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LIGHT-STIMULATED SEED GERMINATION:

Many angiosperms go through a process of seed dormancy in order to make sure that

the environment is suitable for optimal growth when the seed starts to germinate. The seed

needs to be able to determine when is the perfect time to germinate and they do that by

sensing environmental cues. Light is one of the signals that starts the seeds’ germination that is

the beginning of that plant’s life cycle. Once germination starts, the stored nutrients that has

accumulated during maturation which then supports cell expansion and overall growth (4).

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 (2). Another factor that promotes

germination is HFR1 which accumulates in light in some way and forms inactive heterodimers

with PIF1 (3).

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 stabilizes HFR1 in some

way to support the start of germination (2). 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 (1). 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 (5). The switch between seed dormancy

and germination needs to occur at a time when the seed has the best chances of surviving and

an important cue that begins the process of seed germination and overall plant growth is light.

Works Cited:

1.Bethke, Paul C, et al. “The Arabidopsis Aleurone Layer Responds to Nitric Oxide, Gibberellin,

and Abscisic Acid and Is Sufficient and Necessary for Seed Dormancy.” Plant Physiology,

American Society of Plant Biologists, Mar. 2007,

www.ncbi.nlm.nih.gov/pmc/articles/PMC1820924/.

2.de Wit, Mieke, et al. “Light-Mediated Hormonal Regulation of Plant Growth and

Development.” Annual Review of Plant Biology, U.S. National Library of Medicine, 29

Apr. 2016, www.ncbi.nlm.nih.gov/pubmed/26905653.

3.Li, Ruijing, et al. “Nitric Oxide Promotes Light-Initiated Seed Germination by Repressing PIF1

Expression and Stabilizing HFR1.” Plant Physiology and Biochemistry : PPB, U.S. National

Library of Medicine, Feb. 2018, www.ncbi.nlm.nih.gov/pubmed/29248678.

4.Penfield, Steven. “Seed Dormancy and Germination.” Current Biology : CB, U.S. National

Library of Medicine, 11 Sept. 2017, www.ncbi.nlm.nih.gov/pubmed/28898656.

5.Shu, K, et al. “Dormancy and Germination: How Does the Crop Seed Decide?” Plant Biology

(Stuttgart, Germany), U.S. National Library of Medicine, Nov. 2015,

www.ncbi.nlm.nih.gov/pubmed/26095078.