Plant low-K responses are partly due to Ca prevalence and the low-K biomarker putrescine does not protect from Ca side effects but acts as a metabolic regulator.

Potassium (K) deficiency is a rather common situation that impacts negatively on biomass, photosynthesis and N assimilation, making K fertilization often unavoidable. Effects of K deficiency have been investigated for several decades and recently progress has been made in identifying metabolomics signatures thereby offering potential to monitor the K status of crops in the field. However, effects of low K conditions could also be due to the antagonism with other nutrients like calcium (Ca) and the well-known biomarker of K deficiency, putrescine, could be a response to Ca/K imbalance rather than K deficiency per se. To sort this out, we carried out experiments in sunflower grown at either low or high K, at high or low Ca, with or without putrescine added to the nutrient solution. Using metabolomics and proteomics analysis, we show that a significant part of the low K response, such as lower photosynthesis and N assimilation, is due to calcium and can be suppressed by low Ca conditions. Putrescine addition tends to restore photosynthesis and N assimilation but unlike low Ca does not suppress but aggravates the impact of low K conditions on catabolism, including the typical fall-over in pyruvate kinase. We conclude that (a) the effects of K deficiency on key metabolic processes can be partly alleviated by the use of low Ca and not only by K fertilization and (b) in addition to its role as a metabolite, putrescine participates in acclimation to low K via the regulation of the content in enzymes involved in carbon primary metabolism.

δ15 N values in plants are determined by both nitrate assimilation and circulation.

Nitrogen (N) assimilation is associated with 14 N/15 N fractionation such that plant tissues are generally 15 N-depleted compared to source nitrate. In addition to nitrate concentration, the δ15 N value in plants is also influenced by isotopic heterogeneity amongst organs and metabolites. However, our current understanding of δ15 N values in nitrate is limited by the relatively small number of compound-specific data. We extensively measured δ15 N in nitrate at different time points, in sunflower and oil palm grown at fixed nitrate concentration, with nitrate circulation being varied using potassium (K) conditions and waterlogging. There were strong interorgan δ15 N differences for contrasting situations between the two species, and a high 15 N-enrichment in root nitrate. Modelling shows that this 15 N-enrichment can be explained by nitrate circulation and compartmentalisation whereby despite a numerically small flux value, the backflow of nitrate to roots via the phloem can lead to a c. 30‰ difference between leaves and roots. Accordingly, waterlogging and low K conditions, which down-regulate sap circulation, cause a decrease in the leaf-to-root isotopic difference. Our study thus suggests that plant δ15 N can be used as a natural tracer of N fluxes between organs and highlights the potential importance of δ15 N of circulating phloem nitrate.

What is the role of putrescine accumulated under potassium deficiency?

Biomarker metabolites are of increasing interest in crops since they open avenues for precision agriculture, whereby nutritional needs and stresses can be monitored optimally. Putrescine has the potential to be a useful biomarker to reveal potassium (K+ ) deficiency. In fact, although this diamine has also been observed to increase during other stresses such as drought, cold or heavy metals, respective changes are comparably low. Due to its multifaceted biochemical properties, several roles for putrescine under K+ deficiency have been suggested, such as cation balance, antioxidant, reactive oxygen species mediated signalling, osmolyte or pH regulator. However, the specific association of putrescine build-up with low K+ availability in plants remains poorly understood, and possible regulatory roles must be consistent with putrescine concentration found in plant tissues. We hypothesize that the massive increase of putrescine upon K+ starvation plays an adaptive role. A distinction of putrescine function from that of other polyamines (spermine, spermidine) may be based either on its specificity or (which is probably more relevant under K+ deficiency) on a very high attainable concentration of putrescine, which far exceeds those for spermidine and spermine. putrescine and its catabolites appear to possess a strong potential in controlling cellular K+ and Ca2+ , and mitochondria and chloroplasts bioenergetics under K+ stress.

Seed Germination in Oil Palm (Elaeis guineensis Jacq.): A Review of Metabolic Pathways and Control Mechanisms.

Oil palm is an oil-producing crop of major importance at the global scale. Oil palm mesocarp lipids are used for myriads industrial applications, and market demand has been growing for decades. In addition, oil palm seeds are oleaginous, and the oil extracted therefrom can be used for several purposes, from food to cosmetics. As such, there is a huge need in oil palm seeds to maintain the global cohort of more than 2 billion trees. However, oil palm seed germination is a rather difficult process, not only to break dormancy, but also because it is long and often reaches lower-than-expected germination rates. Surprisingly, despite the crucial importance of germination for oil palm plantation management, our knowledge is still rather limited, in particular about germinating oil palm seed metabolism. The present review incorporates different pieces of information that have been obtained in the past few years, in oil palm and in other palm species, in order to provide an overview of germination metabolism and its control. Further insights can also be gained from other oleaginous model plants, such as Arabidopsis or canola, however, palm seeds have peculiarities that must be accounted for, to gain a better understanding of germinating seed metabolism.

Metabolic responses to potassium availability and waterlogging reshape respiration and carbon use efficiency in oil palm.

