Indian Journal of Experimental Biology
Vol. 37, July 1999, pp. 713-717
Ultrastructural changes in rice (Oryza sativa) aleurone cells under NaCI stress
S J Kamath, K S Rao & G Prathapasenan
Department of Botany. The M S University of Baroda, Baroda 390002, Ind ia
Received 19 August 1998; revised 23 March 1999
Ultrastructure of rice ale urone cells after 120 hr of germi nation has been studied. Several changes in the sub-cellular
level s have been noticed in these cells. These changes include occurrence of undi gested aleurone grains, increase in number
of mitochondria, dilation and fragmentation of ER and invigoration of plasmalemma. The ultrastructural changes are
correlated with the disturbances in metabolic functioning of aleurone cells following NaCI salinity.
NaCI is known to cause marked and rapid changes in
the metabolic activities of plant l.2. Plants are more
sensitive to sa linity during germination and early
seedling growth). Ri ce is one of the oldest and most
important cereals. A considerable reductior1 in the
percentage germination, dry matter accumulation and
grain yield is reported in rice when subjected to
NaCI 4.5 . Even though many attempts have been made
to understand the effect of salinity on different
aspects of rice germination 6-8 , little attention is paid
on the structure of aleurone cells Aleurone cells have
important role in the release of hydrolytic enzymes
during germination. The present study was carried out
with an aim to understand the structure of rice
aleurone ce lls under the influence of NaCI.
Materials and Methods
Certified seeds of Olyza sativa L. var GR. I I (salt
sensitive)
were
obtained
from
Agriculture
Department, Gujarat, India. The seeds were presoa ked in distilled water for 2 hr and dehusked
without damag ing the embryo and were surface
sterilised with 0. 1% HgCI 2 for 2 min followed by
thorough washing in di stilled water. The seeds were
blotted and a set of 25 seeds were ITansferred to
sterile petriplates containing Whatman No. I filter
p.a per moistened with 5 ml of--{a) sterile distilled
water for con trol ; and (b) 0.15 M, NaCI solution
(whic h inhibited seedling growth and germination by
50% ).
The seeds were allowed to germinate at 30 o±2°C in
darkness for 120 hr. The embryo alongwith scutellum
wa s separated and the region of grain just above
scutellum was cut. Transverse sections of seeds
measuring 2 to 3 mm thickness were processed for
electron
microscopy
by
fixation
In
2.5%,
glutaraldehyde and 1%, OS04' They were dehydrated
in acetone and embedded in araldite9 . The ultra thin
sections of seeds were stai ned with 2% uranyl
acetate lO and lead citrate II . They were observed under
Philips CM 10 TEM at 80 KY.
Results
Conlrof- The aleurone layer completely surrounded the grain and was tightly bound to the
endosperm and scutellum . The cytoplasm of aleurone
cells had many small vacuoles and a few aleurone
grains at various stages of digestion (Fig. lA, B, C).
The diameter of gra in varied fro m 2.2 to 4.5 11m. A
number of spherosomes measuring 0 .2 to 0.4 11m in
diameter were found arranged in a distinct pattern
around the periphery of the partially digested grain
(Fig I B). They were more concentrated towards the
plasma membrane (F ig . IC). As the aleurone grains,
spherosomes alongwith the vacuoles occupied much
of the space in the aleurone cell , the cytoplasmic
organelles appeared scanty. However, mitochondria,
plastids, ,nicrobodies and free ribosomes were
observed in the cytoplasm (Fig 1D, E). Occasionally
rough ER was observed in the peripheral cytoplasm
of the cells (Fig. 1E). The nucleus was conspicuous
with a single nucleolus and dense nucleoplasm (Fig
IF).
