Dormancy and germination: making every seed count in restoration

From 50 to 90% of wild plant species worldwide produce seeds that are dormant upon maturity, with specific dormancy traits driven by species' occurrence geography, growth form, and genetic factors. While dormancy is a beneficial adaptation for intact natural systems, it can limit plant recruitment in restoration scenarios because seeds may take several seasons to lose dormancy and consequently show low or erratic germination. During this time, seed predation, weed competition, soil erosion, and seed viability loss can lead to plant re‐establishment failure. Understanding and considering seed dormancy and germination traits in restoration planning are thus critical to ensuring effective seed management and seed use efficiency. There are five known dormancy classes (physiological, physical, combinational, morphological, and morphophysiological), each requiring specific cues to alleviate dormancy and enable germination. The dormancy status of a seed can be determined through a series of simple steps that account for initial seed quality and assess germination across a range of environmental conditions. In this article, we outline the steps of the dormancy classification process and the various corresponding methodologies for ex situ dormancy alleviation. We also highlight the importance of record‐keeping and reporting of seed accession information (e.g. geographic coordinates of the seed collection location, cleaning and quality information, storage conditions, and dormancy testing data) to ensure that these factors are adequately considered in restoration planning.     

Polymorphic Homoeolog of Key Gene of RdDM Pathway,ARGONAUTE4_9 class Is Associated with Pre-Harvest Sprouting in Wheat (Triticum aestivum L.)

Resistance to pre-harvest sprouting (PHS) is an important objective for the genetic improvement of many cereal crops, including wheat. Resistance, or susceptibility, to PHS is mainly influenced by seed dormancy, a complex trait. Reduced seed dormancy is the most important aspect of seed germination on a spike prior to harvesting, but it is influenced by various environmental factors including light, temperature and abiotic stresses. The basic genetic framework of seed dormancy depends on the antagonistic action of abscisic acid (ABA) and gibberellic acid (GA) to promote dormancy and germination. Recent studies have revealed a role for epigenetic changes, predominantly histone modifications, in controlling seed dormancy. To investigate the role of DNA methylation in seed dormancy, we explored the role of ARGONAUTE4_9 class genes in seed development and dormancy in wheat. Our results indicate that the two wheat AGO4_9 class genes i.e. AGO802 and AGO804 map to chromosomes 3S and 1S are preferentially expressed in the embryos of developing seeds. Differential expressions of AGO802-B in the embryos of PHS resistant and susceptible varieties also relates with DNA polymorphism in various wheat varieties due to an insertion of a SINE-like element into this gene. DNA methylation patterns of the embryonic tissue from six PHS resistant and susceptible varieties demonstrate a correlation with this polymorphism. These results suggest a possible role for AGO802-B in seed dormancy and PHS resistance through the modulation of DNA methylation.     

MicroRNA在种子发育、休眠与萌发过程中的作用

植物microRNA(miRNA)是一类小分子非编码RNA,对植物的生长发育发挥着重要调控作用。种子发育、休眠与萌发是植物生命进程中的重要阶段。在这一阶段内,种子受各种环境因子及内源激素调控,并且不同植物种子具有不同发育及休眠特性。随着人们对种子发育、休眠及萌发机理的探究,越来越多miRNA被鉴定,它们能够基于植物激素信号传导、抗氧化作用、关键转录因子调控等途径参与种子形态建成、物质代谢及各种胁迫响应。本文主要综述了近年来植物miRNA的形成及调控机理,以及在种子发育、休眠及萌发过程中发挥的调控作用,旨在为今后的研究方向提供参考。     

Differential germination behaviour in seeds of the Indian arid zone

Seed germination studies of a number of Indian arid zone species were conducted and the seeds were classified into groups on the basis of germination behaviour. Results of seed germination were also compared with those of old seed collections. Most of the seeds studied showed some form of dormancy, the hard seed coatedness being the most common. InBorreria articularis, germination inhibitors were found. Seeds of different years required different duration of acid scarification for maximum germination and these differences have been attributed to rainfall and other such environmental conditions in those years.     

The relationship between seed dormancy,seed size and weediness,in Crepis tectorum (Asteraceae)

Summary I examined the germination characteristics of weed and outcrop populations of Crepis tectorum to test the hypothesis that the presumably more ephemeral weed habitat favors the highest levels of seed dormancy. The winter annual habit characterizing most plants of this species was reflected in a rapid germination of seeds sown in late summer. A slightly higher fraction of surface-sown seeds of weed plants delayed germination. Buried seeds of weed plants also survived better than seeds produced by plants in most outcrop populations, supporting the idea that weediness favors seed dormancy and a persistent seed bank. However, the differences in seed dormancy between the two ecotypes were small and not entirely consistent. Furthermore, high levels of seed dormancy were induced during burial in the outcrop group, suggesting that there is a potential for a dormant seed population in this habitat as well. Demographic data from one of the outcrop populations verified the presence of a large between-year seed bank. Possible environmental factors favoring seed dormancy in outcrop populations are discussed. The unusually large seeds of weedy Crepis contrasts with the relatively small difference in seed dormancy between the two ecotypes.     

