Introduction
Since the widespread introduction of super rice
varieties, some large-ear type rice varieties are often used for high-yield or
super-high-yield rice production due to multiple advantages, such as producing
more glumous flowers, large storage capacity and high yield potential.
Although, the yield record per plant has been broken, the total yield in
large-scale production has failed to improve significantly, indicating that
some of these varieties have shortcomings, such as involuntary degradation of
secondary glumous flowers, and pollen sterility or infertility. These problems
lead to more substantial variations in the number of surviving
spikelets, grain number and rate of seed set. Therefore, yield and quality are
not very stable, and their potential is not fully attained (Cheng et al. 2007; Wu et al. 2007; Yang and Zhang 2010; Dong et al. 2015; Hu et al.
2016). Studies have confirmed that the differentiation and degradation of
spikelets are not only controlled by genetics, but also by environmental
ecology and cultivation conditions (Dong
et al. 2015; Tian et al. 2016). The spikelets differentiation and degradation
rate was affected by sowing date. For example, it was found that the spikelets
differentiation decreased, the degradation rate increased, and the total
spikelets were decreased accompanied by the delay of sowing date (the data was
unpublished), especially large panicle rice, spikelet differentiation and
degradation are quite different under different environmental conditions. Even
for the same rice plant, on the secondary branches, the spikelet
differentiation and degradation are very unstable, with large variation in
spikelet numbers (Kovi et al. 2011; Dong et al. 2015; Zhang et al. 2016). Researchers have also found the NSC content and C/N ratio
in the vegetative organs before heading is closely related to spikelet
differentiation and degradation. At 15 days before heading, higher NSC content
is beneficial to spikelet differentiation and reduced degradation, but the
opposite occurs at 20 days or 25 days before heading (Dong et al. 2017). It is known that sugar is an important regulator of
plant growth and gene expression. It is not only an energy source and a
structural substance, but also has a hormone-like primary messenger effect in
signal transduction (Sheen et al.
1999; Horacio and Martinez 2013; Lastdrager
et al. 2014) and regulates plant growth, development, maturation and
senescence. And many other processes have a regulatory role (Koch et al. 2000; Lastdrager et al. 2014). The glucose concentration
is associated with mitogenic activity during the development of Arabidopsis cotyledons, and glucose is a
developmental stimulating factor regulating the expression of cyclin D genes (Riou et al. 2000). Fructan accumulation and consumption at the
pre-flowering stage can strongly influence the formtion of panicles, and
therefore it is believed that the fructan metabolism has an important
physiological significance (Hendrix et
al. 1986). Within 8 days
before flowering, a strong reduction in sugar concentration in the panicle
occurs, and this is closely associated with the final number of spikelets
produced (Wang et al. 1997).
Therefore, spikelet development is sugar-reducing dependent, and this reduction
mainly comes from hydrolysis of fructose and degradation of sucrose. These
studies suggest that soluble sugar, which is an important component of NSC, is
closely related to cell division in seeds or young spikes, and organ
differentiation and development. Sugars are not only an energy source but also
an important signal substance. However, there is little known on how NSC and
its components are related to the differentiation and degradation of young rice
spikelets. The mechanism underpinning this relationship is still unclear. Thus,
it is still a popular topic of current research and worthy of further
investigation.
In this study, the large panicle
hybrid japonica rice variety Yougyou 1540 was used as the crop material, and
the crop was managed using standard high-yield cultivation practices. Different
concentrations of exogenous soluble sugar (glucose and sucrose) were sprayed
during the panicle differentiation period, and the differentiation and
degradation of the spikelets and branches were monitored. These results are
expected to form the basis for further studies on the regulation mechanism of
NSC on rice spikelet differentiation and development.
Materials and Methods
Experimental field and testing variety
The experiments were carried out in the Guli modern
agricultural demonstration zone located in Changshu City, Jiangsu Province
(31°34'53''N, 120°52'25''E) in 2016 and 2017. Soil in the experimental field is
clay, and wheat has been grown and harvested there before. In 2016, the organic
matter content of the soil tillage layer was 2.59% and available nitrogen,
phosphorus and potassium were 127.8, 7.8 and 118.3 mg/kg, respectively. In
2017, the organic matter content of the soil tillage layer was 2.49%, and
available nitrogen, phosphorus and potassium were 135.3, 8.2 and 117.9 mg/kg.
