Phosphorus Nutrition Influence on Starch and Sucrose Accumulation, and Activities of ADP-Glucose Pyrophosphorylase and Sucrose-Phosphate Synthase during the Grain Filling Period in Soybean

Response of Soybean Root to Phosphorus Deficiency under Sucrose Feeding: Insight from Morphological and Metabolome Characterizations

Phosphorus (P) is one the least available essential plant macronutrients in soils that is a major constraint on plant growth. Soybean (Glycine max L.) production is often limited due to low P availability. The better management of P deficiency requires improvement of soybean's P use efficiency. Sugars are implicated in P starvation responses, and a complete understanding of the role of sucrose together with P in coordinating P starvation responses is missing in soybean. This study explored global metabolomic changes in previously screened low-P-tolerant (Liaodou, L13) and low-P-sensitive (Tiefeng 3, T3) soybean genotypes by liquid chromatography coupled mass spectrometry. We also studied the root morphological response to sucrose application (1%) in P-starved soybean genotypes against normal P supply. Root morphology in L13 genotype has significantly improved P starvation responses as compared to the T3 genotype. Exogenous sucrose application greatly affected root length, root volume, and root surface area in L13 genotype while low-P-sensitive genotype, i.e., T3, only responded by increasing number of lateral roots. Root : shoot ratio increased after sucrose treatment regardless of P conditions, in both genotypes. T3 showed a relatively higher number of differentially accumulated metabolites between P-starved and normal P conditions as compared to L13 genotype. Common metabolites accumulated under the influence of sucrose were 5-O-methylembelin, D-glucuronic acid, and N-acetyl-L-phenylalanine. We have discussed the possible roles of the pathways associated with these metabolites. The differentially accumulated metabolites between both genotypes under the influence of sucrose are also discussed. These results are important to further explore the role of sucrose in the observed pathways. Especially, our results are relevant to formulate strategies for improving P efficiency of soybean genotypes with different P efficiencies.

Dithiothreitol decreases in vitro activity of ADP-glucose pyrophosphorylase from leaves of apple (Malus domestica Borkh.) and many other plant species

Inclusion of dithiothreitol (DTT) in the extraction buffer and pre-incubation of apple leaf ADP-glucose pyrophosphorylase (AGPase) with DTT resulted in a decrease in AGPase activity whether the assay was performed in the presence or absence of 3-phosphoglycerate (PGA). When PGA was included in the pre-incubation mixture or when pre-incubation of AGPase with PGA was followed by DTT, the latter did not cause any decrease in AGPase activity. However, once AGPase was decreased by DTT, subsequent incubation of the enzyme with PGA did not reverse the decrease. Pre-incubation of AGPase from leaves of Arabidopsis thaliana, sorghum, soybean, tobacco, spinach, wheat, barley, tomato and potato, and tubers of potato with DTT, generally caused a decrease in AGPase activity when assayed in the presence of PGA. When assayed in the absence of PGA, however, a diverse response of AGPase was observed among species to pre-incubation with DTT. The activity of AGPase from potato tubers was increased by DTT; the activity of AGPase from both potato and tomato leaves was not affected by DTT; the activity of AGPase from leaves of other species was decreased by DTT. It is concluded that DTT decreases in vitro activity of AGPase from leaves of apple and many other plant species such that DTT should not be routinely included in the extraction or assay mixture of leaf AGPase.

Coordination of protein and mRNA abundances of stromal enzymes and mRNA abundances of the Clp protease subunits during senescence of Phaseolus vulgaris (L.) leaves

Our objective was to determine the coordination of transcript and/or protein abundances of stromal enzymes during leaf senescence. First trifolioliate leaves of Phaseolus vulgaris L. plants were sampled beginning at the time of full leaf expansion; at this same time, half of the plants were switched to a nutrient solution lacking N. Total RNA and soluble protein abundances decreased after full leaf expansion whereas chlorophyll abundance remained constant; N stress enhanced the decline in these traits. Abundances of ribulose-1,5-bisposphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39), Rubisco activase and phosphoribulokinase (Ru5P kinase; EC 2.7.1.19) decreased after full leaf expansion in a coordinated manner for both treatments. In contrast, adenosine diphosphate glucose (ADPGlc) pyrophosphorylase (EC 2.7.7.27) abundance was relatively constant during natural senescence but did decline similar to the other enzymes under N stress. Northern analyses indicated that transcript abundances for all enzymes declined markedly on a fresh-weight basis just after full leaf expansion. This rapid decline was particularly strong for the Rubisco small subunit (rbcS) transcript. The decline was enhanced by N stress for rbcS and Rubisco activase (rca), but not for Ru5P kinase (prk) and ADPGlc pyrophosphorylase (agp). Transcripts of the Clp protease subunits clpC and clpP declined in abundance just after full leaf expansion, similar to the other mRNA species. When Northern blots were analyzed using equal RNA loads, rbcS transcripts still declined markedly just after full leaf expansion whereas rca and clpC transcripts increased over time. The results indicated that senescence was initiated near the time of full leaf expansion, was accelerated by N stress, and was characterized by large decline in transcripts of stromal enzymes. The decreased mRNA abundances were in general associated with steadily declining stromal protein abundances, with ADPGlc pyrophosphorylase being the notable exception. Transcript analyses for the Clp subunits supported a recent report (Shanklin et al., 1995, Plant Cell 7: 1713-1722) indicating that the Clp protease subunits were constitutive throughout development and suggested that ClpC and ClpP do not function as a senescence-specific proteolytic system in Phaseolus.

Cloning and developmental expression of the sucrose-phosphate-synthase gene from spinach

A 561-base-pair (bp) polymerase-chain-reaction (PCR) product of sucrose-phosphate synthase (SPS) was amplified using degenerate oligonucleotide primers corresponding to tryptic peptides of SPS (EC 2.4.1.14) from spinach (Spinacia oleracea L). Crucial to the primer specificity and the synthesis of the 561-bp product was the use of primer pools in which the number of degenerate primer species was limited. A full-length cDNA was subsequently obtained by screening a cDNA bacteriophage library with the 561-bp product of SPS and 5' PCR-RACE (Rapid Amplification of cDNA Ends). The 3530-bp cDNA of SPS encoded for a 1056-amino-acid polypeptide of predicted molecular mass of 117 kDa. The deduced amino-acid sequence of spinach SPS showed regions of strong homology with SPS from maize (A.C. Worrell et al., 1991, Plant Cell 3, 1121-1130); amino-acid identity was 54% over the entire protein. Western and Northern analyses of root, petiole and spinach leaf tissue showed that SPS was expressed in an organ-specific manner, being predominantly localized in the leaf. The accumulation of SPS protein and mRNA during leaf development coincided with the early rapid phase of leaf expansion and the apparent transition of the leaf from sink to source status. Levels of SPS mRNA and protein were reduced during the acclimation of leaves to low-irradiance conditions. Transfer of low-irradiance-adapted leaves to higher-irradiance conditions resulted in a gradual increase in SPS protein and mRNA. Diurnal changes in irradiance did not alter SPS protein or transcript levels, indicating that short-term regulation of SPS primarily involves a modulation of enzyme activity.