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Maternal temperature history controls progeny vigour

Periodic Reporting for period 1 - MATHCOV (Maternal temperature history controls progeny vigour)

Reporting period: 2018-07-01 to 2020-06-30

Seed dormancy and germination are complicated transition processes that play important roles in plant life history. It has been shown that seed dormancy is tightly controlled by the temperature experience of mother plants. Whilst there have been studies showing how the seed production temperature affect seed germination and dormancy, less attention has been paid to post-germinative growth. This event has frequently been called ‘seed vigour’, and refers to the ability of seeds to establish a vigorous and uniform stand of seedlings. Field crops generally have high seed vigour and can establish in a wide range of soil conditions. In species without dormancy, such as Brassica spp., variation in seed production temperatures also affects the rate of seedling growth. Thus, these same molecular processes that control dormancy are also relevant to early seedling growth parameters that interest seed companies. Interestingly, our previous work suggests that endosperm may be a site of perception of temperature signals that regulate seed germination and seedling growth. The overall objective is to address how seed production temperature controls progeny germination and seedling growth vigour via endosperm. In details:
Objective 1. Identification of key genes underlying maternal temperature effects on seed development and seedling establishment.
Task 1.1 Reconstruction of temperature-responsive gene network.
Task 1.2 phenotypical verification of identified genes.

Objective 2. Identification of stably-inherited epigenetic changes in seedlings that result from temperature effects during seed maturation.
Task 2.1 Investigation of temperature-induced epigenetic changes.
Task 2.2 Identification of pathways involved in temperature-induced progeny vigour difference.
• Work performed
1) We identified endosperm as a critical tissue that perceives temperature signal to control seed germination and seedling growth;
2) We identified that bent cotyledon stage is the precise stage of seed development in which Brassica seeds are most sensitive to temperature.
3) We constructed transcriptomic networks for endosperm and embryo in response to temperature shifts during seed development;
4) We demonstrated that ABA plays a crucial role in regulating endosperm-dependant seed germination in response to temperature changes.
5) We used TILLING to produce key mutants in Brassica oleracea which are involved in sensing temperature.

• Main results achieved so far
First, we constructed the transcriptomic networks in specific seed tissues responding to temperature by performing the time-series RNA-seq during seed maturation and seed germination. A specific cluster was identified in endosperm, which includes previously reported temperature or seed dormancy regulators, such as bZIP67, FUS3, MFT and SPT, ZOU. bZIP67 directly binds to the promoter of DOG1 and activates its expression. FUS3 is a LEFL transcription factor that plays important roles in seed maturation. In wheat MFT was identified as a temperature sensor to regulate seed germination. MFT also regulates seed dormancy with SPT in Arabidopsis as well. ZOU was recently shown to determine seed dormancy depth with another transcription factor ICE1. The similar transcriptomic pattern of these regulators indicates that there are core genes responds to temperature and controls seed germination in endosperm. Furthermore, we demonstrated a contrasting response of endospermic and embryonic DOG1. In high temperatures, endospermic DOG1 increased, while embryonic DOG1 decreased. This is a consequence of seed development, which is driven by high temperature.
Second, we found that ABA plays a key role in seed maturation process. ABA was reduced in endosperm and embryo when plants were in heat shock for only one day. The differential ABA levels further contribute to the final seed germination difference. By examining the time-series transcriptomic data, we found CYP707As were upregulated in high temperature. This accelerated ABA catabolism. Furthermore, we showed that the seed dormancy phenotype of Arabidopsis mutants aba2 and cyp707a1/2 was not dependant on temperature. Further, we demonstrated that absence of ABA attenuates the temperature effect on MFT and DOG1.
We have produced large-scale tissue-specific (endosperm and embryo) transcriptomic data and hormone data during seed maturation and seed germination for Brassica oleracea. Using this data, we have reconstructed the temperature-responsive gene network in endosperm and embryo. This provides a better understanding that how temperature modulates seed development and seed germination. We also generated homozygous double mft mutants in B.oleracea. Together with related Arabidopsis mutants, these plant materials will benefit seed scientists and seed companies. The data and plant materials will ultimately benefit policymakers and farmers by showing how to add resilience to environmental variation on seed performance.