High Ambient Temperatures Inhibit Ghd7-Mediated Flowering Repression in Rice

Temporal transcriptome analysis reveals the two-phase action of florigens in rice flowering

Two florigen genes, Hd3a and RFT1, are essential for the floral transition in rice. To elucidate the early steps of the transcriptional dynamics during rice floral induction, we compared a set of temporal transcriptome data of SAM (shoot apical meristem)-region samples between the wild-type and a non-flowering line of the hd3a rft1 double mutant during a short-day (SD) treatment after growing under long-day conditions for 42 days, and identified 6,978 DEGs (differentially expressed genes). As expected, FUL-like MADS-box genes were induced just after 4 days SD treatment; meanwhile, SEP-type and AGL-type MADS-box genes were induced after 9 days of SD treatment. We here newly revealed that majority of rhythmic genes including major circadian clock genes were not affected by the florigen genes, implying normal circadian clock phasing at the SAM regions regardless of floral transitions. We found that around two thousands of genes were repressed by Hd3a and RFT1 genes at the SAM regions before the SD treatments and become derepressed and similar to WT expression levels in the double mutants according to the SD treatments. These clearly imply two distinct actions of florigen genes: one for repression of some developmental key genes during vegetative growth possibly by very low level of florigen expression and the other for floral induction by relatively high florigen expressions upon short-day inductions. This repression by low levels of florigens may serve as a maintenance system for vegetative growth before floral induction, which implies a novel role for florigen genes in rice.

Physiological factors influencing climate-smart agriculture: Daylength-mediated interaction between tillering and flowering in rice

BACKGROUND

Controlling rice tillering and flowering is essential for reducing greenhouse gas emissions from paddy fields, a key objective in climate-smart agriculture. However, the interaction between tillering and flowering remains controversial and poorly understood. In this study, we subjected plants of the rice cultivars 'Saenuri' and 'Odae' to short- and long-day conditions and compared their growth and flowering responses after tiller removal.

RESULTS

The effects of tiller removal differed depending on daylength conditions. Under short days, plants in the tiller-removal group flowered earlier than the controls, whereas the opposite trend was observed under long days. This response was associated with changes in florigen gene expression. Under short days, the expression of Hd3a, which promotes flowering, increased in the tiller-removal group compared with that in the controls. In contrast, under long days, the expression of OsMFT1, a gene that delays flowering and promotes spikelet formation, was significantly upregulated, leading to an increased spikelet number. Notably, spikelets per panicle in the tiller-removal groups increased approximately 3.4-fold in 'Saenuri' and 2.2-fold in 'Odae' under long-day conditions compared with those in their respective controls.

CONCLUSIONS

These findings highlight the daylength-dependent variability in tillering and flowering interactions, providing new insights into their regulatory mechanisms. This study offers a foundation for optimizing rice growth strategies under varying photoperiod conditions, contributing to climate-smart agricultural practices and improved breeding programs.

Historic rewiring of grass flowering time pathways and implications for crop improvement under climate change

Grasses are fundamental to human survival, providing a large percentage of our calories, fuel, and fodder for livestock, and an enormous global carbon sink. A particularly important part of the grass plant is the grain-producing inflorescence that develops in response to both internal and external signals that converge at the shoot tip to influence meristem behavior. Abiotic signals that trigger reproductive development vary across the grass family, mostly due to the unique ecological and phylogenetic histories of each clade. The time it takes a grass to flower has implications for its ability to escape harsh environments, while also indirectly affecting abiotic stress tolerance, inflorescence architecture, and grain yield. Here, we synthesize recent insights into the evolution of grass flowering time in response to past climate change, particularly focusing on genetic convergence in underlying traits. We then discuss how and why the rewiring of a shared ancestral flowering pathway affects grass yields, and outline ways in which researchers are using this and other information to breed higher yielding, climate-proof cereal crops.

