Physiological comparison of root trainer and polybag plants of Hevea brasiliensis

The physiology of   root trainer plants in comparison to polybag plants of two popular Hevea clones,   RRII 105 and RRII 430 was studied. Dry matter partitioning and physiological   parameters like leaf water potential, relative water content, chlorophyll   content, PS II activity and photosynthetic efficiency were studied in root   trainer and polybag plants and compared. Root trainer plants had only 55 per   cent less biomass than polybag plants at two whorled stage. No variation was   noticed in water relations among the poly bag and root trainer plants. Total   chlorophyll content and chlorophyll a/b ratio were higher in polybag   plants than root trainer plants of both clones. Polybag plants showed higher   light saturation point when gas exchange was measured at different   photosynthetic photon flux densities. Though there was no variation in   effective quantum yield (Φ PSII), the root trainer plants of RRII 105 showed   more excess electrons (J*) at higher light intensities. Photosystem II   activity and photosynthetic carbon assimilation rates were less in root   trainer plants under open and shaded conditions. The results indicated that   the root confinement in root trainer plants has significant effect on the   physiology of plants. The reduction in plant biomass and other physiological   traits in root trainer plants can be attributed to the limited space in the   container, limiting further root growth, which otherwise is beneficial for   giving a priming effect as long as the plant remains in the container.   Further studies are needed to quantify this ‘stress factor’ in root trainer   plants and its implications in growth and establishment of plants in the   field

Seasonal changes in xylem sap flow rate in mature rubber plants

The rate of flow of xylem sap of mature rubber tree was recorded round the clock continuously for two years using a Granier type thermal dissipation probe (TDP). The measurements were made on 19 year old trees of the clones, RRII 5 and PR 255 with a mean girth of 78 and 82 cm, respectively at 150 cm above bud union. Overall, the average rate of water mining by a tree was to the tune of 22 3 L day-1. The diurnal and seasonal differences in the sap flow rate were very evident which responded to the ambient weather conditions such as intensity of sunlight, temperature, rainfall etc. In the morning hours, as the sunlight intensity increased there was a corresponding sharp increase in sap flow rate which attained maximum level around mid-day. In the evening, as the light intensity declined, the sap flow rate also declined. The maximum rate of sap flow per day was recorded in December and the minimum in February coinciding with complete defoliation of the canopy. Taking a mean water consumption of 22 L tree-1day-1 and assuming there are 400 trees ha-1, the water consumption works out to be in the range of 1-2 mm day-1. This is significantly lesser than the potential evapo-transpiration (ET) of an open field in this traditional region. Taking the long-term average rain fall in the region (3000 mm year-1), it can be seen that the water loss due to transpiration (T) by the trees amounted only to 11 per cent of the annual rainfall

Cloning and expression of hmgr1 gene from Hevea brasiliensis

Biosynthesis of natural rubber (cis-1,4-polyisoprene) takes place through mevalonate pathway in Hevea. The enzyme 3-hydroxy- 3-methyl glutaryl-CoA reductase (HMGR), which catalyses the synthesis of mevalonate from HMG-CoA is a key regulatory enzyme in this pathway. This study aimed to clone and express hmgr1 gene, in order to obtain the HMGR protein in vitro and to further use this protein as a marker for yield potential in Hevea. For this purpose, mRNA was isolated from the latex of Hevea (clone RRII 105). cDNA was synthesized and PCR amplification of coding region of hmgr1 was performed using hmgr1 specific primers. The PCR amplified product (~1.8 kb) was cloned into an expression vector (pRSET-A) and transformed into E. coli (BL21DE3) cells. Protein expression in transformed cells when monitored by SDS-PAGE analysis indicated the presence of HMGR protein (61.6 kDa). The protein would be used for developing specific antibody that could be further utilized for the quantification of HMGR in different Hevea clones for screening the yield potential. The details of cloning and expression of hmgr1 are presented and discussed