Experimental design
The heading time was recorded by counting the
number of remaining leaves. During the initial heading period (25 ± 1 d before
heading), from 7: 00 to 8: 00, glucose solutions (167, 333, 500 and 667 mM, termed as G1, G2, G3 and G4) we sprayed to the rice plants at the top
of the leaves. At the same time, sucrose solutions (87, 175, 263, 351 mM,
termed as S1, S2, S3 and S4) were also applied. The same amount of water was
sprayed as a control (CK). There was no rain during the treatment period. An
RX-60AM high-speed seedling transplanter was used to transplant seedlings (2 to
3 seedlings per hole) during dry conditions. Plot area was 60 m2 (10
m × 6 m) with 3 replicates. The seeds were sown on May 15 and the seedlings
were transplanted on June 15. Seedling age was 31 days with 5.3 to 5.5 leaves. The machine-inserted row spacing was 33 cm and
hole-spacing was 12 cm, with 2 to 3 seedlings per hole. Seedling planting
density is 64×104. The heading period began on September 2nd.
A total of 270 kg/hm2 of
pure nitrogen (N: P2O5:K2O = 2:1:1) was
applied over the entire growth period. The ratio of the nitrogen fertilizer was
4:2:2:2, comprising the following fertilizers: base, tillering, panicle and flower, respectively.
Phosphate fertilizer was applied as base fertilizer once. The ratio of
potassium fertilizer was base fertilizer: panicle fertilizer (flower promotion
fertilizer), 5:5. Appropriate cultivation measures such as water management and
pest control were carried out in accordance with their respective high-yield
cultivation requirements. The experimental design in the first and second year
was identical.
Measurements and methods
Determination of the
differentiation and degradation of branch and spikelet
At heading stage (2/3 spikelets developed), 10 main
stems were selected for measuring the differentiation and degradation of
branches and spikelets. Using the Matsushima Shozo trace method (Matsushima 1966), the primary branches, the number of extant and
degraded primary spikelets, secondary branches, and secondary spikelets were
observed and recorded. The degraded spikelets exhibit a white-small-flower-like
film trace. Some can be counted with the naked eye, but others have smaller
traces, so a stereo microscope was used. The numbers of spikelets and branches
per panicle were the sum of all those which degenerated or were still alive.
The primary branches per spike were evenly divided into upper, middle and lower
parts (if there were 11 or 13 primary branches, then the ratio was 4:3:4 or
4:5:4). The numbers of upper, middle and lower branches and spikelets were
counted. Degradation rates are: primary branch (spikelets) degradation rate (%)
= the number of degraded primary branches (spikelets) / the number of
differentiated primary branches (spikelets) × 100%; secondary branch
(spikelets) degradation rate (%) = the number of secondary branches (spikelets)
/ the number of differentiated secondary branches (spikelets) × 100
Determination of total nitrogen content and NSC
content
Fifteen days before and at heading stage, two
plants with identical growth and average number of tillers were selected; the
ears, leaves and sheaths were separated and packed in kraft paper bags. They
were first placed for 60 min in a constant temperature oven at 105℃, then
dried and weighed at 80℃. Stem sheaths and leaves were crushed and sifted
by a small high-speed mill for the determination of nitrogen (N) and NSC
content. The total N content was determined by the Kjeldahl method, and the
content of NSC (soluble sugar and starch) was determined by anthrone
colorimetry.
NSC content (mg/g) = total soluble sugar content + starch content
C/N = NSC content / total N content
NSC cumulative (kg/hm2) = dry matter weight
×NSC content
Determination of yield and its constituent factors
For each plot, 50 holes of plants were selected for
sampling confirmed panicles. For plants in every 3rd hole, the
number empty or flat grains, 1000-grain weight, and the total weight were
calculated and averaged. The number of grains per panicle, seed rate and
theoretical yield and actual yield were also recorded.
Data processing
The data of differentiated, degraded and extant branches and spikelets
recorded during the two years (Table 1). The exogenous sugar treatment data was
basically the same for two years, and thus it took the average for subsequent
analysis. Microsoft Excel 2007 and SPSS 13.0 were used for statistical analysis
and plotting of the data.