New insights into rice phenology: discovering the effect of insolation on heading response

Precise growth management is required for climate-smart and sustainable crop production in response to climate change, with the heading stage being the most important. Research on the control of heading in rice (Oryza sativa) has mainly focused on day length and temperature; however, research on the effects of insolation is limited. Therefore, this study analyzed the differences in rice growth and heading responses under different light intensity and temperature conditions. Five early-maturing and seven medium-late-maturing rice varieties were used for each japonica heading ecology type. Our results showed that leaf age development, an indirect measure of rice phenological development, was inhibited under low light intensity and low-temperature conditions. Accordingly, the heading date was also delayed by approximately 18 days at low temperatures and 21 days at low light intensity, with no difference among ecotypes. We also found an interaction between temperature and light intensity, with the light intensity-mediated delay in heading date being affected more by high temperatures. This study demonstrated that light intensity and temperature have a major effect on heading date variation, suggesting that the impact of insolation must be considered for the accurate prediction of heading stage variation. These results could shed new light on rice phenology research and contribute to the implementation of precision agriculture.

Photoperiod and temperature synergistically regulate heading date and regional adaptation in rice

Plants must accurately integrate external environmental signals with their own development to initiate flowering at the appropriate time for reproductive success. Photoperiod and temperature are key external signals that determine flowering time; both are cyclical and periodic, and they are closely related. In this review, we describe photoperiod-sensitive genes that simultaneously respond to temperature signals in rice (Oryza sativa). We introduce the mechanisms by which photoperiod and temperature synergistically regulate heading date and regional adaptation in rice. We also discuss the prospects for designing different combinations of heading date genes and other cold tolerance or thermo-tolerance genes to help rice better adapt to changes in light and temperature via molecular breeding to enhance yield in the future.

The molecular basis of heat stress responses in plants

Global warming impacts crop production and threatens food security. Elevated temperatures are sensed by different cell components. Temperature increases are classified as either mild warm temperatures or excessively hot temperatures, which are perceived by distinct signaling pathways in plants. Warm temperatures induce thermomorphogenesis, while high-temperature stress triggers heat acclimation and has destructive effects on plant growth and development. In this review, we systematically summarize the heat-responsive genetic networks in Arabidopsis and crop plants based on recent studies. In addition, we highlight the strategies used to improve grain yield under heat stress from a source-sink perspective. We also discuss the remaining issues regarding the characteristics of thermosensors and the urgency required to explore the basis of acclimation under multifactorial stress combination.

Environmental control of rice flowering time

Correct measurement of environmental parameters is fundamental for plant fitness and survival, as well as for timing developmental transitions, including the switch from vegetative to reproductive growth. Important parameters that affect flowering time include day length (photoperiod) and temperature. Their response pathways have been best described in Arabidopsis, which currently offers a detailed conceptual framework and serves as a comparison for other species. Rice, the focus of this review, also possesses a photoperiodic flowering pathway, but 150 million years of divergent evolution in very different environments have diversified its molecular architecture. The ambient temperature perception pathway is strongly intertwined with the photoperiod pathway and essentially converges on the same genes to modify flowering time. When observing network topologies, it is evident that the rice flowering network is centered on EARLY HEADING DATE 1, a rice-specific transcriptional regulator. Here, we summarize the most important features of the rice photoperiodic flowering network, with an emphasis on its uniqueness, and discuss its connections with hormonal, temperature perception, and stress pathways.

Evolution of flowering time genes in rice: From the paleolithic to the anthropocene

The evolutionary paths of humans and plants have crossed more than once throughout millennia. While agriculture contributed to the evolution of societies in prehistory, human selection of desirable traits contributed to the evolution of crops during centuries of cultivation. Among cereal crops, rice is currently grown around the globe and represents staple food for almost half of the world population. Over time, rice cultivation has expanded from subtropical to temperate regions thanks to artificial selection of mutants with impaired response to photoperiod. Additional regulatory mechanisms control flowering in response to diverse environmental cues, anticipating or delaying the floral transition to produce seeds in more favourable conditions. Nevertheless, the changing climate is threatening grain production because modern cultivars are sensitive to external fluctuations that go beyond their physiological range. One possibility to guarantee food production could be the exploitation of novel varieties obtained by crossing highly productive Asian rice with stress tolerant African rice. This review explores the genetic basis of the key traits that marked the long journey of rice cultivation from the end of the paleolithic to the anthropocene, with a focus on heading date. By 2050, will rice plants of the future flower in the outer space?

