Lettuce seed germination

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(Contributed by Li Bo)

       The timing of seed germination require precise environmentalsensing and integrated responses to multiple inputs so that developmental transitionscan be accurately matched to seasonal conditions[1,2,3]. Seeds use temperature as a signal of the seasonal and currentenvironmental conditions to determine opportune times to germinate with respectto the potential for seedling survival[2, 4, 5].

In their natural setting, wild lettuce seedlings have thehighest probability of survival when germination and emergence occur afterrainfall in autumn or early spring. To avoid premature germination due tooccasional rainfall during summer, lettuce seeds have evolved a hightemperature-induced dormancy mechanism called thermoinhibition. When imbibitionoccurs at warm temperatures (above ~25–30°C for most commercial varieties),germination is prevented until the temperature is lowered[6,7].

LsNCED4, a temperature-regulated gene in the biosynthetic pathway forabscisic acid (ABA), a germination inhibitor, mapped to the center of apreviously detected QTL for high temperature germination (Htg6.1) fromUC96US23[8,9]. Argyris et al.[7] found LsNCED4 as the candidate gene responsible for theHtg6.1 phenotype and indicate that decreased ABA biosynthesis at highimbibition temperatures is a major factor responsible for the increasedgermination thermotolerance of UC96US23 seeds.

       Huo et al.[10] identified two independentgermination mutations in Lettuce seeds in the zeaxanthin epoxidase gene (ABSCISIC ACID DEFICIENT1/ZEAXANTHIN EPOXIDASE, or ABA1/ZEP). The one mutation caused an amino acidreplacement, whereas the other mutation resulted in alternative mRNA splicing.Endogenous abscisic acid contents were reduced in both mutants, and expressionof the ABA1 gene from wild-type lettuce under its own promoter fullycomplemented the two mutant. These results showedthat ABA1/ZEP gene in lettuce regulates seed germination.

       Yoong et al.[11] used RILs population identified a major quantitative trait locus(Htg9.1) associated with the high-temperature germination phenotype. Furthermore,founed a homolog of Arabidopsis ETHYLENE RESPONSE FACTOR1 (termed LsERF1)was differentially expressed between two parents seeds imbibed at hightemperature (30°C). LsERF1 belongs to a large family of transcriptionfactors associated with the ethylenesignaling pathway. The experiment implicate LsERF1 as the gene responsible for the Htg9.1phenotype. Finally, a model is proposed in which LsERF1acts through the promotion of gibberellin biosynthesis to counter theinhibitory effects of abscisic acid and, therefore, promote germination at hightemperatures.

Reference:

[1]Huijser P, Schmid M (2011) The control of developmental phase transitions inplants. Development, 138(19):4117–4129.

[2]Footitt S, Huang Z, Clay HA, Mead A, Finch-Savage WE (2013) Temperature, lightand nitrate sensing coordinate Arabidopsis seed dormancy cycling, resulting inwinter and summer annual phenotypes. Plant J, 74(6):1003–1015.

[3]Chiang GC, Bartsch M, Barua D, Nakabayashi K, Debieu M, Kronholm I, KoornneefM, Soppe WJ, Donohue K, De Meaux J. (2011) DOG1 expression is predicted by theseed-maturation environment and contributes to geographical variation ingermination in Arabidopsis thaliana. Mol Ecol, 20(16):3336–3349.

[4]Batlla D, Benech-Arnold R. (2015) A framework for the interpretation oftemperature effects on dormancy and germination in seed populations showingdormancy. Seed Sci Res, 25:147–158.

[5]Michael M, Katherine K, Tarek E, He HZ, Leónie B, Kathleen D. (2015)DOG1-imposed dormancy mediates germination responses to temperature cues.Environ Exp Bot, 112:33–43.

[6]Argyris J, Dahal P, Hayashi E, Still DW, Bradford KJ. (2008) Genetic Variationfor Lettuce Seed Thermoinhibition Is Associated with Temperature-SensitiveExpression of Abscisic Acid, Gibberellin, and Ethylene Biosynthesis,Metabolism, and Response Genes. Plant Physiology, 148(2):926-947.

[7]Argyris J, Truco MJ, Ochoa O, McHale L, Dahal P, Van Deynze A, Michelmore RW,Bradford KJ. (2011) A gene encoding an abscisic acidbiosynthetic enzyme (LsNCED4) collocates with the high temperature germinationlocus Htg6.1 in lettuce (Lactuca sp.). Theor. Appl. Genet. 122, 95–108.

[8]Argyris J, Truco MJ, Ochoa O, Knapp SJ, Still DW, Lenssen GM, Schut JW,Michelmore RW, Bradford KJ. (2005) Quantitative trait loci associated with seedand seedling traits in Lactuca. Theor Appl Genet 111:1365–1376

[9]Argyris J, Dahal P, Truco MJ, Ochoa O, Still DW, Michelmore RW, Bradford KJ.(2008a) Genetic analysis of lettuce seed thermoinhibition. Acta Hortic782:23–33.

[10]Huo H, Henry IM, Coppoolse ER, Verhoef-Post M, Schut JW, de Rooij H, VogelaarA, Joosen RV, Woudenberg L, Comai L, Bradford KJ. (2016) Rapid identificationof lettuce seed germination mutants by bulked segregant analysis and wholegenome sequencing. Plant J, 88,345-360.

[11]Yoong FY, O'Brien LK, Truco MJ, Huo H, Sideman R, Hayes R, Michelmore RW,Bradford KJ. Genetic Variation for Thermotolerance in Lettuce Seed GerminationIs Associated with Temperature-Sensitive Regulation of ETHYLENE RESPONSEFACTOR1 (ERF1). Plant physiology, 2015, 170(1):472.