Results
Effects on the formation
of rice branches
Compared with the control, the differences in the
number of primary differentiated branches, primary degraded branches and
primary extant branches after glucose and sucrose treatment were not
significant (Table 2). For secondary and total branches, the differences were
significant or extremely significant. With increasing concentration, the number
of differentiated secondary branches and total branches initially increased and
then declined. Different concentrations of exogenous glucose and sucrose always
caused a decrease in number of degraded branches, indicating that the treatment
can negatively regulate panicle degradation. However, different concentrations of glucose and sucrose had different effects,
in the G1 treatment, for example, significantly higher numbers of degraded
branches occurred than the other treatments. And for sucrose, the effects had
no obvious difference among all treatments.
The number of total extant
branches total and the number of total differentiated branches had a
quadratic-curve relation with the concentration of exogenous glucose and
sucrose (r= 0.51**, 0.69**, 0.50**, 0.77**), indicating that an optimal
concentration can promote the differentiation of stems and increase the number
of branches that survive (Fig. 1). Conversely, extremely high concentrations
are not favorable for the differentiation of spikelets.
Effects on the formation
of spikelets in rice concentrations
Compared with the control, the number of primary
differentiated spikelets, primary degraded spikelets and primary extant
spikelets after treatment with different concentrations of exogenous glucose
and sucrose showed no significant differences (Table 3). However, the
difference between the number of differentiated, degraded and extant spikelets
(secondary and total) was very significant.
The number of
differentiated spikelets (secondary and total) initially increased and then
decreased with increasing concentration. The number of secondary differentiated
spikelets peaked when the concentration of exogenous glucose was 333 mM.
The number of differentiated spikelets was always lower than control, except
for high glucose, when it was only slightly lower. The most secondary
differentiated spikelets were found at 263 mM sucrose treatment. At the
same concentration, the number of differentiated spikelets decreased
significantly, to a level slightly lower than in the control. At 333 mM, exogenous glucose level produced significantly higher
number of total differentiated spikelets and extant spikelets than other
treatments.
Correlation analysis showed (Fig.
2) that there was a quadratic curve correlation between the total extant spikelets, differentiated spikelets and the concentration of exogenous
glucose and sucrose (r= 64**, 0.73**, 0.58**,
0.82**). This indicates that an optimal
concentration of sugar can promote panicle differentiation, increasing the
extant spikelets. However, excessive
concentrations are also unfavorable to spikelet differentiation.
Effects on carbon and
nitrogen metabolism during panicle differentiation in rice
Exogenous glucose and sucrose concentrations
significantly reduced the total nitrogen content of rice plants at the Table 1: Statistical table of annual differentiation and degradation of branches
and spikelets of large panicle hybrid
Japonica rice
|
Year |
Differentiated branches |
Degraded branches |
Extant branches |
Differentiated spikelets |
Degraded spikelets |
Extant spikelets |
|