The hot science in rice research: How rice plants cope with heat stress

Global climate change has great impacts on plant growth and development, reducing crop productivity worldwide. Rice (Oryza sativa L.), one of the world's most important food crops, is susceptible to high-temperature stress from seedling stage to reproductive stage. In this review, we summarize recent advances in understanding the molecular mechanisms underlying heat stress responses in rice, including heat sensing and signalling, transcriptional regulation, transcript processing, protein translation, and post-translational regulation. We also highlight the irreversible effects of high temperature on reproduction and grain quality in rice. Finally, we discuss challenges and opportunities for future research on heat stress responses in rice.

How ambient temperature affects the heading date of foxtail millet (Setaria italica)

Foxtail millet (Setaria italica), a short-day plant, is one of the important crops for food security encountering climate change, particularly in regions where it is a staple food. Under the short-day condition in Taiwan, the heading dates (HDs) of foxtail millet accessions varied by genotypes and ambient temperature (AT). The allelic polymorphisms in flowering time (FT)-related genes were associated with HD variations. AT, in the range of 13°C-30°C that was based on field studies at three different latitudes in Taiwan and observations in the phytotron at four different AT regimes, was positively correlated with growth rate, and high AT promoted HD. To elucidate the molecular mechanism of foxtail millet HD, the expression of 14 key FT-related genes in four accessions at different ATs was assessed. We found that the expression levels of SiPRR95, SiPRR1, SiPRR59, SiGhd7-2, SiPHYB, and SiGhd7 were negatively correlated with AT, whereas the expression levels of SiEhd1, SiFT11, and SiCO4 were positively correlated with AT. Furthermore, the expression levels of SiGhd7-2, SiEhd1, SiFT, and SiFT11 were significantly associated with HD. A coexpression regulatory network was identified that shown genes involved in the circadian clock, light and temperature signaling, and regulation of flowering, but not those involved in photoperiod pathway, interacted and were influenced by AT. The results reveal how gene × temperature and gene × gene interactions affect the HD in foxtail millet and could serve as a foundation for breeding foxtail millet cultivars for shift production to increase yield in response to global warming.

LsARF3 mediates thermally induced bolting through promoting the expression of LsCO in lettuce (Lactuca sativa L.)

Lettuce (Lactuca sativa L.) is a leafy vegetable whose edible organs usually are leaf or stems, and thus high-temperature induced bolting followed by flower initiation is an undesirable trait in lettuce production. However, the molecular mechanism that controls lettuce bolting and flowering upon thermal treatments is largely unknown. Here, we identified a Lettuce auxin response factor 3 (LsARF3), the expression of which was enhanced by heat and auxin treatments. Interestingly, LsARF3 is preferentially expressed in stem apex, suggesting it might be associated with lettuce bolting. Transgenic lettuce overexpressing LsARF3 displayed early bolting and flowering, whereas knockout of LsARF3 dramatically delayed bolting and flowering in lettuce under normal or high temperature conditions. Furthermore, Exogenous application of IAA failed to rescue the late-bolting and -flowering phenotype of lsarf3 mutants. Several floral integrator genes including LsCO, LsFT, and LsLFY were co-expressed with LsARF3 in the overexpression and knockout lettuce plants. Yeast one-hybrid (Y1H) experiments suggested that LsARF3 could physically interact with the LsCO promoter, which was further confirmed by a dual luciferase assay in tobacco leaves. The results indicated that LsARF3 might directly modulate the expression of LsCO in lettuce. Therefore, these results demonstrate that LsARF3 could promote lettuce bolting in response to the high temperature by directly or indirectly activating the expression of floral genes such as LsCO, which provides new insights into lettuce bolting in the context of ARFs signaling and heat response.

A Daylength Recognition Model of Photoperiodic Flowering

The photoperiodic flowering pathway is crucial for plant development to synchronize internal signaling events and external seasons. One hundred years after photoperiodic flowering was discovered, the underlying core signaling network has been elucidated in model plants such as Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), and soybean (Glycine max). Here, we review the progress made in the photoperiodic flowering area and summarize previously accepted photoperiodic flowering models. We then introduce a new model based on daylength recognition by florigen. By determining the expression levels of the florigen gene, this model can assess the mechanism of daylength sensing and crop latitude adaptation. Future applications of this model under the constraints of global climate change are discussed.