Expression analysis of rubber biosynthetic pathway genes in Hevea brasiliensis

Hevea brasiliensisis, the primary commercial source of natural rubber (cis-1, 4-polyisoprene), is a fundamental raw material used for manufacturing a wide range of industrial and domestic rubber products in automobile, medical and defense industries. In Hevea, biosynthesis of rubber takes place through mevalonate pathway. Clonal variations in the productivity of rubber may be the result of variations in the activities of the enzymes involved in rubber biosynthesis in different Hevea clones. In this study, expression of 14 genes corresponding to enzymes/regulatory proteins involved in rubber biosynthesis was analyzed in high and low latex yielding clones of Hevea brasiliensis. The level of expression of HbSUT3, a sucrose transporter and enzymes related to the synthesis of rubber such as hydroxymethyl glutaryl-CoA synthase (hmgs), HMG-CoA reductase (hmgr) and mevalonate diphosphate decarboxylase (MVD) were found to be significantly higher in high rubber yielding clones compared to the low rubber yielding clones. The higher expression of these genes might result in an increased supply of IPP, the isoprenoid monomer, required for rubber biosynthesis. Expression of genes in the downstream rubber biosynthetic pathway such as FPPS, RuT and REF2 were also found to be significantly higher in high rubber yielding clones than low yielders. The results suggest that high rubber yield is associated with high expression of these genes and these genes can be used as markers for high yield potential in Hevea

ATP concentration in latex as an indicator for early evaluation of yield in Hevea brasiliensis

Adenosine 5’ triphosphate (ATP) in the laticiferous tissues of Hevea brasiliensis plays a major role in rubber biosynthesis through its direct involvement in metabolic pathways and indirectly through H+ ATPase activity. A strong positive correlation was observed between dry rubber yield and latex ATP, when ATP was measured in the latex of ten Hevea clones with different yield potentials such as low, medium and high. The latex ATP was significantly high in clones with higher yield than medium and low yielding clones. The correlation between same day latex yield and ATP during the peak yielding season (September-November) also showed a direct relationship. This study was extended to immature plants of the same clones to confirm the practical application of this finding in crop improvement programme. Latex ATP was analysed in two year old young plants and continued for five years to correlate with rubber yield of the mature trees in field trial. A positive correlation was noticed between ATP in young plants and mature tree rubber yield. Young plants of high yielding clones always showed higher latex ATP concentration. In view of the direct relationship with yield, significant differences between clones and its seasonal insensitivity, latex ATP could be used as an indicator for early prediction of high yield in Hevea

Drought tolerance in MnSOD transgenic Hevea brasiliensis in a dry sub-humid environment

One year old bud-grafted plants of MnSOD transgenic Hevea lines (L1 and L2) and an untransformed line of clone RRII 105 were used in the present study to evaluate their physiological performance in a dry sub-humid environment by withholding irrigation and to assess the recovery by re-watering. The dry matter partitioning was more towards the root in transgenic lines (55% and 60% in Ll and L2, respectively) but, was less in the untransformed RRII 105 (43%). After six days of moisture stress in polybags, pre-dawn leaf water potential and relative water content declined in all the lines, however, transgenic line L1 showed higher tissue water content throughout the drought as well as recovery period. Chlorophyll content did not show any significant reduction. Net photosynthetic rate (PN) declined rapidly and it reached near zero on the third day of drought imposition except for line L1, which showed lesser decline in PN. The decline in stomatal conductance (gs) was more rapid than PN in all the lines. On re-watering, recovery of PN and gs was better in the transgenic lines than untransformed RRII 105, which did not recover fully from the drought impact. Antioxidant enzymes, superoxide dismutase and peroxidase did not show a definite trend in their activities in these lines. However, it was found that the transgenic line L1 had better drought tolerant capacity in terms of lesser inhibition of photosynthetic rate under drought and faster recovery on re-watering

Expression of miRNAs of <i>Hevea brasiliensis</i> under drought stress is altered in clones with varying levels of drought tolerance

153-160MicroRNAs (miRNAs) in plants play critical role in regulating gene expression at the post-transcriptional level in a number of biological processes including plant growth, development and defense responses against biotic and abiotic stresses. Recent reports indicate the role of miRNAs in regulating genes associated with various metabolic as well as abiotic stress responsive pathways in Hevea. The present study was initiated with an objective to identify drought responsive miRNAs from Hevea and to elucidate their role in stress response/tolerance in four clones of Hevea with varying levels of drought tolerance. miRNAs were isolated from leaves of Hevea and annotated after sequencing. The selected miRNAs were quantified using real time PCR. Four of the miRNAs displayed definite pattern of expression. The miR164, miR167 and the novel miRNA (HbmiRn_42) were down regulated in susceptible clones. HbmiRn_42 got upregulated in tolerant clones, indicating its strong association with stress tolerance. On the contrary, miR482 was down regulated in tolerant clones, while there was no change in its expression level in susceptible clones. This study throws light on stress responsive miRNAs of Hevea and their regulation under drought conditions