2016 |
132.5a |
58.2a |
74.3b |
478.3a |
147.6a |
330.7b |
|
2017 |
123.2ab |
40.4b |
82.8a |
463.9b |
109.2b |
354.7a |
|
F |
25.32** |
39.47** |
19.74** |
18.29** |
145.26** |
30.05** |
** represent significance at P < 0.01
Table 2: Effects of different
concentrations of exogenous glucose and sucrose on formation of branches
|
Treatment |
Primary branches |
Secondary branches |
Total branches |
|||||||||
|
Differentiated branches |
Extant branches |
Degraded branches |
Degraded percentage |
Differentiated branches |
Extant branches |
Degraded branches |
Degraded percentage |
Differentiated branches |
Extant branches |
Degraded branches |
Degraded percentage |
|
|
G1 |
20.9ab |
19.3ab |
1.6a |
7.8bc |
101.5bc |
58.5c |
43.0a |
42.4a |
122.4cd |
77.8de |
44.6a |
36.5a |
|
G2 |
22.3a |
20.6a |
1.6a |
7.3bc |
113.0a |
76.3a |
36.8b |
32.5d |
135.3a |
96.9a |
38.4c |
28.4c |
|
G3 |
21.0ab |
19.1ab |
1.9a |
9.0b |
106.0b |
67.9b |
38.1ab |
36.0cd |
127.0b |
87.0 b |
40.0bc |
31.5bc |
|
G4 |
20.9bc |
19.4ab |
1.5a |
7.2bc |
102.9bc |
63.8bc |
39.1ab |
38.0b |
123.8bc |
83.1bc |
40.6bc |
32.8bc |
|
S1 |
21.4ab |
19.3ab |
2.1a |
5.3c |
99.4c |
58.6c |
40.8ab |
41.0ab |
120.8d |
77.9de |
41.9abc |
35.5a |
|
S2 |
21.5ab |
20.3a |
1.3a |
5.8c |
107.1b |
67.8b |
39.4ab |
36.8bc |
128.6b |
88.0b |
40.6bc |
33.1ab |
|
S3 |
21.6ab |
20.0a |
1.6a |
7.5bc |
103.1bc |
62.8bc |
40.4ab |
39.2ab |
124.8bc |
82.8bc |
42.0abc |
34.2ab |
|
S4 |
21.5ab |
20.0a |
1.5a |
7.0bc |
102.1bc |
61.9c |
40.3ab |
39.4ab |
123.6bc |
81.9cd |
41.8abc |
35.9a |
|
CK |
20.1b |
17.8b |
2.4a |
11.8a |
100.8c |
57.8c |
43.0a |
42.7a |
120.9d |
75.5e |
45.4a |
37.5a |
G1, G2, G3, G4
represent 167, 333, 500 and 667 mmol/l glucose solutions, respectively. S1, S2,
S3, S4 represent 87, 175, 263, 351 mmol/l sucrose solutions, respectively. CK
represents water solution. Values followed by different letters are
significantly different at P ＜ 0.05. The same as below
Table 3: Effects of different
concentrations of exogenous glucose and sucrose on formation of spikelets
|
Treatment |
Primary
spikelets |
Secondary
spikelets |
Total
spikelets |
|||||||||
|
Differentiated spikelets |
Extant spikelets |
Degraded spikelets |
Degraded percentage |
Differentiated spikelets |
Extant spikelets |
Degraded spikelets |
Degraded percentage |
Differentiated spikelets |
Extant spikelets |
Degraded spikelets |
Degraded percentage |
|
|
G1 |
120.8a |
111.4a |
9.4ab |
7.7a |
357.9b |
243.5b |
114.4b |
32.0a |
478.6b |
354.9b |
123.8c |
25.9a |
|
G2 |
129.1a |
121.0a |
8.1ab |
6.3a |
393.9a |
263.6a |
130.3a |
34.7a |
523.0a |
384.6a |
138.4a |
27.8a |
|
G3 |
120.8a |
113.3a |
7.5ab |
6.2a |
342.9c |
225.5bcd |
117.4b |
34.2a |
463.6c |
338.8bc |
124.9c |
26.9a |
|
G4 |
119.5a |
111.4a |
8.1ab |
6.8a |
344.6c |
215.6d |
129.0ab |
37.4a |
464.1c |
327.0c |
137.1ab |
29.5a |
|
S1 |
124.1a |
113.5a |
10.6a |
8.6a |
344.6c |
222.4bcd |
122.3b |
35.4a |
468.8bc |
335.9bc |
132.9bc |
28.3a |
|
S2 |
124.8a |
118.5a |
6.3b |
5.0a |
355.8b |
237.6bc |
118.1b |
33.2a |
480.5b |
356.1b |
124.4bc |
25.9a |
|
S3 |
129.6a |
117.1a |
12.5a |
9.6a |
358.4b |
231.6bc |
126.8b |
35.4a |
488.0b |
348.8b |
139.3a |
28.5a |
|
S4 |
125.0a |
116.9a |
8.1ab |
6.5a |
350.8b |
229.6bcd |
121.1b |
34.5a |
475.8b |
346.5b |
129.3bc |
27.2a |
|
CK |
125.5a |
113.6a |
11.9a |
9.5a |
344.1c |
215.1d |
129.0ab |
37.5a |
469.6bc |
328.8c |
140.9a |
30.0a |
Table 4: Effects of exogenous glucose and
sucrose concentrations on carbon and nitrogen metabolism during panicle
