Testing intra-species variation in allocation to growth and defense in rubber tree (Hevea brasiliensis).

Background: Plants allocate resources to growth, defense, and stress resistance, and resource availability can affect the balance between these allocations. Allocation patterns are well-known to differ among species, but what controls possible intra-specific trade-offs and if variation in growth vs. defense potentially evolves in adaptation to resource availability. Methods: We measured growth and defense in a provenance trial of rubber trees (Hevea brasiliensis) with clones originating from the Amazon basin. To test hypotheses on the allocation to growth vs. defense, we relate biomass growth and latex production to wood and leaf traits, to climate and soil variables from the location of origin, and to the genetic relatedness of the Hevea clones. Results: Contrary to expectations, there was no trade-off between growth and defense, but latex yield and biomass growth were positively correlated, and both increased with tree size. The absence of a trade-off may be attributed to the high resource availability in a plantation, allowing trees to allocate resources to both growth and defense. Growth was weakly correlated with leaf traits, such as leaf mass per area, intrinsic water use efficiency, and leaf nitrogen content, but the relative investment in growth vs. defense was not associated with specific traits or environmental variables. Wood and leaf traits showed clinal correlations to the rainfall and soil variables of the places of origin. These traits exhibited strong phylogenetic signals, highlighting the role of genetic factors in trait variation and adaptation. The study provides insights into the interplay between resource allocation, environmental adaptations, and genetic factors in trees. However, the underlying drivers for the high variation of latex production in one of the commercially most important tree species remains unexplained.

Genetic analysis of agronomic and physiological traits associated with latex yield revealed complex genetic bases in Hevea brasiliensis.

Hevea brasiliensis, a natural rubber producing species, is widely cultivated due to its high rubber yield potential. Natural rubber is synthesised in the rubber particles of laticifers. Latex diagnosis (LD) was established to characterise the physiological state of the laticiferous system by measuring its physiological parameters, i.e., sucrose, inorganic phosphorous (Pi), thiols and total solid content (TSC). Rubber clones are often classified in three groups i.e., quick starters, medium starters and slow starters. To better understand the genetic bases of latex yield, a biparental population was generated from a cross between the quick-starter clone PB 260 and the medium-starter clone SP 217. LD was performed during the peak latex production season and used to calculate sucrose loading. The agronomic and physiological parameters associated with latex yield led to the classification of genotypes according to the rubber clonal typology and to the identification of quantitative trait loci (QTL) using a high-density map. Inorganic phosphorous content was positively associated with yield during the first year of production thus enabling identification of quick-starter clones. In addition, the LD-based clonal typology led to determine the long-term yield potential and the use of appropriate ethephon stimulation. QTL analysis successfully identified several QTLs related to yield, sucrose, Pi and TSC. One QTL related to sucrose loading was identified in the same position as the QTL for sucrose on linkage group 1. To our knowledge, this is the first study to report QTL analysis for this trait. The use of a high-density map enables the identification of genes underlying QTLs. Several putative genes underlying QTLs related to yield, sucrose and TSC were identified.

Involvement of ethylene-responsive microRNAs and their targets in increased latex yield in the rubber tree in response to ethylene treatment.

The rubber tree is an economically important plant that produces natural rubber for various industrial uses. The application of ethylene contributes to increased latex production in rubber trees; however, the molecular biology behind the effects of ethylene on latex yield remains to be elucidated. Recently, the intersection between microRNA (miRNA) regulation and phytohormone responses has been revealed. Insight into the regulation of miRNAs and their target genes should help to determine the functional importance of miRNAs as well as the role of miRNAs in signaling under ethylene stimulation in the rubber tree. In this study, hbr-miR159 and hbr-miR166 were down-regulated in bark under ethylene treatment. The ethylene also down-regulated ATHB15-like (Class III Homeodomain Leucine Zipper, HD-ZIP III) which have been extensively implicated in the regulation of primary and secondary vascular tissue pattern formation. The strong negative-regulation of ARF6/ARF8 caused by hbr-miR167 involved in an attenuation of vascular development and may gradually lead to bark dryness syndrome in the long term ethylene treatment. The negative correlation of hbr-miR172 and its target REF3 in the inner soft bark under ethylene treatment results in dramatic increases in latex yield in the ethylene-sensitive clone of the rubber tree. The overall results suggested that the differential expression of HD-ZIP III, miR167/ARF6, ARF8, and miR172/REF3 and related genes may play possible roles in the response to ethylene treatment, resulting in longer latex flow and increased latex yield.