Oil palm is by far the major oil-producing crop on the global scale, with c. 62 Mt oil produced each year. This species is a strong potassium (K)-demanding species cultivated in regions where soil K availability is generally low and waterlogging due to tropical heavy rains can limit further nutrient absorption. However, the metabolic effects of K and waterlogging have never been assessed precisely. Here, we examined the metabolic response of oil palm saplings in the glasshouse under controlled conditions (nutrient composition with low or high K availability, with or without waterlogging), using gas exchange, metabolomics and proteomics analyses. Our results showed that both low K and waterlogging have a detrimental effect on photosynthesis but stimulate leaf respiration, with differential accumulation of typical metabolic intermediates and enzymes of Krebs cycle and alternative catabolic pathways. In addition, we found a strong relationship between metabolic composition, the rate of leaf dark respiration, and cumulated respiratory loss. Advert environmental conditions (here, low K and waterlogging) therefore have an enormous effect on respiration in oil palm. Leaf metabolome and proteome appear to be good predictors of carbon balance, and open avenues for cultivation biomonitoring using functional genomics technologies.

Responses to K deficiency and waterlogging interact via respiratory and nitrogen metabolism.

K deficiency and waterlogging are common stresses that can occur simultaneously and impact on crop development and yield. They are both known to affect catabolism, with rather opposite effects: inhibition of glycolysis and higher glycolytic fermentative flux, respectively. But surprisingly, the effect of their combination on plant metabolism has never been examined precisely. Here, we applied a combined treatment (K availability and waterlogging) to sunflower (Helianthus annuus L.) plants under controlled greenhouse conditions and performed elemental quantitation, metabolomics, and isotope analyses at different sampling times. Whereas separate K deficiency and waterlogging caused well-known effects such as polyamines production and sugar accumulation, respectively, waterlogging altered K-induced respiration enhancement (via the C5 -branched acid pathway) and polyamine production, and K deficiency tended to suppress waterlogging-induced accumulation of Krebs cycle intermediates in leaves. Furthermore, the natural 15 N/14 N isotope composition (δ15 N) in leaf compounds shows that there was a change in nitrate circulation, with less nitrate influx to leaves under low K availablity combined with waterlogging and more isotopic dilution of lamina nitrates under high K. Our results show that K deficiency and waterlogging effects are not simply additive, reshape respiration as well as nitrogen metabolism and partitioning, and are associated with metabolomic and isotopic biomarkers of potential interest for crop monitoring.

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.

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.

Changes in 13C/12C of oil palm leaves to understand carbon use during their passage from heterotrophy to autotrophy.

The carbon isotope composition of leaf bulk organic matter was determined on the tropical tree Elaeis guineensis Jacq. (oil palm) in North Sumatra (Indonesia) to get a better understanding of the changes in carbon metabolism during the passage from heterotrophy to autotrophy of the leaves. Leaf soluble sugar (sucrose, glucose and fructose) contents, stomatal conductance and dark respiration, as well as leaf chlorophyll and nitrogen contents, were also investigated. Different growing stages were sampled from leaf rank -6 to rank 57. The mean values for the delta(13)C of bulk organic matter were -29.01 +/- 0.9 per thousand for the leaflets during the autotrophic stage, -27.87 +/- 1.08 per thousand for the petioles and -28.17 +/- 1.09 per thousand for the rachises, which are in the range of expected values for a C(3) plant. The differences in delta(13)C among leaf ranks clearly revealed the changes in the origin of the carbon source used for leaf growth. Leaves were (13)C-enriched at ranks below zero (around -27 per thousand). During this period, the 'spear' leaves were completely heterotrophic and reserves from storage organs were mobilised for the growth of these young emerging leaves. (13)C-depletion was then observed when the leaf was expanding at rank 1, and there was a continuous decrease during the progressive passage from heterotrophy until reaching full autotrophy. Thereafter, the delta(13)C remained more or less constant at around -29.5 per thousand. Changes in sugar content and in delta(13)C related to leaf ranks showed an interesting similarity of the passage from heterotrophy to autotrophy of oil palm leaves to the budburst of some temperate trees or seed germination reported in the literature.

Effects of Potassium Fertilization on Oil Palm Fruit Metabolism and Mesocarp Lipid Accumulation.

Potassium fertilization is commonly practiced in oil palm (Elaeis guineensis) plantations to increase yield. However, its effects on fruit oil content and composition are not well documented. Here, we conducted bunch, metabolomics, and oil composition analyses in two contrasting crosses (Deli × La Mé and Deli × Yangambi) grown under different K fertilization conditions. K availability impacted bunch oil content, resulting in lower water content and higher oil proportion in fruit mesocarp, in Deli × La Mé only, thus showing differential responses of crosses to K. Oil composition at maturity did not significantly change under low K conditions despite clear alterations in fruit metabolism associated with lipid production during maturation, demonstrating the resilience of oil biosynthetic metabolism. However, the analysis of variance in oil content (across K treatments and crosses) demonstrates that sugar availability, lipid synthesis rates, and metabolic recycling are all important in determining the oil content.