DELAY OF GERMINATION 1-LIKE 4 acts as an inducer of seed reserve accumulation

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.

Introduction

Seed development can be divided into two major phases: embryogenesis and maturation. Events during embryogenesis include histodifferentiation, which establishes the foundation of the major tissues, organs and a body plan of plants (Bewley et al., 2013). In the maturation phase, the differentiated embryo accumulates seed reserves, such as proteins, lipids and polysaccharides (Bewley et al., 2013).

The seed maturation program is regulated mainly by abscisic acid (ABA). 9-cis-Epoxycarotenoid dioxygenase (NCED) is a rate-limiting enzyme for ABA biosynthesis (Nambara and Marion-Poll, 2005). NCED9 is expressed in the testa, the seed coat, and the embryo of developing Arabidopsis seeds, whereas NCED6 is expressed in the endosperm (Lefebvre et al., 2006). NCED5 is expressed in the embryo and the endosperm (Frey et al., 2012). All of these genes contribute to ABA production in developing seeds, which is essential for normal seed maturation (Meurs et al., 1992).

Another event during seed maturation that is induced by ABA is seed dormancy, which is an intrinsic pause in germination in otherwise favorable germination conditions (Bentsink and Koornneef, 2008; Bewley et al., 2013; Nonogaki, 2014). DELAY OF GERMINATION 1 (DOG1) is a central regulator of seed dormancy (Bentsink et al., 2006), which encodes for a plant-specific protein that enhances ABA signaling through its binding to protein phosphatase 2C (PP2C) ABA HYPERSENSITIVE GERMINATION1 (AHG1) and AHG3 (Née et al., 2018; Nishimura et al., 2018). There are five DOG1-LIKE (DOGL) genes – DOGL1, DOGL2, DOGL3, DOGL4 and DOGL5 – in the Arabidopsis genome (Nishimura et al., 2018). DOGL3 and DOGL5 (DOGL5.2, alternative spliced form similar to DOG1) also bind to PP2C (Nishimura et al., 2018). Overexpression of DOGL3 causes ABA hypersensitivity in seed germination, like DOG1, whereas DOGL5 overexpression does not result in increased ABA sensitivity in seeds (Nishimura et al., 2018). No phenotypes were observed for the dogl1, dogl2 and dogl3 single mutants (Bentsink et al., 2006). The dogl4 mutants exhibit moderately enhanced seed dormancy (Zhu et al., 2018), suggesting instead that DOGL4 is a negative regulator of seed dormancy, unlike DOG1. The dog1 single knock-out mutant seeds exhibit a non-dormant phenotype (Bentsink et al., 2006). Taken together, DOG1 and DOGLs do not seem to share a great deal of redundancy in terms of seed dormancy imposition.

An additional function of DOG1 outside seed dormancy has been demonstrated through genetic and molecular approaches. DOG1 has some synergistic effects with ABA INSENSITIVE 3 (ABI3), a major factor in ABA signaling. The dog1-1 mutation enhances weak phenotypes of the abi3-1 mutants and causes downregulation of the seed maturation genes (Dekkers et al., 2016). These results suggest that DOG1 has additional biological roles in the seed maturation program to dormancy. The involvement of DOGLs in the seed maturation program, other than seed dormancy, is not known.

In this study, we identified DOGL4 as a DOG1 family gene that is induced by ABA and found that DOGL4 induction was robust enough to cause the expression of a number of seed maturation-specific genes, even in germinating seeds. The biological role of DOGL4 will be discussed in the context of the evolution of diverged functions of the DOG1 family proteins in seed maturation and dormancy mechanisms.