Insight into Phloem Sap Metabolomics: Comparison
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Phloem sap transport is essential for plant nutrition and development since it mediates redistribution of nutrients, metabolites and signaling molecules. However, its biochemical composition is not so well-known because phloem sap sampling is difficult and does not always allow extensive chemical analysis. In the past years, eEfforts have been devoted to metabolomics analyses of phloem sap using either liquid chromatography or gas chromatography coupled with mass spectrometry. Phloem sap metabolomics is of importance to understand how metabolites can be exchanged between plant organs and how metabolite allocation may impact plant growth and development.

  • phloem
  • sap
  • metabolome
  • metabolic cycle

1. Introduction

Plant organs do not grow and operate separately since they are connected via long-distance transport systems (xylem and phloem). While xylem mostly conducts water and nutrients (along with organic molecules at low concentration, such as amino acids, iron-chelates or hormones) from roots to aboveground organs, phloem carries inorganic ions and many organic molecules coming from mature leaves, sustaining growth and development of heterotrophic organs, and from remobilized material, especially in perennials. In effect, in addition to playing a crucial role in allocation of photosynthates, phloem sap transport is crucial to communicate the nutritional status (shoot to root signaling), trigger defense responses or regulate developmental processes [1]. Specialized cells enable the phloem tissue to achieve this: sieve-tube elements (SE; non-lignified cellulosic transport tubes without nucleus, vacuoles, and ribosomes), companion cells (CC; supporting cells capable of gene expression and translation), and phloem parenchyma cells. CC and parenchyma cells are also crucial for (un)loading of transported molecules via plasmodesmata and specialization into transfer cells, respectively. Importantly, SE are connected via sieve plates (SP) coming from the selective hydrolysis of cell walls [2][3]. Phloem sap contains not only metabolites but also proteins (mostly P-proteins) and nucleic acids. P-Proteins may have several morphological forms (granular, filamentous or fibrillar, tubular, and crystalline) depending on developmental stage, environmental conditions and species. Filaments formed by P-proteins play several roles during cell specialization, and sap transport and P-protein aggregation is involved in the rapid sealing of SP pores [4][5].
This overview of phloem structure and function highlights key metabolic features of phloem sap transport (here, the term “phloem sap” refers to the circulating fluid in SE, and does not refer to total phloem fluid, i.e., contained by all cell types: CC, phloem parenchyma cells and SE. First, it redistributes sugars synthesized during photosynthesis from source organs (mature leaves) to sink tissues such as roots, flowers, fruits and seeds [6][7]. In general, sugars are at a very high concentration (hundreds of millimolar) and, combined with P-proteins, impacts sap viscosity. Second, signalling mediated by phloem sap may involve metabolites that are known to act as regulators or allosteric effectors, such as sucrose or hormones [1][7][8] (e.g., abscisic acid or cytokinins). Third, N- and S-containing molecules (amino acids and others) are also amongst phloem metabolites resulting from nitrate and sulphate assimilation in leaves [9][10][11][12][13]. For these reasons, metabolite transport via phloem sap is essential for plant growth and development, since it redistributes carbon, nitrogen and sulfur, and is involved in long-distance signalling.

