328
BIOCHEMICAL SOCIETY TRANSACTIONS
Takahashi, M. & Asada, K. (1975)Plant Cell Physiol. 16,191-194
Wilson, M. T . & Greenwood, C. (1971) Eur. J. Biochem. 22,11-18
Wood, P. M. (1974)Biochim. Biophys. Acta 357,370-379
Aspects of Nucleic Acid Metabolism in Wheat Aleurone Tissue
K. VARTY' and D. L. LAIDMAN
Department of Biochemistry and Soil Science, University College of North Wales,
Bangor, Gwynedd LL57 2UW, Wales, U.K.
During germination in barley, the action of gibberellic acid in the aleurone
tissue induces the synthesis de nouo of hydrolytic enzymes (Bennett & Chrispeels, 1972).
A similar system of induced enzyme synthesis has been characterized in wheat (Collins
et al., 1972). It has been suggested that RNA synthesis is necessary for the
gibberellic acid action, and the hypothesis has consequently arisen that gibberellic acid
action is concerned with the control of gene expression by inducing the synthesis of
new mRNA.
Although earlier studies on barley reported a marked stimulation by gibberellic
acid of rRNA synthesis and of both the ribosome and polyribosome population
of the aleurone cell (Evins, 1971 ;Evins & Varner, 1972), later work has demonstrated
little or no stimulation of rRNA synthesis and no stimulation of total RNA content
(Jacobsen & Zwar, 1974). The confusing situation has been clarified to some extent by
the demonstration that gibberellic acid specifically causes a large increase in the
synthesis of a heterodisperse type of RNA, which consists mainly of a poly(A) RNA
of widely varying molecular sizes (Jacobsen & Zwar, 1974). Since many mRNA species
contain a poly(A) segment (Adesnik et al., 1972), and poly(A)-containingRNA has been
found in plants (Manahan et al., 1973), the possibility arises again that gibberellic acid
initiates hydrolase synthesis in aleurone tissue by specifically inducing the synthesis of
mRNA. The present paper presents results from our investigations on wheat, which
support the findings from barley.
During the first 2 days of germination, the total nucIeic acid content of the wheat
aleurone tissue increased by about 35% (Fig. 1). Thereafter, the content remained
constant up to the sixth day of germination. This pattern of nucleic acid content in the
aleurone tissue was independent of the presence of the embryo, since incubating endosperms gave an identical pattern. Further, gibberellic acid did not significantly affect
the pattern in the incubating endosperms. Fractionation of the total nucleic acid by
chromatography on methylated albumin/silica gel suggested that the increase observed
during the fkst 2 days took place in the RNA fraction and that both the transfer
and ribosomal species underwent similar percentage increases during this period
Warty, 1975).
Coincidentally with the rise in total nucleic acid content during the first 2 days of
germination there was a rapid increase in the rate at which [U-14C]uridine was
incorporated into the total nucleic acid fraction. After the second day, however, when
the total nucleic acid content was no longer increasing, the rate of [U-14C]uridine
incorporation remained high, indicating a maintained high turnover. Removal of the
embryo caused a decrease in the rate of [U-14C]uridineincorporation compared with
that in the germinated grain, and gibberellic acid-stimulated incorporation. These
effectsof the embryo and of gibberellic acid were, however, small compared with the
overall increase in nucleic acid synthesis during the first 2 days. Dual-labelling experiments and fractionation by chromatography on methylated albumin/silica gel did not
reveal incorporation into any particular species of RNA as being specifically influenced
by the embryo or by gibberellic acid Warty, 1975). Thus general nucleic acid
metabolism in wheat aleurone tissue is initiated by the imbibition of water during early
germination, and this process is independent of control by gibberellic acid or any other
* Present address: Departamento de Bioquimica, Division de Estudios Superiores,
Facultad de Quimica, Ciudad Universitaria, Mexico 20, D.F.
1977
329
566th MEETING, CAMBRIDGE
1
0
I
I
I
I
I
I
I
2
3
4
5
6
I
2
3
4
Duration of incubation (days)
Fig. 1. Concentrations of total nucleic acids in wheat aleurone tissue
0 , Tissue of germinating grain; 0 , tissue of endosperms incubated alone; A, tissue of
endosperms incubated with 1 PM-gibberellic acid.
0
Duration of incubation
Fig. 2. Incorporation of [2-3H]udenosine into the poly(A) mRNA fraction of wheat
aleurone tissue
0,Tissue
of endosperms incubated alone;
1 pkf-gibberellic acid.
Vol. 5
0,
tissue of endosperms incubated with
12
330
BIOCHEMICAL SOCIETY TRANSACTIONS
factor emanating from the embryo. Our finding that the increase in total nucleic acid
concentrations, which occurs during the first 2 days of germination in wheat, is not
influenced by gibberellic acid and that the incorporation of labelled precursors into the
total nucleic acids is only slightly stimulated by gibberellicacid,is in broad agreement with
reports of Jacobsen and his colleagues for barley (Jacobsen & Zwar, 1974).
Although gibberellic acid had only a small effect on the incorporation of labelled
uridine into the total nucleic acids of aleurone tissue, it caused a dramatic
increase in the incorporation of [2-3H]adenosine into the fraction of nucleic acid
retained by poly(UtSepharose columns (Fig. 2). These results are also in agreement
with Jacobsen’s results from barley (Jacobsen & Zwar, 1974). Dual-labellingexperiments
using [3H]- and [14C]-uridinehave confirmed the findings obtained with labelled adenosine (Varty, 1975). Since Jacobsen & Zwar (1974) have demonstrated the presence of
poly(A) segments in the material retained by the affinity chromatography, it is most
probable that the nucleic acid synthesis, which is strongly stimulated by gibberellic acid
in both barley and wheat, is that of mRNA. In considering a possible mechanism for
the action of gibberellic acid in aleurone tissue it is important to note that the stimulation
of nucleic acid synthesis by gibberellic acid is evident by the first day of incubation
before the appearance of gibberellic acid-induced a-amylase synthesis, which becomes
manifest on the second day (Varty, 1975) in wheat. This is one of the earliest responses to
gibberellic acid that we have observed in the wheat grain, and it is consistent with the
hypothesis that the induction of mRNA synthesis is an early event in the gibberellic
acid response leading to the induction of a-amylase synthesis.
Adesnik, M., Salditt, M., Thomas, W. & Darnell, J. E. (1972)J. Mol. Biol. 71,21-30
Bennett, P. A. & Chrispeels, M.J. (1972)Plant Physiol. 49,445-447
Collins, C.G.,Jenner, C. F. & Paleg, L. G. (1972)Plant Physiol. 49,404-410
Evins, W.H.(1971)Biochemistry 10,42954303
Evins, W.H.& Varner, J. E. (1972)Plant Physiof. 49,348-352
Jacobsen, J. V. & Zwar, J. A. (1974)Aust. J. Plant Physiol. 1,343-356
Manaham, C . O.,App, A. A. & Still, C. C. (1973) Biochem. Biophys. Res. Commun. 53,
588-595
Varty, K.(1975) Ph.D. Thesis, University of Wales
1977