Regulation of Protein Degradation and Protease Expression by Mannose in Maize Root Tips. Pi Sequestration by Mannose May Hinder the Study of Its Signaling Properties

Abstract

The effects of mannose (Man) and glucose (Glc) on central metabolism, proteolysis, and expression of the root starvationinduced protease (RSIP; F. James, R. Brouquisse, C. Suire, A. Pradet, P. Raymond [1996] Biochem J 320: 283-292) were investigated in maize (Zea mays L. cv DEA) root tips. Changes in metabolite concentrations (sugars, ester-phosphates, adenine nucleotides, and amino acids) were monitored using in vivo and in vitro 13 C-and 31 P-NMR spectroscopy, in parallel with the changes in respiration rates, protein contents, proteolytic activities, and RSIP amounts. The inhibition of proteolysis, the decrease in proteolytic activities, and the repression of RSIP expression triggered by Man, at concentrations usually used to study sugar signaling (2 and 10 mm), were found to be related to a drop of energy metabolism, primarily due to a Man-induced Pi sequestration. However, when supplied at low concentration (2 mm) and with the adequate phosphate concentration (30 mm), energy metabolism was restored and Man repressed proteolysis similarly to Glc, when provided at the same concentration. These results indicate that Man should be used with caution as a Glc analog to study signalization by sugars in plants because possible signaling effects may be hindered by Pi sequestration.

A metabolic study of the regulation of proteolysis by sugars in maize root tips: effects of glycerol and dihydroxyacetone

“…We showed previously that, in maize root tips, mannose might be considered to have the same down-regulation effect on proteolysis as Glc, provided that Pi status is maintained (Brouquisse et al 2001). We later showed that, although very slowly phosphorylated by hexokinase, 3-OMG does not repress proteolysis (Cortès et al 2003).…”

“…Maize seeds (Zea Mays L. cv DEA, from Pioneer France Maïs, Toulouse, France) germination, root tip preparation and incubation conditions have already been described (Brouquisse et al 2001). Incubation medium A was composed of a mineral medium (Saglio and Pradet 1980), plus 10 mM Mes-KOH at pH 6.0.…”

“…To monitor growth, samples of 20 root tips were harvested at different times, dried on filter paper, weighed on tinfoil, and root tip lengths were measured. In-vivo NMR experiments were performed as described (Brouquisse et al 2001) except that 4000 excised root tips were used for each experiment and the partial oxygen pressure was regulated at 80% in the reservoir of external medium. The renewal of the medium around the sample varied from 24 to eight times a minute between the start and the end of the experiment, because of the growth (the average root tip length was 2.5 mm at the start and 3.7 mm after 48 h for the Glyc or DHA-fed samples).…”

“…In excised maize root tips, a vacuolar serine endopeptidase (EP) is induced in response to glucose (Glc) starvation (James et al 1996). This protease, named Root Starvation Induced Protease (RSIP), accounts for about 80% of the total EP activity in the starved roots (James et al 1996;Brouquisse et al 1998Brouquisse et al , 2001. Cysteine protease and chymotrypsin/subtilisin-like protease inhibitor are also induced in sugar starved maize root tips (Chevalier et al 1995).…”

“…Inorganic phosphate was also reported to inhibit the expression of sugar-inducible genes encoding vegetative storage protein, lipoxygenase A, chalcone synthase, proteinase inhibitor II and class I patatin, in the presence or absence of sugars (Sadka et al 1994), and could be part of, or directly interact with, sugar sensing mechanisms. However, Pi did not relieve the repression by sugars of genes involved in glyoxylic cycle (Graham et al 1994), photosynthesis (Jang and Sheen 1994), or proteolysis (Brouquisse et al 2001). …”