Genetic Variation for Thermotolerance in Lettuce Seed Germination Is Associated with Temperature-Sensitive Regulation of ETHYLENE RESPONSE FACTOR1 (ERF1)

Abstract

ORCID IDs: 0000-0002-1928-9719 (F.-Y.Y.); 0000-0002-9521-6124 (K.J.B.). Seeds of most lettuce (Lactuca sativa) cultivars are susceptible to thermoinhibition, or failure to germinate at temperatures above approximately 28°C, creating problems for crop establishment in the field. Identifying genes controlling thermoinhibition would enable the development of cultivars lacking this trait and, therefore, being less sensitive to high temperatures during planting. Seeds of a primitive accession (PI251246) of lettuce exhibited high-temperature germination capacity up to 33°C. Screening a recombinant inbred line population developed from PI215246 and cv Salinas identified a major quantitative trait locus (Htg9.1) from PI251246 associated with the high-temperature germination phenotype. Further genetic analyses discovered a tight linkage of the Htg9.1 phenotype with a specific DNA marker (NM4182) located on a single genomic sequence scaffold. Expression analyses of the 44 genes encoded in this genomic region revealed that only a homolog of Arabidopsis (Arabidopsis thaliana) ETHYLENE RESPONSE FACTOR1 (termed LsERF1) was differentially expressed between PI251246 and cv Salinas seeds imbibed at high temperature (30°C). LsERF1 belongs to a large family of transcription factors associated with the ethylene-signaling pathway. Physiological assays of ethylene synthesis, response, and action in parental and near-isogenic Htg9.1 genotypes strongly implicate LsERF1 as the gene responsible for the Htg9.1 phenotype, consistent with the established role for ethylene in germination thermotolerance of Compositae seeds. Expression analyses of genes associated with the abscisic acid and gibberellin biosynthetic pathways and results of biosynthetic inhibitor and hormone response experiments also support the hypothesis that differential regulation of LsERF1 expression in PI251246 seeds elevates their upper temperature limit for germination through interactions among pathways regulated by these hormones. Our results support a model in which LsERF1 acts through the promotion of gibberellin biosynthesis to counter the inhibitory effects of abscisic acid and, therefore, promote germination at high temperatures.

DELAY OF GERMINATION1 ( DOG1 ) regulates both seed dormancy and flowering time through microRNA pathways

“…S1B), indicating that these homologs are all functional and that their role in regulation of seed thermoinhibition is conserved. The presence of functional DOG1 alleles in US96UC23 and PI251246 is consistent with lack of association of this locus with quantitative trait loci for high-temperature germination in these genotypes (27,34).…”

“…On the contrary, GA can alleviate the effects of overexpression of DOG1 on thermoinhibition (SI Appendix, Fig. S3) (20), as might be expected from the suppression of expression of GA biosynthetic enzymes by high temperatures in Arabidopsis and lettuce (25,28,34). Ecological genetics studies have shown that, under natural seasonal variation, DOG1 can influence seed germination and flowering times (48).…”

Rapid identification of lettuce seed germination mutants by bulked segregant analysis and whole genome sequencing

“…Loss of function of ABA1 in Arabidopsis caused a germination thermotolerance phenotype (Tamura et al ., ). Interestingly, seeds that are thermotolerant due to loss of function in other hormonal pathways or genetic factors generally also have altered expression of ABA1 or NCED s (Kim et al ., ; Piskurewicz et al ., ; Cheng et al ., ; Gabriele et al ., ; Rizza et al ., ; Chiu et al ., ; Yoong et al ., ), further indicating that ABA indeed plays a primary role in regulating seed thermoinhibition.…”

“…The upper temperature limit for lettuce seed germination is genetically heritable and is influenced by environmental cues such as temperature and light during seed maturation and germination (Argyris et al ., , ; Contreras et al ., , ). Molecular genetic studies have revealed that seed thermoinhibition is regulated by hormonal pathways, particularly abscisic acid (ABA), gibberellins (GA) and ethylene (Tamura et al ., ; Argyris et al ., ; Toh et al ., ; Dong et al ., ; Huo et al ., ; Yoong et al ., ). Among the hormonal regulators, ABA plays a leading role in induction and maintenance of primary seed dormancy (Bewley et al ., ).…”

“…To improve seed germination traits, quantitative trait locus (QTL) mapping and map‐based cloning have been widely used to identify genetic loci underlying mutant phenotypes or desirable seed phenotypes (Alonso‐Blanco et al ., ; Osa et al ., ; Argyris et al ., , ; Bentsink et al ., ; Tamura et al ., ; Nakamura et al ., ; Sugimoto et al ., ; Gu et al ., ; Ye et al ., ; Yoong et al ., ). In addition to gene discovery from germplasm exhibiting natural variation, mutagenesis of commercial varieties or well studied genotypes is widely used for crop improvement and functional analyses (Henikoff and Comai, ; Parry et al ., ).…”