4. DISCUSSION
“Early flowering” and “high yielding” are important goals for crop
domestication and improvement, but they are contradictory. The duration
and fate of floret primordia development in wheat have a significant
impact on this contradiction; for example, the shorter the development
duration, the fewer floret primordia survive to reproduce (Ghiglione et
al. , 2008; Watson et al. , 2018). A thorough understanding
of floret development can assist in overcoming the trade-off between
“early flowering” and “high yielding” and achieving faster and more
wheat production.
From the terminal spikelet to flowering, wheat floret primordia in SW
treatment went through differentiation (Stage I), differentiation and
morphology development concurrently (Stage II), morphology development
(Stage III), and polarization (Stage IV), according to the high
temporal-resolution investigation of floret primordia number,
morphology, and spike transcriptome (Figure 2, Figure 4). During Stage
IV, there were two morphologies of floret primordia, including fertile
and infertile floret primordia (Figure S1b). Although the initiation of
polarization varied between treatments (24 DAT in SW and 21 DAT in LW
and LR; Figure 2e), all infertile floret primordia degenerated rapidly
and finally became invisible (Figure 2d, 2g). This result was consistent
with previous research, which proposed that initiating abortive floret
primordia would inevitably degenerate and fail to develop into fertile
florets even when the plant was grown under optimal conditions, such as
adequate water and fertilizer (Zhang et al. , 2021). Meanwhile, we
also found that there was a stable difference in the number of fertile
floret primordia between treatments in the polarization, and almost all
fertile floret primordia in the three light regimens continued their
development and finally became fertile florets at flowering (Figure 2f).
These results showed that infertile floret primordia could not be
reversed to fertile floret primordia and that the number of fertile
floret primordia could not be increased by light treatments in the
polarization, suggesting that Stage IV had little effect on improving
NFFs.
Fertile floret primordia in Stage IV, which originated from a portion of
floret primordia in the first three stages, have greatly determined
NFFs. Therefore, promoting the growth and development of floret
primordia during Stage I-III was essential for increasing NFFs.
Assimilate availability in the spike significantly affected NFFs; for
example, the higher the dry spike weight at flowering, the more the NFFs
(Figure 3), which was consistent with previous studies (González et
al. , 2011; Zhang et al. , 2020). Hence, the accelerated
growth and development of the most advanced floret primordium (i.e., F1;
Figure 2d), as well as the increased dry spike weight and soluble sugar
contents (Figure 3), observed in LW and LR treatments, were required for
high NFFs (Ghiglione et al. , 2008; Ugarte et al. , 2010).
However, comparisons between treatments (LW versus SW and LRversus LW) showed that NFFs and the number of fertile floret
primordia were decreased in LW treatment but increased in LR treatment
(Figure 2b, 2f). The maximum number of differentiable floret primordia
did not differ between treatments (Figure 2e), supporting historical
findings that the number of floret primordia was not a limiting factor
for NFFs improvement (Guo & Schnurbusch, 2015; Zhang et al. ,
2021). Some floret primordia, such as F6 (Figure 2d), were either
interrupted by the advance of degradation initiation in LW treatment,
resulting in a shorter growth duration and lower dry spike weight, or
did not develop as quickly in LW treatment as in LR treatment due to
inadequate assimilate supply, which prevented primordia from acquiring
fertile potential comparable to SW and LR treatment. Increasing the
available assimilates and promoting the development of more floret
primordia to the scale of fertile floret primordia during the shortened
Stage I-III became crucial for increasing wheat grains.
In comparison to SW, LW treatment accelerated flowering but sacrificed
NFFs (Figure 2a, 2b), which was consistent with previous findings
(Ghiglione et al. , 2008; Watson et al. , 2018).
Furthermore, our findings suggested that the shortened Stage III in LW
treatment was mainly responsible for flowering acceleration (Figure 4).
In analogy with the conserved photoperiodic flowering pathway in
Arabidopsis and rice (Fornara et al. , 2010; Zhou et al. ,
2021), we also constructed a GI -CO -like-HD3Aframework during wheat floret development that played a pivotal role in
accelerated flowering (Figure 5). First, LW treatment significantly
regulated GI and its directly related IAA16 , which
contributed to accelerated differentiation and growth of floret
primordia. Then, LW treatment regulated the CO 3-centered gene
network, which included several MYB transcription factor genes; this
accelerated anther development, induced the up-regulation ofCOL2 , COL5 , and GHD7 and the down-regulation ofCOL3 , and caused Stage IV initiation to be advanced from 24 DAT
to 21 DAT. Finally, HD3A synergized with its directly related
genes involved in pollen tube growth to induce early flowering in LW
treatment. Since our study found that wheat plants grown in LR treatment
had earlier flowering and higher NFFs than those grown in LW treatment
(Figure 2a, 2b), it would be worthwhile to explore how LR treatment
regulated floret development at different stages.
Floret primordia under LR treatment mainly underwent Stage I, III, and
IV, which corresponded to Stage I, II, and IV under LW treatment,
respectively, based on the sampling times (Figure 4), indicating that LR
treatment modulated gene expression at 15 DAT and 18 DAT, allowing
spikes to reach the Stage III criterion more quickly. Compared to LW, LR
treatment significantly up-regulated the expression of genes encoding
ubiquitin and calcium signaling, which accelerated cell proliferation
and histogenesis during Stage I (Figure 6e, Figure 7). Almost all floret
primordia were generated in Stage I under LR treatment, whereas they
required Stage I to II under LW treatment (Figure 2e). Hence, LR
treatment mainly shortened the development time of floret primordia in
Stage II, inducing early flowering. During Stage III, LR treatment
significantly increased the response of the gene network centered on
ubiquitin and ALDH s involved in primary and secondary metabolite
production, cell wall biogenesis, and anther development (Figure 6e,
Figure 7). This showed that floret primordia in LR treatment underwent
adequate morphological development in Stage III, so that more floret
primordia acquired fertile potential before polarization. During Stage
IV, wheat spikes were directly exposed to light, requiring plants to
coordinate growth, development, and defense (Lazzarin et al. ,
2021). The gene network centered on ZF s and HSP s was
strongly regulated by LR treatment, which not only accelerated
environmental adaptation by promoting abscisic acid and jasmonic acid
signaling pathways, but also improved stamen filament and pollen tube
development in preparation for successful fertilization (Figure 6e,
Figure 7). Furthermore, significant down-regulation of genes involved in
carbohydrate, lipid, and fatty acid metabolism under LR treatment did
not prevent the continued development of fertile floret primordia,
indicating that LR treatment acted to maintain fertile floret primordia
development during Stage IV until flowering (Figure 2f, Figure 6e). In
summary, LR treatment accelerated floret primordia differentiation in
Stage I, shortened Stage II, enhanced morphological development of
floret primordia and increased the number of fertile floret primordia in
Stage III, and maintained fertile floret primordia development in Stage
IV through a gene regulatory network centered on ubiquitin, calcium
signaling, ALDH s, ZF s, and HSP s, ultimately
achieving the dual promoting effects of accelerating flowering and
increasing NFFs.
Collectively, we proposed a modulated light regimen (i.e., long
photoperiod supplemented with red light) and a dynamic regulatory model
that could remove the barrier between
“early
flowering” and “high yielding” during wheat floret development. Early
flowering could be achieved by shortening the development time of floret
primordia in Stage II or Stage III. High yield could be obtained by
increasing assimilate availability and promoting the morphological
development of more floret primordia to acquire fertile potential during
Stage III. These results provide important resources for future
improvements in the “early flowering” and “high yielding” of wheat.