differentiation in rice
|
Treatment |
15 days before
heading |
Heading date |
||||
|
N content (mg/g) |
NSC content (mg/g) |
C/N |
N content (mg/g) |
NSC content (mg/g) |
C/N |
|
|
G1 |
22.5b |
101.8b |
4.5 bc |
19.0abc |
201.2b |
10.5ab |
|
G2 |
21.6c |
105.2a |
4.8 ab |
18.8bc |
208.5a |
11.0ab |
|
G3 |
20.6d |
106.3b |
5.1a |
18.2c |
209.6a |
11.5a |
|
G4 |
19.5e |
106.8a |
5.4 a |
18.1c |
209.1a |
11.5a |
|
S1 |
23.2ab |
102.5ab |
4.4 bc |
19.4ab |
198.7c |
10.2bc |
|
S2 |
22.9b |
104.7a |
4.5 bc |
19.2ab |
202.3b |
10.5ab |
|
S3 |
21.2c |
104.9a |
4.9 ab |
18.2c |
203.4b |
11.1b |
|
S4 |
21.0cd |
105.1a |
5.0a |
18.0c |
204.2b |
11.3a |
|
CK |
23.7a |
100.2b |
4.2c |
19.9a |
195.6c |
9.83c |
panicle differentiation stage (Table 4). Exogenous
glucose and sucrose decreased by 11.18 and 6.86% on average at 15 days before
heading, and decreased by 6.91 and 6.03% respectively, at heading date.
Simultaneously, with an increase in exogenous sugar concentration, the total
nitrogen content of plants decreased, in the order G1 > G2 > G3 > G4,
S1 > S2 > S3 > S4. Exogenous glucose and sucrose significantly
increased NSC content and C/N ratio of rice plants at the panicle
differentiation stage, which contrasted with the total nitrogen content and
showed that NSC content increased with increasing exogenous sugar
concentration. Glucose and sucrose increased the NSC content by 4.82 and 4.11%
and increased the C/N by 18.38 and 11.94% at 15 days before heading. It also
increased the NSC content by 5.88 and 3.35% on average at the heading stage and
the C/N by 13.81 and 10.12%.
The correlation
analysis showed that the surviving and differentiation number of total branches
and spikelets had a quadratic correlation with the C/N ratio of plants 15 days before heading and at heading stage
(Fig. 3). %20247-254_files/image002.png)
Fig. 1: Correlation between
differentiation and extant number of total branches and the concentration of
exogenous glucose and sucrose
%20247-254_files/image004.png)
Fig. 2: Correlation between the
differentiation and existing number of total spikelets and the concentration of
exogenous glucose and sucrose
%20247-254_files/image006.png)
Fig. 3: Correlation between C/N Ratio of
Rice Plants and Branch and Spikelet Formation at the Panicle Differentiation
Stage
Furthermore, the correlation
coefficients were significant indicating that the C/N ratio of plants at
panicle differentiation stage was closely related to the formation of spikelets
and the appropriate C/N ratio could promote the formation of branches and
spikelets.
Effects on NSC accumulation
and distribution during panicle differentiation of rice
During panicle differentiation, more than 85% of
NSCs were distributed in stem sheath, about 8% in green leaves and about 3% in
panicles. Different concentrations of exogenous glucose and sucrose could
increase the accumulation of NSC in different organs of plants at panicle
differentiation stage. Although the accumulation of NSC in stem and sheath did
not reach a significant level at 15 days before heading, it reached significant
levels in other organs. The results also showed that with increasing exogenous
sugar concentration, changes in NSC accumulation in stem and sheath was not
obvious 15 days before heading, it increased gradually at the heading stage.
Furthermore, with increasing exogenous sugar concentration, the NSC
accumulation in green leaves and spike at 15 days before heading and at the
heading stage initially increased and then declined.
