Abstract
Epistasis analysis of gid1 single and double mutants revealed that GID1c is a key positive regulator of seed germination, whereas the GID1b receptor can negatively regulate germination in dormant seeds and in the dark. The GID1 GA receptors were expected to positively regulate germination because the plant hormone gibberellin (GA) is required for seed germination in Arabidopsis thaliana. The three GA hormone receptors, GID1a, GID1b, and GID1c, positively regulate GA responses via GA/GID1‐stimulated destruction of DELLA (Asp‐Glu‐Leu‐Leu‐Ala) repressors of GA responses. The fact that the gid1abc triple mutant but not gid1 double mutants fail to germinate indicates that all three GA receptors can positively regulate non‐dormant seed germination in the light. It was known that the gid1abc triple mutant fails to lose dormancy through the dormancy breaking treatments of cold stratification (moist chilling of seeds) and dry after‐ripening (a period of dry storage). Previous work suggested that there may be some specialization of GID1 gene function during germination because GID1b mRNA expression was more highly induced by after‐ripening, whereas GID1a and GID1c mRNA levels were more highly induced by cold stratification. In light‐germinated dormant seeds, the gid1b mutation can partly rescue the germination efficiency of gid1a but not of gid1c seeds. Thus, GID1b can function as an upstream negative regulator GID1c, a positive regulator of dormant seed germination. Further experiments showed that GID1b can negatively regulate dark germination. Wild‐type Arabidopsis seeds do not germinate well in the dark. The gid1b and gid1ab double mutants germinated much more efficiently than wild type, gid1c, or gid1ac mutants in the dark. The observation that the gid1ab double mutant also shows increased dark germination suggests that GID1b, and to some extent GID1a, can act as upstream negative regulators of GID1c. Since the gid1abc triple mutant failed to germinate in the dark, it appears that GID1c is a key downstream positive regulator of dark germination. This genetic analysis indicates that the three GID1 receptors have partially specialized functions in GA signaling.
Highlights
Plant embryos can survive for extended periods of time in a dry seed, in a state resembling suspended animation
Using the same gid1 alleles, this study examined the effect of gid1 single and double mutants on initial dormancy, after‐ripening, dark germination, and GA sensitivity
GA hormone stimulates germination in many plant species, and it is known that GA hormone biosynthesis is required for germination in Arabidopsis and tomato
Summary
Plant embryos can survive for extended periods of time in a dry seed, in a state resembling suspended animation (reviewed in Bewley, Bradford, Hilhorst, & Nonogaki, 2013). GE AND STEBER biosynthesis mutant ga in the light, but complete rescue of ga dark germination required mutations in DELLA genes RGL2, RGA, and GAI. Rescue of sly germination by after‐ripening and by overexpression of the GID1 receptor genes on the CaMV 35S promoter is associated not with decreased DELLA protein accumulation, but with increased formation of the GID1‐DELLA protein complex (Ariizumi et al, 2013; Fukazawa, Ito, Kamiya, Yamaguchi, & Takahashi, 2015). It appears that GID1‐binding to DELLA blocks DELLA interaction with downstream regulators of GA signaling. For the GA dose–response experiments, 2 and 4 weeks (wk) after‐ ripened seeds were incubated with gentle agitation in 10 μM paclobutrazol (PAC) in 0.5× MS salts and 5 mM MES (2‐(N‐morpholino) ethanesulfonic acid), pH 5.5; Sigma) for 48 hr in dark at 4°C to suppress endogenous GA biosynthesis, washed 6 times with sterile water before plating on MS‐agar containing 0 μM, 0.01 μM, 0.1 μM and 0.5 μM gibberellin A3 (GA3, Phytotechnology) Germination was scored daily for 5 days and averaged over 4 biological replicates
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