2. Phloem Sap Metabolites

Metabolic analyses carried out in the past decades with different techniques have shown that phloem sap is not just a concentrated sucrose/amino acid solution. In fact, many compounds of different metabolite families have been found, suggesting that phloem sap has metabolic functions beyond C, N and S redistribution from source leaves to sink organs. Nevertheless, it must be recognized that metabolites found in phloem sap depend on the analytical technique (typically, HPLC, GC-MS or LC-MS), and thus our current knowledge might not be strictly representative. Methods coupled to mass spectrometry allow analyses of many more metabolites than just HPLC, and thus GC-MS and LC-MS usually give access to a more representative picture of the metabolome. That said, GC-MS is usually dedicated to primary metabolites while LC-MS is more adapted to metabolites of higher molecular mass (such as secondary metabolites). In terms of concentration, current metabolomics techniques (such as GC-MS and LC-MS) are semi-quantitative and adapted to comparing samples rather than providing absolute concentrations (in mM). In general, sugars are the major component of phloem sap, representing more than 70% of phloem sap metabolites. In herbaceous plants, sucrose concentrations range from 400 to 1400 mM [14], with some variation (maize (Zea mays) 900 mM, [15]; 844 mM, [16]; rice (Oryza sativa) 574 mM, [17]; barley (Hordeum vulgare) 1030 mM, [18]; wheat (Triticum aestivum) 251 mM, [19]; and castor bean (Ricinus communis) 270 mM, [20]). Likewise, sucrose concentration varies considerably between tree species from 65 mM to 1 M (for instance, oak (Quercus robur) ~1 M, [21]; beech (Fagus sylvatica) 790 mM, [22]; magnolia (Magnolia kobus) 850 mM, [22]; Eucalyptus (Eucalyptus globulus) 220 mM, [23]; and lemon tree (Citrus limon) 65 mM, [24]). It is believed that other soluble sugars such as fructose and glucose are also present in phloem sap, and sometimes at relatively high concentrations. Evidence for the occurrence of hexoses has been provided with reliable phloem sap sampling techniques, including stylectomy [18][21][25][26], the EDTA method [26][27][28][29][30][31], the incision method [32][33] and centrifugation [34]. Fructose and glucose content depends on species (e.g., found in lemon tree, wheat and maize [27][28] but absent in castor bean [20][35]) and developmental stage; for instance, it depends on grain filling stage, N fertilization and CO2 concentration in wheat [31][36][37]. When hexose concentration is high, it likely reflects the action of sucrose-cleaving enzymes during sampling (invertase, sucrose synthase) and/or the contribution of hexose from petiole tissues to the extract obtained by exudation [38][39]. Amino acids are the second most abundant metabolites in phloem sap, representing a small percentage of the total sap concentration with substantial variations between species, from about 5% to 15% [15][31][40], up to 360 mM in maize [28]. The prevalent amino acids are glutamate (Glu), glutamine (Gln), aspartate (Asp) and asparagine (Asn) [17][20][23][32][36][41][42][43][44][45]. However, in other species, other amino acids have been found to be more abundant: proline (Pro), alanine (Ala) or glycine (Gly) in Arabidopsis, Citrus species or maize [15][24][27][28][40]. Interestingly, amino acid phloem sap composition changes with the photoperiod. Under long days, Glu, Asp, and Ser decrease while Gln and Asn increase compared to short days, in Arabidopsis [46].  Organic acids are also present in phloem sap, at rather small concentrations, depending on the species. In Arabidopsis, total organic acid concentration is less than 0.5 mM [31], i.e., about 5% of the total metabolites. The same amount (5%) has been found in maize [28] and Eucalyptus (5 mM, [23]) but higher content has been found in others, e.g., castor bean (30 to 47 mM [20]) and lemon tree (44 to 232 mM, [24]). Most represented organic acids are from the tricarboxylic acid pathway: malate, citrate and aconitate [23][28][32][35]. Quinate has also been found in Prunus [34] and Citrus [24] species. In squash (Cucurbita maxima), lactate is as concentrated as malate [32], and propanoate and maleate are as concentrated as malate in Arabidopsis [40]. Free fatty acids are often overlooked in phloem sap composition. However, they may be present at a higher content than organic acids in most species tested so far. For example, fatty acids and their derivatives reach 13% of total metabolites in Arabidopsis [40]. Their concentration has been found to be up to 5 mM in Citrus species [24][27]. Palmitate and oleate appear to be prevalent fatty acids, along with stearate (Citrus, Arabidopsis) [27][40][47] and specific oxylipins, and phospholipids were detected only when the EDTA method was used (in Arabidopsis) [48]. It has been suggested that sap collection methods can change fatty acid composition or perhaps explain their presence in sap samples.

3. Metabolic Pathways Reflected by Phloem Sap Metabolome

It is worth noting that the diversity of metabolites found in phloem sap (and summarized in Section 3 above) reflects different metabolic pathways such as sugar metabolism, nitrogen and sulfur assimilation and amino acid synthesis, the tricarboxylic acid pathway, arginine metabolism and polyamine synthesis. There is presently some uncertainty as to whether some of these pathways take place in phloem cells themselves (CC and SE) or only reflect source cell metabolism. It is well-accepted that major amino acids and sugars come from source cell N assimilation and photosynthesis, respectively. Specific amino acid synthesis may occur in phloem cells themselves such as asparagine since a phloem isoform of asparagine synthetase has been found in Arabidopsis [49][50]. Pioneering studies have shown that several enzymatic activities are absent from phloem sap in particular those involved in sugar cleavage and utilization [51]. This is consistent with the fact that phloem sugar transport should not compete with sugar catabolism. Accordingly, it is generally believed that SE mitochondria are small and not numerous probably reflecting limited catabolic activity [52]. GC-MS-based metabolomics analysis of phloem sap in Citrus cultivars has shown that many metabolite contents change along with the total sap osmolarity [24]. This indicates that the increase in total sap concentration is not simply associated with a general increase in loading capability of source leaves but is also associated with modifications in metabolic pathways. This is readily visible via combined univariate-multivariate statistics, with total sap concentration as a response variable (Figure 1). As phloem sap concentration changes, sucrose covaries with organic acids including malate (cluster 2, Figure 1a, red arrow). Even so, the amino acids hexoses and inositol are more correlated to the total concentration than sucrose (Figure 1b). Amongst amino acids, arginine, aspartate and asparagine are good markers of a high phloem sap concentration while cysteine appears to be a marker of low phloem sap concentration. Metabolites that increase concurrently with total phloem sap concentration include pyruvate and glycolate (cluster 4, Figure 1a, blue arrow). Such changes are reminiscent of the aspartate ‘cycle’ (illustrated in Figure 1c, in red) which connects organic acid metabolism (malate) to aspartate via arginine biosynthesis.
/media/item_content/202304/643796d5a64e5ijms-24-06917-g002.png
Figure 1. Metabolic analysis of phloem sap in Citrus. (a) Hierarchical clustering analysis showing covariation groups of metabolites when sap composition changes between cultivars. Sucrose and pyruvate are shown by red and blue arrows, respectively. (b) Volcano plot combining univariate (–log p-values) and multivariate (orthogonal partial least square (OPLS) loadings, pq1) analyses to identify best drivers of total phloem concentration (which increases from left to right). p-value thresholds (0.01 and Bonferroni) are shown with dash-dotted lines. (c) Simplified aspartate cycle (red) and its connections to other pathways. Abbreviations: 2-OG, 2-oxoglutarate; GABA, 4-aminobutyrate; Glu, glutamate; MTA, S-methyl thioadenosine; OAA, oxaloacetate; PEP, phosphoenolpyruvate; SAM, S-adenosyl methionine; SMA, S-adenosyl methioninamine. This figure has been generated by re-analysis of the original dataset in [24], using MeV (ANOVA) and Simca® (OPLS).

