SUMMARYAbscisic acid is an essential hormone for seed dormancy. Our previous study using the plant gene switch system, a chemically induced gene expression system, demonstrated that induction of 9-cis-epoxycarotenoid dioxygenase (NCED), a rate-limiting ABA biosynthesis gene, was sufficient to suppress germination in imbibed Arabidopsis seeds. Here, we report development of an efficient experimental system that causes amplification of NCED expression during seed maturation. The system was created with a Triticum aestivum promoter containing ABA responsive elements (ABREs) and a Sorghum bicolor NCED to cause ABAstimulated ABA biosynthesis and signaling, through a positive feedback mechanism. The chimeric gene pABRE:NCED enhanced NCED and ABF (ABRE-binding factor) expression in Arabidopsis Columbia-0 seeds, which caused 9-to 73-fold increases in ABA levels. The pABRE:NCED seeds exhibited unusually deep dormancy which lasted for more than 3 months. Interestingly, the amplified ABA pathways also caused enhanced expression of Arabidopsis NCED5, revealing the presence of positive feedback in the native system. These results demonstrated the robustness of positive feedback mechanisms and the significance of NCED expression, or single metabolic change, during seed maturation. The pABRE:NCED system provides an excellent experimental system producing dormant and non-dormant seeds of the same maternal origin, which differ only in zygotic ABA. The pABRE:NCED seeds contain a GFP marker which enables seed sorting between transgenic and null segregants and are ideal for comparative analysis. In addition to its utility in basic research, the system can also be applied to prevention of pre-harvest sprouting during crop production, and therefore contributes to translational biology.

Summary More than 70% of global food supply depends on seeds. The major seed reserves, such as proteins, lipids, and polysaccharides, are produced during seed maturation. Here, we report that DELAY OF GERMINATION 1‐LIKE 4 (DOGL4) is a major inducer of reserve accumulation during seed maturation. The DOGL family proteins are plant‐specific proteins of largely unknown biochemical function. DOGL4 shares only limited homology in amino acid sequence with DOG1, a major regulator of seed dormancy. DOGL4 was identified as one of the outstanding abscisic acid (ABA)‐induced genes in our RNA sequencing analysis, whereas DOG1 was not induced by ABA. Induction of DOGL4 caused the expression of 70 seed maturation‐specific genes, even in germinating seeds, including the major seed reserves ALBUMIN, CRUCIFERIN and OLEOSIN. Although DOG1 affects the expression of many seed maturation genes, the major seed reserve genes induced by DOGL4 are not altered by the dog1 mutation. Furthermore, the reduced dormancy and longevity phenotypes observed in the dog1 seeds were not observed in the dogl4 mutants, suggesting that these two genes have limited functional overlap. Taken together, these results suggest that DOGL4 is a central factor mediating reserve accumulation in seeds, and that the two DOG1 family proteins have diverged over the course of evolution into independent regulators of seed maturation, but retain some overlapping function.

Induction of nine-cis-epoxycarotenoid dioxygenase 6 (NCED6), an abscisic acid (ABA) biosynthesis gene, alone is sufficient to suspend germination in testa-ruptured seeds, which are at the final step of germination. Molecular consequences of NCED6 induction in imbibed seeds were investigated by RNA sequencing. The analysis identified many unknown and uncharacterized genes that were up-regulated by NCED6 induction, in addition to the major regulators of ABA signalling. Interestingly, other NCEDs were up-regulated by NCED6 induction, suggesting that the major rate-limiting enzymes in the ABA biosynthesis pathway are subject to positive-feedback regulation. ZEAXANTHIN EPOXIDASE and ABSCISIC ALDEHYDE OXIDASE3, which function upstream and downstream of NCED, were also up-regulated in seeds by NCED6 induction, which suggests that the distinct layers of positive feedback loops are coordinately operating in the NCED6-induced seeds. SOMNUS (SOM), which was also up-regulated by NCED6 induction, was the major mediator of enhanced ABA signalling in NCED6-induced seeds. SOM exerted negative effects on GA biosynthesis, which also contributes to a high ABA–GA ratio and reinforces the suppressive state of germination. Besides these coding genes, long intergenic non-coding RNAs (lincRNAs) were also up-regulated upon NCED6 induction (termed N6LINCRs). Conditional expression of N6LINCR1 altered gene expression profiles in seeds. Twenty-six genes were up-regulated and 66 genes were down-regulated by the induction of N6LINCR1. These results suggest that some of N6LINCRs have a regulatory role in gene expression in seeds, which potentially contributes to the regulation of germination by ABA.