Ecology of seed dormancy and germination in sedges (Carex)

The genus Carex, with its wide distribution and large number of species yet with a rather uniform life history, is a very convenient group for comparative studies of germination ecology at the generic level. The combination of a strict or conditional primary dormancy, a light requirement for germination, low germination at constant temperatures, a positive response to diurnal temperature fluctuations and an induction of secondary dormancy in late spring by increasing environmental temperatures are attributes that were found to be characteristics shared by almost all the Carex species investigated, though there was variation between species in the degree to which these characters were expressed. In almost all species, dormancy was broken by stratification at low temperatures, though few species gained the ability to germinate at temperatures <10 °C. There is evidence that long-term physiological changes and the structure of seed coats can play a decisive role in delaying germination. High dormancy levels were found mainly in Carices with large seeds (>0.9 mg), probably due to a thicker seed coat and hence a higher resistance to germination. Differences in primary dormancy between sedges of various habitats could not be established. However, there was a tendency for temperature limits to be low in forest sedges. Many species of wetlands and open sites showed a greater capability to respond to fluctuating temperatures than species of dry sites. These dormancy and germination traits not only enable the accumulation of seeds in the soil, but also constitute seasonal seed regeneration strategies that rely on the high longevity of seeds and the formation of persistent seed banks. Temperate Carices are mainly adapted to exploit the temporally and spatially infrequent occurrence of canopy gaps that become available only in late spring or early summer, whereas the colonization of gaps at the beginning of the vegetation period is largely prevented by a high temperature requirement for germination. Many of the dormancy and germination characteristics of Carices are important in Cyperaceae generally. A greater diversity of germination responses, however, can be found in the related families, Juncaceae and Poaceae. Our present knowledge is not sufficient to determine unequivocally whether a phylogenetic component contributes significantly to the germination behaviour of the genus Carex, but certain tendencies are clearly indicated.     

Molecular mechanisms of seed dormancy

Seed dormancy is an important component of plant fitness that causes a delay of germination until the arrival of a favourable growth season. Dormancy is a complex trait that is determined by genetic factors with a substantial environmental influence. Several of the tissues comprising a seed contribute to its final dormancy level. The roles of the plant hormones abscisic acid and gibberellin in the regulation of dormancy and germination have long been recognized. The last decade saw the identification of several additional factors that influence dormancy including dormancy-specific genes, chromatin factors and non-enzymatic processes. This review gives an overview of our present understanding of the mechanisms that control seed dormancy at the molecular level, with an emphasis on new insights. The various regulators that are involved in the induction and release of dormancy, the influence of environmental factors and the conservation of seed dormancy mechanisms between plant species are discussed. Finally, expected future directions in seed dormancy research are considered.     

干旱胁迫下白刺花种子大小与萌发对策

种子大小与种子萌发及其与环境因子的关系是植物种子萌发对策研究中的重要科学问题之一。采用PEG模拟干旱法研究不同干旱胁迫强度(0,5%,10%,15%,20%)下,白刺花(Sophora davidii)种子萌发进程、种子大小与种子萌发及种子命运的关系。结果表明:不同干旱胁迫下,白刺花种子具有相似的萌发进程,但中度干旱处理(10%PEG)萌发率显著高于零干旱(0%PEG)和重度干旱处理(P0.05),重度干旱处理(20%PEG)种子萌发开始时间晚于零干旱和中度干旱处理;种子大小与种子萌发开始时间的关系表现为零干旱处理下呈极显著负线性关系,中度干旱处理(5%PEG,10%PEG)下无相关关系,重度干旱处理(15%PEG,20%PEG)下呈负二次曲线关系;种子大小对种子命运的影响表现为零干旱处理有利于大、小种子萌发和小种子休眠,中度干旱处理(10%PEG)增加中等种子萌发、大种子休眠和小种子死亡风险,重度干旱处理(15%PEG,20%PEG)增加大种子死亡风险、中等种子和小种子休眠。综合分析表明,白刺花种子大小与萌发行为及种子命运的关系具有较强的环境依赖性,即种子萌发行为表现为顺境下种子越大萌发越快,逆境下小种子和大种子较中等种子萌发更快;种子命运表现为顺境增加种子死亡的风险,中度干扰有利于种子萌发,逆境则有利于种子休眠。