Functional analysis of Arabidopsis NCED6 and NCED9 genes indicates that ABA synthesized in the endosperm is involved in the induction of seed dormancy

Abstract

SummaryThe cleavage of 9-cis-epoxycarotenoids to xanthoxin, catalyzed by 9-cis-epoxycarotenoid dioxygenases, is considered to be the key regulatory step of abscisic acid (ABA) biosynthesis. In Arabidopsis, genes for these enzymes form a multigene family with nine members, only five of which are thought to be involved in ABA production. In contrast to the prominent function of AtNCED3 in stress responses, the physiological and developmental role of the other 9-cis-epoxycarotenoid dioxygenases (NCEDs) remain unknown. Our functional and expression analyses have revealed that AtNCED6 and AtNCED9 are required for ABA biosynthesis during seed development. Reverse genetic analysis showed that ABA levels were reduced in Atnced6 and Atnced9 mutant seeds. In addition, transgenic plants overexpressing the AtNCED6 gene overproduced ABA. In accordance with mutant phenotypes, both AtNCED6 and AtNCED9 exhibited seed-specific expression. Detailed cytological studies were carried out, either by using transcriptional fusions of the promoter with GUS and GFP reporter genes, or by in situ hybridization. Expression of AtNCED6 was observed exclusively in the endosperm during seed development, that of AtNCED9 in both embryo and endosperm at mid-development. In addition to reduced ABA levels, Atnced6 and Atnced9 mutant seeds were also resistant to paclobutrazol, a gibberellin biosynthesis inhibitor. Although seeds of single mutants were still dormant, reduced dormancy was observed in the Atnced6 Atnced9 double-mutant seeds. These demonstrate that ABA synthesized in both the endosperm and the embryo participates in the hormonal balance that controls seed dormancy and germination.

ArabidopsisABA INSENSITIVE4 Regulates Lipid Mobilization in the Embryo and Reveals Repression of Seed Germination by the Endosperm

“…At the transcript level, the endosperm also expresses many genes required for ABA synthesis and catabolism, suggesting that the endosperm might also be able to make and degrade ABA. This has been further demonstrated by a recent report that two carotenoid cleavage dioxygenase isoforms are expressed in the endosperm of maturing seeds and are required for dormancy (Lefebvre et al, 2006). One of these, NCED9, was still expressed in an endosperm-specific manner in germinating seeds (Table 3).…”

“…The endosperm is also vital for normal seed dormancy and may be the principal site for the synthesis of dormancy inducing ABA during seed development (Lefebvre et al, 2006). It is possible that the reduced ABA response observed in the endosperm has evolved as a consequence of higher endogenous levels of ABA in that tissue.…”

High Temperature-Induced Abscisic Acid Biosynthesis and Its Role in the Inhibition of Gibberellin Action in Arabidopsis Seeds

“…In this study, we found that expression of NCED2, NCED5, and NCED9 was up-regulated by high temperature in after-ripened seeds under continuous illumination. NCED6 and NCED9 are responsible for ABA synthesis during seed development and dormancy (Lefebvre et al, 2006). In contrast to NCED9, NCED6 showed weak expression during imbibition, and T-DNA insertion mutants of NCED6 showed no resistance to thermoinhibition (Supplemental Fig.…”

“…In lettuce seeds, thermoinhibition is alleviated by removing or puncturing the endosperm . From spatial expression analysis of NCED genes during development of Arabidopsis seeds, Lefebvre et al (2006) suggested that ABA synthesized in the endosperm contributes to the induction of seed dormancy. We found that expression of NCED9 was predominant over other NCED genes in the embryo, whereas NCED5 expression was predominant in the endosperm of thermoinhibited seeds (Fig.…”

“…NCED6 expression is regulated in a photoreversible manner via phytochrome in seeds, and it plays a role in far-red light-induced suppression of seed germination . NCED6 and NCED9 play a major role in the control of seed development, including dormancy (Lefebvre et al, 2006). Real-time PCR and reporter gene expression analyses indicated that NCED2 and NCED5 are expressed in specific tissues and developmental stages (Tan et al, 2003), but physiological functions of these two NCED genes remain obscure.…”