Accumulation of NSC in stem and
sheath at 15 days before heading and at heading stage was weakly correlated
with differentiated, surviving and retrograded branches and spikelets (Table 5). The accumulation
of NSC in green leaves and spikelets at 15 days before heading and at the
heading stage was positively correlated with the differentiated and surviving
spikelets (R2= 0.80*, 0.90**, 0.75*, 0.83**, 0.75*, 0.88**, 0.81**
and 0.80**, respectively). The retrograded branches and spikelets were
negatively correlated with the NSC accumulation in all organs at 15 days before
heading and at the heading stage, but not significantly.
Effects on yield related
factors of rice
Changes caused by different concentrations of
exogenous glucose and sucrose treatment on rice yield was highly significant
(Table 6), both were higher than the control. However, increasing exogenous
glucose and sucrose concentration, showed an initial increase in yield,
followed by a decline. At a concentration of 333 mM glucose and 175 mM
sucrose treatment, the yield of both exogenous sugar treatments was higher than
others and rate of seed set was also high. Effects of different concentrations
of exogenous glucose and sucrose per panicle were extremely significant (F = 25.49 **), whereas for rate of seed
set, 1000-grain weight and degree of fullness, there was no significant change,
indicating that field changes caused by exogenous glucose and sucrose were
mainly due to changes in the number of grains per panicle.
Discussion
Table 5: The correlation between NSC
accumulation and spikelet formation at the panicle differentiation stage in
Rice
|
|
15 days before
heading |
Heading date |
||||
|
|
stems and leaf sheath |
leaf |
spike |
stems and leaf sheath |
leaf |
spike |
|
Differentiated branches |
-0.126 |
0.720* |
0.695* |
-0.248 |
0.749* |
0.578 |
|
Extant branches |
-0.272 |
0.669 |
0.670 |
-0.362 |
0.695* |
0.497 |
|
Degraded branches |
-0.514 |
-0.510 |
-0.575 |
-0.517 |
-0.527 |
-0.249 |
|
Differentiated spikelets |
-0.346 |
0.796* |
0.752* |
-0.196 |
0.751* |
0.811** |
|
Extant spikelets |
-0.329 |
0.897** |
0.829 ** |
-0.168 |
0.878** |
0.804** |
|
Degraded spikelets |
-0.080 |
-0.181 |
-0.128 |
-0.091 |
-0.254 |
-0.197 |
* , ** represent
significance at P < 0.05 and P < 0.01. The same as below
Table 6: Effects of different
concentrations of exogenous glucose and sucrose on yield of rice
|
Treatment |
Spikelets per panicle |
Rate of seed set (%) |
1000-grain weight (g) |
Grain plumpness (%) |
Theoretical yield (t/hm2) |
|
G1 |
283.9b |
86.8a |
21.7a |
85.1a |
11.6b |
|
G2 |
297.7a |
86.9a |
21.8a |
85.6a |
12.3a |
|
G3 |
271.0bc |
87.0a |
21.3a |
83.9a |
10.9cd |
|
G4 |
261.6c |
87.0a |
21.9a |
84.3a |
10.9cd |
|
S1 |
277.2b |
86.5a |
21.4a |
84.9a |
11.2bc |
|
S2 |
284.9b |
87.2a |
21.9a |
85.3a |
11.8ab |
|
S3 |
279.0b |
85.6a |
21.6a |
84.6a |
11.4bc |
|
S4 |
268.7bc |
85.5a |
21.7a |
83.3a |
11.0c |
|
CK |
263.0c |
85.9a |
21.3a |
85.0a |
10.6d |
The number of grains per panicle is one of the
important indicators of rice yield. Especially during the promotion and
implementation of super rice, the number of secondary branches, total branches,
and number of grains on secondary branches were the main indicators for the
number of grains per panicle (Kato and Katsura 2010). For differentiation and
degradation of spikelets, studies suggest that these processes are closely
related to the health of plants during
differentiation and development stages, as well as carbon and nitrogen
nutrition and metabolism. Meanwhile, physiological activity of young spikes is
also important. From vegetative to reproductive growth of rice, a great deal of
carbohydrates is needed as a carbon and energy source for the formation of
young spikelets, pollen and embryo sacs. In particular, at the meiosis stage of
rice pollen mother cells, carbon metabolism is beneficial to the reproductive
growth of crops (Fu et al. 2015). Studies (Tian et al. 2016) show that concentration
and content level are two factors that are closely associated with the
differentiation and degradation of spikelets and
branches. The 12 d or 4 d before heading, or
during the heading stage, stem and sheath growth is high, thus, a high
concentration of non-structural carbohydrate is not conducive to the
differentiation and degradation of young branches and spikelets. From 16 d
before heading to 8 d before heading, a higher NSC accumulation in young
spikelets is necessary for the formation of large spikes. NSC is closely
correlated with the number of grains. NSC, like sucrose, glucose, fructose, and
fructan influence plant metabolism and yield (Yoshinagaa et al. 2013; Wang et al. 2017).
