The role of the Arabidopsis FUSCA3transcription factor during inhibition of seed germination at high temperature

Abstract

BackgroundImbibed seeds integrate environmental and endogenous signals to break dormancy and initiate growth under optimal conditions. Seed maturation plays an important role in determining the survival of germinating seeds, for example one of the roles of dormancy is to stagger germination to prevent mass growth under suboptimal conditions. The B3-domain transcription factor FUSCA3 (FUS3) is a master regulator of seed development and an important node in hormonal interaction networks in Arabidopsis thaliana. Its function has been mainly characterized during embryonic development, where FUS3 is highly expressed to promote seed maturation and dormancy by regulating ABA/GA levels.ResultsIn this study, we present evidence for a role of FUS3 in delaying seed germination at supraoptimal temperatures that would be lethal for the developing seedlings. During seed imbibition at supraoptimal temperature, the FUS3 promoter is reactivated and induces de novo synthesis of FUS3 mRNA, followed by FUS3 protein accumulation. Genetic analysis shows that FUS3 contributes to the delay of seed germination at high temperature. Unlike WT, seeds overexpressing FUS3 (ML1:FUS3-GFP) during imbibition are hypersensitive to high temperature and do not germinate, however, they can fully germinate after recovery at control temperature reaching 90% seedling survival. ML1:FUS3-GFP hypersensitivity to high temperature can be partly recovered in the presence of fluridone, an inhibitor of ABA biosynthesis, suggesting this hypersensitivity is due in part to higher ABA level in this mutant. Transcriptomic analysis shows that WT seeds imbibed at supraoptimal temperature activate seed-specific genes and ABA biosynthetic and signaling genes, while inhibiting genes that promote germination and growth, such as GA biosynthetic and signaling genes.ConclusionIn this study, we have uncovered a novel function for the master regulator of seed maturation, FUS3, in delaying germination at supraoptimal temperature. Physiologically, this is important since delaying germination has a protective role at high temperature. Transcriptomic analysis of seeds imbibed at supraoptimal temperature reveal that a complex program is in place, which involves not only the regulation of heat and dehydration response genes to adjust cellular functions, but also the activation of seed-specific programs and the inhibition of germination-promoting programs to delay germination.

Identification of Direct Targets of FUSCA3, a Key Regulator of Arabidopsis Seed Development

“…Yamamoto et al (2010) documented transcriptomes in developing seed of fus3 compared with the wild type and assessed transcript accumulation in response to an inducible form of FUS3. Chiu et al (2012) examined transcript accumulation in response to supraoptimal imbibition temperatures in wild-type seeds, an environment where FUS3 has been found to have a protective function. Lumba et al (2012) examined the consequences of inducible FUS3 in seedlings where FUS3 has been shown to have roles in vegetative phase transitions.…”

“…Here, we report the genome-wide identification of in vivo binding sites for FUS3 in an embryonic culture tissue system that allows robust ChIP. The data sets generated by Yamamoto et al (2010), Chiu et al (2012), and Lumba et al (2012) were used to identify FUS3-responsive genes. Yamamoto et al (2010) documented transcriptomes in developing seed of fus3 compared with the wild type and assessed transcript accumulation in response to an inducible form of FUS3.…”

Inhibition of FUSCA3 degradation at high temperature is dependent on ABA signaling and is regulated by the ABA/GA ratio

“…11,12,13 Overexpression of FUS3 at 32 C completely inhibit germination through de novo ABA synthesis, this allows higher seedling survival when seeds are transferred to optimal temperature. 13 To elucidate the mechanism behind FUS3 protein accumulation under heat stress, we recently characterized FUS3 degradation rates in extracts of seed imbibed at optimal temperature (21 C) in the presence of ABA and at supra-optimal (32 C) temperatures. 14 By using ABA biosynthetic mutants and fluridone, an inhibitor of ABA biosynthesis, we showed that FUS3 degradation in vitro CONTACT Sonia Gazzarrini gazzarrini@utsc.utoronto.ca Addendum to: Chiu RS, Pan S, Zhao R, Gazzarrini S. ABA-dependent inhibition of the ubiquitin proteasome system during germination at high temperature in Arabidopsis.…”

“…5,10 At elevated temperatures, ABA levels increase and this leads to a delay or inhibition of germination (thermoinhibition) and FUS3 accumulation. 11,12,13 Overexpression of FUS3 at 32 C completely inhibit germination through de novo ABA synthesis, this allows higher seedling survival when seeds are transferred to optimal temperature. 13 To elucidate the mechanism behind FUS3 protein accumulation under heat stress, we recently characterized FUS3 degradation rates in extracts of seed imbibed at optimal temperature (21 C) in the presence of ABA and at supra-optimal (32 C) temperatures.…”

“…17 Together with ABI3/ABI5/DELLA complex, FUS3 regulates the expression of heat-induced genes to promote thermoinhibition. 13,18,19 It will be important to determine if the accumulation of the ABI3/ABI5/DELLA complex at high temperature is also due to inhibition of proteasome activity by ABA. Given that GA partially rescues thermoinhibition and increases FUS3 degradation, it is expected that GA would have a stimulatory effect on proteasome activity and protein ubiquitination during germination, by antagonizing ABA action.…”