Molecular Mechanisms of Photoperiod Control of Plant Seed Size

As the most stable environmental factor in nature, photoperiod widely controls many aspects of plant growth and development. Over the years, people have had a clear understanding of how photoperiod affects plant flowering and the molecular mechanism behind it, but how it affects post-flowering development, especially seed development, remains unclear, and its underlying mechanism needs to be resolved urgently.

On February 6, 2023, Nature Plants published a paper entitled "Photoperiod controls plant seed size in a CONSTANS-dependent manner" by Xingliang Hou's team from the South China Botanical Garden, Chinese Academy of Sciences, which revealed the universal law of photoperiod controls plant seed size.

Based on the response of flowering transition to different day lengths, photoperiod-sensitive plants are mainly classified into long-day (LDs)-induced flowering plants and short-day (SDs)-induced flowering plants. To explore whether photoperiod affects plant seed development, the researchers selected six plants with different photoperiodic properties, including the long-day plants Lotus japonicus, Pisum sativum and Arabidopsis thaliana, and the short-day plants Glycine max, Vigna umbellata and Phaseolus vulgaris, and observed their seed phenotypes under different photoperiod conditions. Interestingly, three long-day plantsand three short-day plants  produced larger seeds under LDs and SDs, respectively, consistent with their respective photoperiodic flowering characteristics.

In-depth research on the long-day plant Arabidopsis thaliana and the short-day plant soybean found that when the photoperiodic response factor CONSTANS (CO) was mutated, plants produced seeds of the same size under different photoperiods, i.e., seed size development loses sensitivity to photoperiod, indicating a key role of CO in photoperiod regulation of seed development. Phenotypic observations of circadian clock genes and photoreceptor gene mutants upstream of CO in the Arabidopsis photoperiod pathway, as well as a series of photoperiod shift experiments further confirmed the central function of CO in mediating photoperiod signaling to regulate seed size.

Through transcriptome, gene expression, genetic analysis and cytological observation, they found that the seed development negative regulatory gene APETALA2 (AP2) is an important target gene of CO. In Arabidopsis grown under LDs and soybean grown under SDs, CO directly repressed AP2 transcription and regulated seed size in a photoperiod-dependent manner. Further analysis revealed that CO-AP2 functions in a maternal-dependent manner by regulating seed coat epidermal cell proliferation.

Based on these results, this study reveals the direct regulation of photoperiod on seed development and elucidates the core function of CO-AP2 in this process. This discovery will deepen people's understanding of how plants with different photoperiod characteristics sense seasonal changes to optimize their reproductive growth, and provide new insights and evidence for the mechanism by which environmental factors directly affect seed development. In addition, seed size is an important agronomic trait that affects crop yield and quality. This study can provide important theoretical guidance for regional planting of crops at different latitudes.

Reference:

Yu, B., He, X., Tang, Y. et al. Photoperiod controls plant seed size in a CONSTANS-dependent manner. Nat. Plants 9, 343–354 (2023). https://doi.org/10.1038/s41477-023-01350-y