NaCf treated- the aleurone cells of the seeds,
germinated in NaCI solution for 120 hr, had dense
cytoplasm with many small vacuoles (Fig 2A). The
cytoplasmic organelles were in abundance. Aleurone
grains were more in number and they were either in
71 4
INDIAN J EXP BIOL, JULY 1999
Fig. I- (A) A Icurone cell of control rice seed (after 120 hr of germ ination); (B) Partiall y digested aleuro ne grain surrounded by
spherosomcs: (C, 0 , E) Cytop las m of aleu rone showing aleuro ne grain (ag), mitochcrnd ri a (m), rilJosomes (1'), rough endopla"mlc
endoplasmic reticulum (rer), mitochondria (mb); and (F) Aleurone cell nucleus (n) with nucleolus (nu), plastid (pi)
715
KAMATH et al.: ULTRA STRUCTURAL CHANGES IN RICE ALEURONE CELL
undigested fonn or showing just the beginning of
digestion (Fig. 2B, C). The globoid cavities (gc) of
the aleurone grains were partially filled with phytin
globoids (pg); (Fig 2B, C). Usually, the aleurone
grain contained one globoid , but occasionally two
(Fig.- 2B, C). Their size varied from 0.5 to 3.0 m in
diameter. The aleurone grains also contained granular
membrane bound protein carbohydrate bodies (Fig.
2C). A well defined nucleus was seen in the center of
aleurone cell. The chromatin in the nucleus appeared
condensed compared to that of control (Fig. 3A).
Conspicuous changes were seen in distribution of
cytoplasmic organelles such as 'mitochondria and ER.
Mitochondria with indistinct cristae often appeared in
large number in the cytoplasm (Fig. 3B). Besides,
some of the mitochondria were also observed with
vesiculated cristae (Fig. 3C). ER was cistemed and
frequently dilated (Fig. 2D). Few ribosomes were
seen free in the cytoplasm and also attached to ER
Plasmalemma
fonned
vesicular
(Fig.
3A).
invagination into cytoplasm. Vacuoles were often
noticed with intravacuolar membranes In the
cytoplasm of aleurone cells (Fig. 3D, E) .
Discussion
Water uptake during gennination process brings.
about changes in the cytoplasm and its organelles.
The ultrastructural changes of membranes and
organelles such as ER, mitochondria, ribosomes and
microbodies during gennination has been extensivel y
studied and correlated to their metabolic activities in
different plant systems I2. IS.
Aleurone grains are the major sites of protein
storage in aleurone cells. The vacuolation in these
~
Fig. 2- (A) Aleuron e ce ll of NaC I exposed rice seed (after 120 hr of genni nation) ; (8) Undigested a le urone grain showing g loboid
cavit y (gc) and ph yti n g lob ule (pg); (C) Aleurone grain wi th two phytin g lobul es (pg) ; and (D) A leu ro ne cytop lasm showin g fra g ments
of rough endop lasmic reticululll ( rer), protein carbohydrate bodies (pc b) and phytin glob ule (pg).
716
INDIAN J EX P SIOL, JULY 1999
Fi g . 3- Aleurone ce ll of aC I exposed rice seed showin g- (A) Nuc leu s (n) with nucl eous (nu) and condensed chromatin ; (8) Group of
mi toc hondri a (m); (C) Mi tochondri a (m) wi th vesiculated cristae; (0, E) In vag ination of membrane (arrow) into the vac uol es (v).
ce ll s reflects mobili zation of proteins for the growing
embryo. Presence of numerous undi gested aleurone
grain s indicated that NaC I inhibited the synthesis' of
proteolytic en zymes.
Stru ctural changes in tbe aleurone cells of NaCI
trea ted rice seeds clearly indicate their disturbed
phys iol og ica l functioning. Petruzze lli et af. 16
suggested that the high density mitochondria
observed In NaCI exposed cells may be a
compensation for the damage in mitochondrial
fu nction. Vacuolation in mitochondria under stress is
also reported by Smith et al. 17. According to them
vac uolation could be due to hi gh accumulation of Na+
an d/or CI- di fferences in ion compartmentalisation.
Dictyosomes are reported to be active during
germination 18 • The failure to find them in the rice
aleurone cells may be that they are few in number.·
The reduced occurrence of ribosomes could be due to
water stress l9 .
Salinity is known to cause changes in the
membrane lipids 20 and selective permeability of
membranes. Invaginations of plasmalemma into the
vacuoles to form endocytotic vesicles may be a
means to reduce NaCI toxicity22. The chromatin
condenses in the nuclei due to NaCI has been al so
reported in the root cells of barJey 23 and wheat
embryosl 7. According to them the chromatin
condensation is due to suppression of nucleic acid
biosynthesis in NaCI exposed cells. Thus it is
concluded from the present observation that NaCI
KAMATH et al.: ULTRA STRUCTURAL CHANGES IN RI CE ALEURONE CE LL
inhibits seedling growth of rice by altering the cell
structure and its metabolic functioning.
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