Role of transitory carbon reserves during adjustment to climate variability and source-sink imbalances in oil palm (Elaeis guineensis)

Transcriptomic responses of oil palm (Elaeis guineensis) stem to waterlogging at plantation in relation to precipitation seasonality

Global warming-induced climate change causes significant agricultural problems by increasing the incidence of drought and flooding events. Waterlogging is an inevitable consequence of these changes but its effects on oil palms have received little attention and are poorly understood. Recent waterlogging studies have focused on oil palm seedlings, with particular emphasis on phenology. However, the transcriptomic waterlogging response of mature oil palms remains elusive in real environments. We therefore investigated transcriptomic changes over time in adult oil palms at plantations over a two-year period with pronounced seasonal variation in precipitation. A significant transcriptional waterlogging response was observed in the oil palm stem core but not in leaf samples when gene expression was correlated with cumulative precipitation over two-day periods. Pathways and processes upregulated or enriched in the stem core response included hypoxia, ethylene signaling, and carbon metabolism. Post-waterlogging recovery in oil palms was found to be associated with responses to heat stress and carotenoid biosynthesis. Nineteen transcription factors (TFs) potentially involved in the waterlogging response of mature oil palms were also identified. These data provide new insights into the transcriptomic responses of planted oil palms to waterlogging and offer valuable guidance on the sensitivity of oil palm plantations to future climate changes.

Occurrence of triploids in oil palm and their origin

BACKGROUND AND AIMS

Oil palms showing exceptional vigour and dubbed as 'giant palms' were identified in some progeny during breeding. A panel of phenotypical traits were studied to characterize these trees. The hypothesis that gigantism and other anomalies might be linked to polyploidy was investigated.

METHODS

Twenty sib pairs of palms from different crosses, each comprising a giant and a normal oil palm, were studied by flow cytometry with rice 'Nipponbare' as standard reference. In parallel, palms were assessed in the field using 11 phenotypic traits. A principal component analysis (PCA) was conducted to define relationships between these phenotypical traits, and a linear discriminant analysis (LDA) to predict ploidy level and giant classification. Finally, a co-dominant molecular marker study was implemented to highlight the sexual process leading to the formation of 2n gametes.

KEY RESULTS

The first group of oil palms presented an oil palm/rice peak ratio of around 4.8 corresponding to diploid oil palms, whereas the second group presented a ratio of around 7, classifying these plants as triploid. The PCA enabled the classification of the plants in three classes: 21 were normal diploid palms; ten were giant diploid palms; while 11 were giant triploid palms. The LDA revealed three predictors for ploidy classification: phyllotaxy, petiole size and circumference of the plant, but surprisingly not height. The molecular study revealed that triploid palms arose from 2n gametes resulting from the second division restitution of meiosis in parents.

CONCLUSIONS

This study confirms and details the process of sexual polyploidization in oil palm. It also identifies three phenotypical traits to assess the ploidy level of the giant oil palms in the field. In practical terms, our results provide a cheap scientific method to identify polyploid palms in the field.

Non-structural carbohydrate dynamics and growth in tomato plants grown at fluctuating light and temperature

Fluctuations in light intensity and temperature lead to periods of asynchrony between carbon (C) supply by photosynthesis and C demand by the plant organs. Storage and remobilization of non-structural carbohydrates (NSC) are important processes that allow plants to buffer these fluctuations. We aimed to test the hypothesis that C storage and remobilization can buffer the effects of temperature and light fluctuations on growth of tomato plants. Tomato plants were grown at temperature amplitudes of 3 or 10°C (deviation around the mean of 22°C) combined with integration periods (IP) of 2 or 10 days. Temperature and light were applied in Phase (high temperature simultaneously with high light intensity, (400 μmol m^-2 s^-1), low temperature simultaneously with low light intensity (200 μmol m^-2 s^-1) or in Antiphase (high temperature with low light intensity, low temperature with high light intensity). A control treatment with constant temperature (22°C) and a constant light intensity (300 μmol m^-2 s^-1) was also applied. After 20 days all treatments had received the same temperature and light integral. Differences in final structural dry weight were relatively small, while NSC concentrations were highly dynamic and followed changes of light and temperature (a positive correlation with decreasing temperature and increasing light intensity). High temperature and low light intensity lead to depletion of the NSC pool, but NSC level never dropped below 8% of the plant weight and this fraction was not mobilizable. Our results suggest that growing plants under fluctuating conditions do not necessarily have detrimental effects on plant growth and may improve biomass production in plants. These findings highlight the importance in the NSC pool dynamics to buffer fluctuations of light and temperature on plant structural growth.

