Transient dissociation of polyribosomes and concurrent recruitment of calreticulin and calmodulin transcripts in gravistimulated maize pulvini

The dynamics of polyribosome abundance were studied in gravistimulated maiz e (Zea mays) stem pulvini. During the initial 15 min of gravistimulation, t he amount of large polyribosomes transiently decreased. The transient decre ase in polyribosome levels was accompanied by a transient decrease in polyr ibosome-associated mRNA. After 30 min of gravistimulation, the levels of po lyribosomes and the amount of polyribosome-associated mRNA gradually increa sed over 24 h up to 3- to 4-fold of the initial value. Within 15 min of gra vistimulation, total levels of transcripts coding for calreticulin and calm odulin were elevated 5-fold in maize pulvinus total RNA. Transcripts coding for calreticulin and calmodulin were recruited into polyribosomes within 1 5 min of gravistimulation. Over 4 h of gravistimulation, a gradual increase in the association of calreticulin and calmodulin transcripts with polyrib osomes was seen predominantly in the lower one-half of the maize pulvinus; the association of transcripts for vacuolar invertase with polyribosomes di d not change over this period. Our results suggest that within 15 min of gr avistimulation, the translation of the majority of transcripts associated w ith polyribosomes decreased, resembling a general stress response. Recruitm ent 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 com plete.