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Photoblasticism

Photoblastic response of Japanese katsura tree seed germination[1]

Photoblasticism is a mechanism of seed dormancy. Photoblastic seeds require light in order to germinate.[2] Once germination starts, the stored nutrients that have accumulated during maturation start to be digested which then supports cell expansion and overall growth.[3] Within light-stimulated germination, Phytochrome B (PHYB) is the photoreceptor that is responsible for the beginning stages of germination. When red light is present, PHYB is converted to its active form and moves from the cytoplasm to the nucleus where it upregulates the degradation of PIF1. PIF1, phytochrome-interaction-factor-1, negatively regulates germination by increasing the expression of proteins that repress the synthesis of gibberellin (GA), a major hormone in the germination process.[4] Another factor that promotes germination is HFR1 which accumulates in light in some way and forms inactive heterodimers with PIF1.[5]

Although the exact mechanism is not known, nitric oxide (NO) plays a role in this pathway as well. NO is thought to repress PIF1 gene expression and stabilises HFR1 in some way to support the start of germination.[3] Bethke et al. (2006) exposed dormant Arabidopsis seeds to NO gas and within the next 4 days, 90% of the seeds broke dormancy and germinated. The authors also looked at how NO and GA effects the vacuolation process of aleurone cells that allow the movement of nutrients to be digested. A NO mutant resulted in inhibition of vacuolation but when GA was later added the process was active again leading to the belief that NO is prior to GA in the pathway. NO may also lead to the decrease in sensitivity of Abscisic acid (ABA), a plant hormone largely responsible for seed dormancy.[6] The balance between GA and ABA is important. When ABA levels are higher than GA then that leads to dormant seeds and when GA levels are higher, seeds germinate.[7]

References

  1. ^ Grbić M., Skočajić D., Đukić M., Đunisijević-Bojović D., Marković M. (2015): Photoblastic response of seed germination of Japanese katsura tree (Cercidiphyllum japonicum Siebold & Zucc). 2nd International Conference on Plant Biology, 21st Syposium of the Serbian Plant Physiology Society; Cost Action FA1106 Qualityfruit Workshop; Petnica, 17-20 June, 2015; p. 50
  2. ^ Yang, Liwen; Liu, Shuangrong; Lin, Rongcheng (September 2020). "The role of light in regulating seed dormancy and germination". Journal of Integrative Plant Biology. 62 (9): 1310–1326. doi:10.1111/jipb.13001. ISSN 1672-9072. S2CID 220878159. Retrieved 13 January 2023.
  3. ^ a b Penfield S (September 2017). "Seed dormancy and germination". Current Biology. 27 (17): R874–R878. doi:10.1016/j.cub.2017.05.050. PMID 28898656.
  4. ^ de Wit M, Galvão VC, Fankhauser C (April 2016). "Light-Mediated Hormonal Regulation of Plant Growth and Development". Annual Review of Plant Biology. 67: 513–37. doi:10.1146/annurev-arplant-043015-112252. PMID 26905653.
  5. ^ Li R, Jia Y, Yu L, Yang W, Chen Z, Chen H, Hu X (February 2018). "Nitric oxide promotes light-initiated seed germination by repressing PIF1 expression and stabilizing HFR1". Plant Physiology and Biochemistry. 123: 204–212. doi:10.1016/j.plaphy.2017.11.012. PMID 29248678.
  6. ^ Bethke PC, Libourel IG, Aoyama N, Chung YY, Still DW, Jones RL (March 2007). "The Arabidopsis aleurone layer responds to nitric oxide, gibberellin, and abscisic acid and is sufficient and necessary for seed dormancy". Plant Physiology. 143 (3): 1173–88. doi:10.1104/pp.106.093435. PMC 1820924. PMID 17220360.
  7. ^ Shu K, Meng YJ, Shuai HW, Liu WG, Du JB, Liu J, Yang WY (November 2015). "Dormancy and germination: How does the crop seed decide?". Plant Biology. 17 (6): 1104–12. doi:10.1111/plb.12356. PMID 26095078.
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