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.

The dynamics of polyribosome abundance were studied in gravistimulated maize (Zea mays) stem pulvini. During the initial 15 min of gravistimulation, the amount of large polyribosomes transiently decreased. The transient decrease in polyribosome levels was accompanied by a transient decrease in polyribosome-associated mRNA. After 30 min of gravistimulation, the levels of polyribosomes and the amount of polyribosome-associated mRNA gradually increased over 24 h up to 3-to 4-fold of the initial value. Within 15 min of gravistimulation, total levels of transcripts coding for calreticulin and calmodulin were elevated 5-fold in maize pulvinus total RNA. Transcripts coding for calreticulin and calmodulin were recruited into polyribosomes within 15 min of gravistimulation. Over 4 h of gravistimulation, a gradual increase in the association of calreticulin and calmodulin transcripts with polyribosomes was seen predominantly in the lower one-half of the maize pulvinus; the association of transcripts for vacuolar invertase with polyribosomes did not change over this period. Our results suggest that within 15 min of gravistimulation, the translation of the majority of transcripts associated with polyribosomes decreased, resembling a general stress response. Recruitment of calreticulin and calmodulin transcripts into polyribosomes occurred predominantly in the lower pulvinus one-half during the first 4 h when the presentation time for gravistimulation in the maize pulvinus is not yet complete.The vector of the gravitational force provides a constant cue for the direction of plant growth. Changes in the orientation of a plant relative to the gravity vector result in positive or negative gravitropic growth of roots and shoots, respectively. The gravity vector is thought to be perceived through changes in tensegrity or the pressure exerted by statoliths (Sack, 1991; Kaufman et al., 1995;Yoder et al., 2001) or by the entire protoplast (Staves, 1997). Sedimentation of statoliths in starch-containing cells can occur within seconds to minutes of gravistimulation (Sack, 1991; Kaufman et al., 1995;Yoder et al., 2001). A cascade of coordinated biochemical events subsequently amplifies and distributes the signal through a responsive tissue, resulting in the redistribution of auxin between upper and lower sides of the gravistimulated organ and initiating the bending response (for review, see Kaufman et al., 1995; Lomax et al., 1995;Sinclair and Trewavas, 1997;Chen et al., 1999;Rosen et al., 1999). Although the gravitropic response of plants has been the subject of intensive research, our understanding of the signaling processes linking perception of gravity to differential growth is still limited.The stem pulvini of cereal grasses have previously been used as model systems for the investigation of gravitropic growth in a number of studies (Dayanandan and Kaufman, 1984; Kaufman et al., 1987 Kaufman et al., , 1995Winter et al., 1997; Collings et al., 1998;Perera et al., 1999Perera et al., , 2001 Johannes et al., 2001). The pulvini are d...