A Diurnal Component to the Variation in Sieve Tube Amino Acid Content in Wheat

Abstract

We have used high-sensitivity capillary electrophoresis coupled to a laser-induced fluorescence detection method to quantify 16 amino acids in wheat (Triticum aestivum) sieve tube (ST) samples as small as 2 nL collected by severing the stylets of feeding aphids. The sensitivity of the method was sufficient to determine a quantitative amino acid profile of individual STs without the need to bulk samples to produce larger volumes for analysis. This allowed the observation of the full range of variation that exists in individual STs. Some of the total concentrations of amino acids recorded are higher than those reported previously. The results obtained show variation in the concentrations of phenylalanine (Phe), histidine/valine (His/Val), leucine/ isoleucine (Leu/Ile), arginine, asparagine, glutamine, tyrosine (Tyr), and lysine (Lys) across the ST samples. These could not be explained by plant-to-plant variation. Statistical analyses revealed five analytes (Tyr, Lys, Phe, His/Val, and Leu/Ile) that showed striking covariation in their concentrations across ST samples. A regression analysis revealed a significant relationship between the concentrations of Tyr, Lys, Phe, Leu/Ile, His/Val, asparagine, arginine, and proline and the time of collection of ST samples, with these amino acids increasing in concentration during the afternoon. This increase was confirmed to occur in individual STs by analyzing samples obtained from stylet bundles exuding for many hours. Finally, an apparent relationship between the exudation rate of ST sap and its total amino acid concentration was observed: samples containing higher total amino acid concentrations were observed to exude from the severed stylet bundles more slowly.

Diurnal changes in assimilate concentrations and fluxes in the phloem of castor bean (Ricinus communis L.) and tansy (Tanacetum vulgare L.)

“…There was no obvious external or internal factor, which could explain the large variation from sample to sample and from plant to plant. A similar conclusion had been reached for amino acid concentrations in single sieve tubes of wheat (Gattolin et al 2008). The least variance was observed in phloem bleeding sap obtained from the cut terminal inXorescence of 3-month-old Ricinus plants, whereas the data from exudates of 10-month-old Ricinus were much more scattered.…”

“…The analysis of the time courses and the concentrations of phloem solutes are further complicated by inexplicable variations between individual plants (Peuke et al 2001) and between individual sieve tubes of the same plant (Gattolin et al 2008). In addition, Fiehn (2003) reported a strong variation of metabolite levels between the phloem sap of individual leaves of the same plant and more so between leaves of diVerent plant individuals, although they were of the same developmental stage.…”

A speciation model of essential trace metal ions in phloem

“…A better methodology utilizes aphid stylets to facilitate the exudation of larger volumes of phloem sap [58][59][60][61][62]. Aphids extract phloem sap from plants by inserting their stylets into the plant.…”

Symbiotic bacteria enable insect to use a nutritionally inadequate diet

“…Sandström & Pettersson 1994;Girousse et al 1996;Corbesier et al 2001;Karley et al 2002). Compounding this variation, the amino acid profile of phloem sap also varies at the fine scale of the individual sieve elements, as is highlighted by the threefold variation in total phloem amino acid content among the uniformly aged plants reared under tightly controlled conditions in this study (figure 1b), and the fourfold variation in total amino acid concentration among sieve elements from a single wheat plant studied by Gattolin et al (2008). We can be confident that aphids use this range of sieve elements because the phloem samples are collected by stylectomy of feeding aphids (see §2), and an individual pea aphid can feed from a single sieve element of V. faba for many hours ( Wilkinson & Douglas 1998).…”