Seed Biology Updates – Highlights and New Discoveries in Seed Dormancy and Germination Research

Abstract

An understanding of the biology of seeds has been greatly advanced in recent years. The progresses, particularly in the field of seed dormancy and germination research, have been made at a remarkable speed. Some of the possible epigenetic mechanisms, including an involvement of non-coding RNA, which were predicted for DELAY OF GERMINATION1 just a few years ago, have now been demonstrated with strong molecular and genetic evidence. Imprinting, or parent-of-origin-specific gene silencing/expression, which was characterized particularly for developing seeds, was also found in imbibed seeds and suggested for dormancy mechanisms. Hormone biology in seeds, which is the most advanced and almost a traditional area of seed research, also presents a new dimension. Upstream regulators of hormone metabolism and hormone transporters, such as abscisic acid and gibberellin influx/efflux carriers, have been identified. Characterization of the novel posttranslational modification pathways, including the N-end rule and S-nitrosylation pathways, which play a critical role in turnover of the major hormone signal transduction proteins, also expanded our knowledge about the complexity of hormone signaling in seeds. These progresses made at the molecular level are significant steps toward a better understanding of how seeds translate soil and other environmental signals into their internal hormone biology and make an important decision to stay dormant or commence with germination.

Seed germination and dormancy: The classic story, new puzzles, and evolution

“…ABA biosynthesis and deactivation play important roles in seed dormancy (Bewley et al ). The continuous ABA biosynthesis in imbibed seeds, which is catalyzed by the rate‐limiting enzyme NCED (Lefebvre et al ; Millar et al ; Frey et al ), seems to be an essential “final output” of seed dormancy (Nonogaki ). Application of fluridone, a carotenoid biosynthesis inhibitor that averts ABA biosynthesis, releases seed dormancy in most occasions, including the deep dormancy imposed by the DOG1 transgene (Née et al ) ( DOG1 discussed below).…”

“…This gene is expressed when dormancy is released (Kushiro et al ; Millar et al ; Barrero et al ). Nodule Inception (NIN)‐like protein 8 (NLP8) has been identified as a direct inducer of CYP707A2 in imbibed seeds in response to nitrate and dormancy release (Yan et al ; summarized in Nonogaki ). On the other hand, direct upstream regulators of NCED that are responsible for the maintenance of dormancy in imbibed seeds are still missing, although the transcription factor (NGATHA1) involved in NCED3 induction during drought responses has been recently identified (Sato et al ).…”

“…Based on Fujii et al ; Ma et al ; Park et al ; Cutler et al ; Antoni et al ; Née et al . Modified from Nonogaki and Nonogaki ; Nishimura et al . See text for details.…”

“…Redox state changes in seeds have been proposed to be a key to controlling seed dormancy and germination (Nee et al ). Nitric oxide triggers S ‐nitrosylation of ABI5, which results in the destabilization of this germination suppressor (Albertos et al ; summarized in Nonogaki ). Hemoglobin, a heme‐containing protein, is a scavenger of NO and negatively affects S ‐nitrosylation of ABI5 (hence stabilizing ABI5) (summarized in Nonogaki ).…”

“…For comprehensive understanding of seed germination and dormancy, it is crucial to understand how the seed covering tissues are modified during imbibition to cause radicle emergence and what molecular and biochemical events impose the inability of embryo growth during dormancy. Recent reviews summarized epigenetic regulation of seed dormancy and germination (Nonogaki ; Nonogaki ). In this article, a specific focus will be placed on the new developments in the classic story of endosperm weakening, which is a core mechanism of seed germination.…”