Potassium deficiency reconfigures sugar export and induces catecholamine accumulation in oil palm leaves

Metabolic effects of potassium (K) deficiency have been described for nearly 70 years but specific effects of low K availability on sugar composition, sugar export rate and its relationship with other leaf metabolites are not very well documented. Having such pieces of information is nevertheless essential to identify metabolic signatures to monitor K fertilization. This is particularly true in oil-producing crop species such as oil palm (Elaeis guineensis), which is strongly K-demanding and involves high sugar dependence for fruit formation because of low carbon use efficiency in lipid synthesis. Here, we used metabolic analyses, measured sugar export rates with ¹³C isotopic labeling and examined the effects of K availability on both leaflet and rachis sugar metabolism in oil palm seedlings. We show that low K leads to a modification of sugar composition mostly in rachis and decreased sucrose and hexose export rates from leaflets. As a result, leaflets contained more starch and induced alternative pathways such as raffinose synthesis, although metabolites of the raffinose pathway remained quantitatively minor. The alteration of glycolysis by low K was compensated for by an increase in alternative sugar phosphate utilization by tyrosine metabolism, resulting in considerable amounts of tyramine and dopamine.

The case for improving crop carbon sink strength or plasticity for a CO2-rich future

Atmospheric CO₂ concentration [CO₂] has increased from 260 to 280μmolmol^-1 (level during crop domestication up to the industrial revolution) to currently 400 and will reach 550μmolmol^-1 by 2050. C3 crops are expected to benefit from elevated [CO₂] (e-CO₂) thanks to photosynthesis responsiveness to [CO₂] but this may require greater sink capacity. We review recent literature on crop e-CO₂ responses, related source-sink interactions, how abiotic stresses potentially interact, and prospects to improve e-CO₂ response via breeding or genetic engineering. Several lines of evidence suggest that e-CO₂ responsiveness is related either to sink intrinsic capacity or adaptive plasticity, for example, involving enhanced branching. Wild relatives and old cultivars mostly showed lower photosynthetic rates, less downward acclimation of photosynthesis to e-CO₂ and responded strongly to e-CO₂ due to greater phenotypic plasticity. While reverting to such archaic traits would be an inappropriate strategy for breeding, we argue that substantial enhancement of vegetative sink vigor, inflorescence size and/or number and root sinks will be necessary to fully benefit from e-CO₂. Potential ideotype features based on enhanced sinks are discussed. The generic 'feast-famine' sugar signaling pathway may be suited to engineer sink strength tissue-specifically and stage-specifically and help validate ideotype concepts. Finally, we argue that models better accounting for acclimation to e-CO₂ are needed to predict which trait combinations should be targeted by breeders for a CO₂-rich world.

Small temperature variations are a key regulator of reproductive growth and assimilate storage in oil palm (Elaeis guineensis)

Oil palm is an important crop for global vegetable oil production, and is widely grown in the humid tropical regions of Southeast Asia. Projected future climate change may well threaten palm oil production. However, oil palm plantations currently produce large amounts of unutilised biological waste. Oil palm stems – which comprise two-thirds of the waste - are especially relevant because they can contain high levels of non-structural carbohydrates (NSC) that can serve as feedstock for biorefineries. The NSC in stem are also considered a potent buffer to source-sink imbalances. In the present study, we monitored stem NSC levels and female reproductive growth. We then applied convergent cross mapping (CCM) to assess the causal relationship between the time-series. Mutual causal relationships between female reproductive growth and the stem NSC were detected, with the exception of a relationship between female reproductive organ growth and starch levels. The NSC levels were also influenced by long-term cumulative temperature, with the relationship showing a seven-month time lag. The dynamic between NSC levels and long-term cumulative rainfall showed a shorter time lag. The lower temperatures and higher cumulative rainfall observed from October to December identify this as a period with maximum stem NSC stocks.