4. Phloem Sap Metabolites and Plant Resistance to Environmental Cues

Amino acid composition in phloem sap appears to respond to environmental factors such as nutrient availability. When supplied with high levels of nitrogen (N), Glu content increase in Arabidopsis phloem sap and conversely, Pro, Gln and γ-aminobutyrate (4-aminobutyrate; GABA) increase under low N availability [31]. Total amino acid concentration also changes with N supply as has been shown in canola [53]. Other conditions such as low phosphorus (P) availability and water deficit also affect amino acid composition [31][54]. Polyol content in phloem sap (such as sorbitol and erythritol) have also been found to impact drought and cold tolerance [24][28]. Recently, metabolomics analyses of cucumber plants subjected to phosphorus deficiency have shown important changes in phloem sap metabolome, not only in carbohydrates (galactitol, fructose) but also in organic acids (e.g., oxalate, citrate, fatty acids) along with nitrogenous compounds (e.g., ethanolamine, 4-aminobutyrate and pyroglutamate) [33]. Metabolomics analyses of phloem sap exudates during root waterlogging have found a change in the sugar to organic acid ratio, suggesting (i) a crucial role of the balance between loading in shoots and unloading in roots for sap composition and (ii) potentially, a negative feedback of metabolite accumulation in phloem sap onto shoot metabolism, such as sugar interconversions and respiration [55].

5. Phloem Sap Metabolome: Unforeseen Whole-Plant Metabolic Cycles?

The presence of many metabolites in phloem sap raises the question of their fate and role when they reach sink organs, including the roots. While the general principle of sugar and amino acid utilization is well-accepted (C and N redistribution from source leaves to sinks), uncertainty remains as to whether other metabolites are simply consumed or converted to derivatives that can be recycled back to shoots via xylem sap. It has been argued that a metabolic cycle between roots and shoots takes place with some amino acids exported by roots to shoots (including in non-legumes) [56][57]. It has also been shown that organic acids such as malate can be found in xylem sap at a significant concentration, partly explaining the CO2 release by shoots via the malic enzyme which produces pyruvate (in addition to CO2 liberation from dissolved CO2 and bicarbonate) [58][59]. Since many organic acids can be found in phloem sap, they could be converted to malate (via the Krebs cycle) in roots and then exported back to shoots via the xylem. It is also intriguing to find free pyruvate in phloem sap [24]. Its origin could be linked to either transamination activity (via alanine transaminase) or glycolytic activity in SE. Pyruvate could then be used by sink organs to resynthesize alanine or used by mitochondrial metabolism. Taken as a whole, a pyruvate cycle could take place at the whole plant scale, whereby (i) pyruvate reaching roots via the phloem sap could be recycled to organic acids such as malate; (ii) malate would then be exported back to shoots, where it could be partly cleaved to pyruvate and carbon dioxide (Figure 2).
/media/item_content/202304/643796fb649b1ijms-24-06917-g003.png
Figure 2. Two examples of possible metabolic cycles suggested by phloem sap metabolome. (a) Pyruvate cycle. Alanine and pyruvate are interconvertible via alanine transaminases with glutamate (Glu) and 2-oxoglutarate (2OG). They are present in phloem sap and could feed malate production in sink organs (roots) via the tricarboxylic acid pathway (TCAP), and malate can then be sent back to shoots via xylem sap. In shoots, malate can be cleaved by the malic enzyme (ME) to produce pyruvate. (b) Putrescine cycle. Arginine, methionine and putrescine synthesized in shoots can be used to feed higher degree polyamine synthesis (spermine, spermidine) in roots, and putrescine can be oxidized to 4-aminobutyrate (GABA) via polyamine oxidase (PAO) and aldehyde dehydrogenase (ALDH) and then to malate via the TCAP. As in (a), malate can be exported back to shoots to feed organic acid synthesis, and thus glutamate and aspartate production via N assimilation, sustaining arginine synthesis. The asterisk stands for the contribution of methionine derivatives: SMM and SAM.

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