The doubled haploid system is the fastest way of hybrid variety production and plays an important role in breeding programs and developmental studies. The most commonly used methods of haploid induction, leading to haploid plants in vitro through the plant tissue/cell culture, are named as the in vitro -based (IVB) methods. These methods have been established in many of the important crops, such as barley, pepper, rapeseed, rice, sugar beet, and wheat. There are ongoing researches to optimize and improve the efficiency of these methods by focusing on factors involved in induction and regeneration phases. These factors mainly include plant genotype, the surrounding environment of parental plants, components of culture medium, the developmental stage of initial gametophytic cells, physical treatments (cold pre-treatment, heat shock) of cultured gametophytic cells, and application of different additives and plant growth regulators. Stress treatment is one of the important prerequisites for stimulation of gametophytic cells to switch towards the sporophytic pathway. However, autophagy and programmed cell death, oxidative stress, and production of reactive oxygen species (ROS) are the major limiting factors in stress-induced embryogenesis. The positive effect of different additives, such as plant growth regulators, chlormequat, polyamines (putrescine, spermidine, and spermine), stress hormones (abscisic acid, jasmonic acid, salicylic acid), DNA demethylating agents and histone deacetylase inhibitors, cellular antioxidants, cell wall remodeling agents (arabinogalactan-proteins), and compatible solutes (proline and chitosan), has been proved on the efficiency of haploid induction through IVB methods. Different mechanisms have been reported through which the aforementioned additives can enhance tolerance to embryo-inducing stresses in plants, and subsequently increase the efficiency of induction phase of IVB methods of haploid induction. Finding the best combination/interaction of inductive stresses and their corresponding chemical enhancers is crucial for successful haploid induction through IVB methods. In the present review, we highlighted recently applied additives to enhance the efficiency of the major IVB methods of haploid induction in different plants. Other potentially applicable additives, those are involved in preventing ROS accumulation, ethylene inhibitors, activating of antioxidant enzyme activity, detoxification capacity, and defense response signal pathway, which could be useful in IVB haploid induction are also discussed. The presented information could be useful to improve the efficiency of developed IVB protocols and/or to develop new protocols in recalcitrant species/genotypes.

中文翻译：

---

基于体外的双单倍体生产：最近的改进

双单倍体系统是杂交品种生产最快的方式，在育种计划和发育研究中发挥着重要作用。最常用的单倍体诱导方法，通过植物组织/细胞培养在体外产生单倍体植物，被称为基于体外（IVB）的方法。这些方法已在许多重要作物中建立，例如大麦、胡椒、油菜籽、水稻、甜菜和小麦。通过关注诱导和再生阶段所涉及的因素，正在进行研究以优化和提高这些方法的效率。这些因素主要包括植物基因型、亲本植物周围环境、培养基成分、初始配子体细胞的发育阶段、物理处理（冷预处理、热休克）培养的配子体细胞，以及不同添加剂和植物生长调节剂的应用。应激处理是刺激配子体细胞转向孢子体途径的重要先决条件之一。然而，自噬和程序性细胞死亡、氧化应激和活性氧 (ROS) 的产生是应激诱导胚胎发生的主要限制因素。不同添加剂的积极作用，如植物生长调节剂、百草枯、多胺（腐胺、亚精胺和精胺）、应激激素（脱落酸、茉莉酸、水杨酸）、DNA 去甲基化剂和组蛋白去乙酰化酶抑制剂、细胞抗氧化剂、细胞壁重塑剂（阿拉伯半乳聚糖蛋白）和相容的溶质（脯氨酸和壳聚糖），已经通过IVB方法证明了单倍体诱导的效率。已经报道了不同的​​机制，通过这些机制，上述添加剂可以增强植物对胚胎诱导胁迫的耐受性，并随后提高单倍体诱导的 IVB 方法的诱导阶段的效率。寻找诱导应激及其相应化学增强剂的最佳组合/相互作用对于通过 IVB 方法成功诱导单倍体至关重要。在本综述中，我们强调了最近应用的添加剂，以提高不同植物中单倍体诱导的主要 IVB 方法的效率。其他潜在适用的添加剂，涉及防止 ROS 积累、乙烯抑制剂、抗氧化酶活性的激活、解毒能力、还讨论了可用于 IVB 单倍体诱导的防御反应信号通路。所提供的信息可能有助于提高已开发的 IVB 协议的效率和/或在顽固的物种/基因型中开发新的协议。