Natural (13) C distribution in oil palm (Elaeis guineensis Jacq.) and consequences for allocation pattern

Oil palm has now become one of the most important crops, palm oil representing nearly 25% of global plant oil consumption. Many studies have thus addressed oil palm ecophysiology and photosynthesis-based models of carbon allocation have been used. However, there is a lack of experimental data on carbon fixation and redistribution within palm trees, and important C-sinks have not been fully characterized yet. Here, we carried out extensive measurement of natural (13) C-abundance (δ(13) C) in oil palm tissues, including fruits at different maturation stages. We find a (13) C-enrichment in heterotrophic organs compared to mature leaves, with roots being the most (13) C-enriched. The δ(13) C in fruits decreased during maturation, reflecting the accumulation in (13) C-depleted lipids. We further used observed δ(13) C values to compute plausible carbon fluxes using a steady-state model of (13) C-distribution including metabolic isotope effects ((12) v/(13) v). The results suggest that fruits represent a major respiratory loss (≈39% of total tree respiration) and that sink organs such as fruits are fed by sucrose from leaves. That is, glucose appears to be a quantitatively important compound in palm tissues, but computations indicate that it is involved in dynamic starch metabolism rather that C-exchange between organs.

Carbohydrate regulation of photosynthesis and respiration from branch girdling in four species of wet tropical rain forest trees

How trees sense source-sink carbon balance remains unclear. One potential mechanism is a feedback from non-structural carbohydrates regulating photosynthesis and removing excess as waste respiration when the balance of photosynthesis against growth and metabolic activity changes. We tested this carbohydrate regulation of photosynthesis and respiration using branch girdling in four tree species in a wet tropical rainforest in Costa Rica. Because girdling severs phloem to stop carbohydrate export while leaving xylem intact to allow photosynthesis, we expected carbohydrates to accumulate in leaves to simulate a carbon imbalance. We varied girdling intensity by removing phloem in increments of one-quarter of the circumference (zero, one--quarter, half, three-quarters, full) and surrounded a target branch with fully girdled ones to create a gradient in leaf carbohydrate content. Light saturated photosynthesis rate was measured in situ, and foliar respiration rate and leaf carbohydrate content were measured after destructive harvest at the end of the treatment. Girdling intensity created no consistent or strong responses in leaf carbohydrates. Glucose and fructose slightly increased in all species by 3.4% per one-quarter girdle, total carbon content and leaf mass per area increased only in one species by 5.4 and 5.5% per one-quarter girdle, and starch did not change. Only full girdling lowered photosynthesis in three of four species by 59-69%, but the decrease in photosynthesis was unrelated to the increase in glucose and fructose content. Girdling did not affect respiration. The results suggest that leaf carbohydrate content remains relatively constant under carbon imbalance, and any changes are unlikely to regulate photosynthesis or respiration. Because girdling also stops the export of hormones and reactive oxygen species, girdling may induce physiological changes unrelated to carbohydrate accumulation and may not be an effective method to study carbohydrate feedback in leaves. In three species, removal of three-quarters of phloem area did not cause leaf carbohydrates to accumulate nor did it change photosynthesis or respiration, suggesting that phloem transport is flexible and transport rate per unit phloem can rapidly increase under an increase in carbohydrate supply relative to phloem area. Leaf carbohydrate content thus may be decoupled from whole plant carbon balance by phloem transport in some species, and carbohydrate regulation of photosynthesis and respiration may not be as common in trees as previous girdling studies suggest. Further studies in carbohydrate regulation should avoid using girdling as girdling can decrease photosynthesis through unintended means without the tested mechanisms of accumulating leaf carbohydrates.

Oil palm natural diversity and the potential for yield improvement

African oil palm has the highest productivity amongst cultivated oleaginous crops. Species can constitute a single crop capable to fulfill the growing global demand for vegetable oils, which is estimated to reach 240 million tons by 2050. Two types of vegetable oil are extracted from the palm fruit on commercial scale. The crude palm oil and kernel palm oil have different fatty acid profiles, which increases versatility of the crop in industrial applications. Plantations of the current varieties have economic life-span around 25-30 years and produce fruits around the year. Thus, predictable annual palm oil supply enables marketing plans and adjustments in line with the economic forecasts. Oil palm cultivation is one of the most profitable land uses in the humid tropics. Oil palm fruits are the richest plant source of pro-vitamin A and vitamin E. Hence, crop both alleviates poverty, and could provide a simple practical solution to eliminate global pro-vitamin A deficiency. Oil palm is a perennial, evergreen tree adapted to cultivation in biodiversity rich equatorial land areas. The growing demand for the palm oil threatens the future of the rain forests and has a large negative impact on biodiversity. Plant science faces three major challenges to make oil palm the key element of building the future sustainable world. The global average yield of 3.5 tons of oil per hectare (t) should be raised to the full yield potential estimated at 11-18t. The tree architecture must be changed to lower labor intensity and improve mechanization of the harvest. Oil composition should be tailored to the evolving needs of the food, oleochemical and fuel industries. The release of the oil palm reference genome sequence in 2013 was the key step toward this goal. The molecular bases of agronomically important traits can be and are beginning to be understood at the single base pair resolution, enabling gene-centered breeding and engineering of this remarkable crop.