Previously plant hormones have
been shown to play a regulatory role at very low concentrations, while sugars
are more active when concentrations are high. In this study, the effects of
exogenous sugars on the degradation of branches and spikelets at the early
stage of panicle differentiation indicate that the sugar signal may induce the
synthesis of NSC in plants and even at low concentrations, they can regulate
the C/N ratio during the panicle differentiation stage, facilitating the
differentiation of branches and spikelets. However, when the concentration is
high, the balance of C and N is disrupted, suppressing the differentiation of
young spikelets and branches. To date, we have only hypotheses on how NSC
(probably together with hormones) might regulate spikelet formation. Little is
known about the links between sugar signaling substances (soluble sugar part of
NCS) and hormones, especially the relationship between protein level and enzyme
activity in the process of spikelet formation. Therefore, it is important to
identify the relation between sugar and hormonal signals. Two open questions
are: (1) what is the synergistic regulation of rice panicle differentiation and
development and, (2) what is the signal regulation network of rice spikelet
growth and development? To answer these questions, further research on signal
levels is needed.
The results of this study showed
that appropriate concentrations of exogenous glucose and sucrose could
significantly promote rice spikelet differentiation. When the concentration was
low, the interaction between exogenous sugar and hormone signaling reduced ABA
content in the young panicle, promoting differentiation (Radchuk et al.
2010). At high concentration, spikelet numbers were lower than in the control
treatment, which may also reflect a sugar-hormone interaction (Arenas et al.
2000; Rook et al. 2001). Adding exogenous sugars can reduce the number
of degradations, however, it is not known if ethylene is involved (Zhou et
al. 1998). The differentiation and development of young panicles are the
result of a combination of internal and external factors, and a series of
complex physiological and biochemical processes. Yet, there are still many
processes that are not fully understood, such as how the external environmental
conditions regulate sugar metabolism, physiology and hormones. It is suggested
that the mechanism of how multiple interactions regulate young panicle
formation deserves further in-depth research.
A thorough and detailed study of
the internal regulatory network between the expression of soluble sugar in rice
(NSC) - plant hormone – protein (Arenas et al. 2000) and the
activity of metabolic enzymes is necessary. In particular, the differentiation
and development of young panicles and the size of the sink volume by NSC should
clarify the internal relationship between sugar signals and hormones and the
mechanism of the formation of young panicles. An understanding of the signal
regulation network of rice panicle growth and development will provide better
theoretical and practical guidance for the cultivation of large panicles in
production.
A thorough and detailed study of
the internal regulatory network between the expression of soluble sugar in rice
(NSC) - plant hormone – protein, and the activity of metabolic enzymes is
necessary. In particular, the differentiation and development of young panicles
and the size of the sink volume by NSC should clarify the internal relationship
between sugar signals and hormones and the mechanism of the formation of young
panicles. An understanding of the signal regulation network of Rice Panicle
growth and development will provide better theoretical and practical guidance
for the cultivation of large panicles in production.
Conclusion
Exogenous glucose and sucrose application at
booting stage demonstrated that NSC involved in the regulation of
differentiation and development of spikelets in rice. NSC is not only a kind of
energy material, but also a signaling molecule. However, the mechanism of NSC
regulating spikelets differentiation and degradation is not clear, which is
worth further study.
Acknowledgements
This study was funded by Special Grain Science and Technology Innovation
Project (No. 2017YFD0300102), National Key Research and Development Program of
China (No. 2017YFD0301206); National Natural Science Foundation of China (No.
31471447); Jiangsu Province Six Talents Summit Project (No. NY-129) and Jiangsu
Agricultural Science and Technology Innovation and Promotion Project.
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