative damage, with the lowest hydrogen peroxide (H2O2), superoxide (O2−) and malondialdehyde (MDA) contents in the roots compared to stems, followed by “Maxchata”. The enzyme activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) and peroxidase (POD) were found to be increased with heat stress in tolerant genotypes. The significant inductions of FeSOD, MnSOD, APX2, CAT1 and CAT3 isoforms in tolerant genotypes suggested their participation in heat tolerance. The differential isoform patterns of SOD, APX and CAT between stems and roots also indicated their tissue specificity. Furthermore, despite the sequence similarity of the studied antioxidant genes among “C. maxima” and “Maxchata”, most of these genes were highly induced under heat stress in “Maxchata”, which contributed to its heat tolerance. This phenomenon also indicated the involvement of other unknown genetic and/or epigenetic factors in controlling the expression of these antioxidant genes in squashes, which demands further exploration