Using plant growth modeling to analyze C source-sink relations under drought: inter- and intraspecific comparison

The ability to assimilate C and allocate non-structural carbohydrates (NSCs) to the most appropriate organs is crucial to maximize plant ecological or agronomic performance. Such C source and sink activities are differentially affected by environmental constraints. Under drought, plant growth is generally more sink than source limited as organ expansion or appearance rate is earlier and stronger affected than C assimilation. This favors plant survival and recovery but not always agronomic performance as NSC are stored rather than used for growth due to a modified metabolism in source and sink leaves. Such interactions between plant C and water balance are complex and plant modeling can help analyzing their impact on plant phenotype. This paper addresses the impact of trade-offs between C sink and source activities and plant production under drought, combining experimental and modeling approaches. Two contrasted monocotyledonous species (rice, oil palm) were studied. Experimentally, the sink limitation of plant growth under moderate drought was confirmed as well as the modifications in NSC metabolism in source and sink organs. Under severe stress, when C source became limiting, plant NSC concentration decreased. Two plant models dedicated to oil palm and rice morphogenesis were used to perform a sensitivity analysis and further explore how to optimize C sink and source drought sensitivity to maximize plant growth. Modeling results highlighted that optimal drought sensitivity depends both on drought type and species and that modeling is a great opportunity to analyze such complex processes. Further modeling needs and more generally the challenge of using models to support complex trait breeding are discussed.

Simulation of inflorescence dynamics in oil palm and estimation of environment-sensitive phenological phases: a model based analysis

For oil palm, yield variation is in large part due to variation in the number of harvested bunches. Each successively-produced phytomer carries a female (productive), male or aborted inflorescence. Since phytomer development takes 3-4 years and nearly two phytomers are produced per month, many inflorescences develop in parallel but have different phenological stages. Environment-dependent developmental rate, sex and abortion probability determine bunch productivity, which, in turn, affects other phytomers via source-sink relationships. Water deficit, solar radiation, temperature and day length are considered key external factors driving variation. Their impact is difficult to predict because of system complexity. To address this question we built a simple model (ECOPALM) to simulate the variation in number of harvested bunches. In this model, trophic competition among organs, expressed through a plant-scale index (Ic), drives sex determination and inflorescence abortion during specific sensitive phases at phytomer level. As a supplemental hypothesis, we propose that flowering is affected by photoperiod at phytomer level during a sensitive phase, thus, contributing to seasonal production peaks. The model was used to determine by parameter optimisation the influence of Ic and day length on inflorescence development and the stages at which inflorescences are sensitive to these signals. Parameters were estimated against observation of number of harvested bunches in Ivory Coast using a genetic algorithm. The model was then validated with field observations in Benin and Indonesia. The sensitive phases determined by parameter optimisation agreed with independent experimental evidence, and variation of Ic explained both sex and abortion patterns. Sex determination seemed to coincide with floret meristem individualisation and occurred 29-32 months before bunch harvest. The main abortion stage occurred 10 months before harvest - at the beginning of rapid growth of the inflorescence. Simulation results suggest involvement of photoperiod in the determination of bunch growth dynamics. This study demonstrates that simple modelling approaches can help extracting ecophysiological information from simple field observations on complex systems.

Environmental regulation of sex determination in oil palm: current knowledge and insights from other species

BACKGROUND

The African oil palm (Elaeis guineensis) is a monoecious species of the palm subfamily Arecoideae. It may be qualified as 'temporally dioecious' in that it produces functionally unisexual male and female inflorescences in an alternating cycle on the same plant, resulting in an allogamous mode of reproduction. The 'sex ratio' of an oil palm stand is influenced by both genetic and environmental factors. In particular, the enhancement of male inflorescence production in response to water stress has been well documented.

SCOPE

This paper presents a review of our current understanding of the sex determination process in oil palm and discusses possible insights that can be gained from other species. Although some informative phenological studies have been carried out, nothing is as yet known about the genetic basis of sex determination in oil palm, nor the mechanisms by which this process is regulated. Nevertheless new genomics-based techniques, when combined with field studies and biochemical and molecular cytological-based approaches, should provide a new understanding of the complex processes governing oil palm sex determination in the foreseeable future. Current hypotheses and strategies for future research are discussed.