It is difficult to separate the capacity for memory from the act of thinking. Indeed, memories are held as so fundamental an act of cognition that the concept of an organism without a brain (let alone a nervous system) storing and acting upon memories is profoundly outlandish. Despite this, research emerging from the field of plant science is continuing to reveal the sophisticated ways plants interact with their environments; discerning friend from foe, communicating information to neighbouring plants, and even recalibrating future stress responses based on the memories of past afflictions. In this issue of Physiologia Plantarum do Amaral et al. (2020 ) investigated the mechanisms by which rice can recall exposure to an earlier stress event to better cope with future hardships; a phenomenon that resonates with the quote by poet W. S. Merwin: ‘What you remember saves you’.

All organisms exist in a world of change. From the imperceptibly slow march of geological processes to the rapid cycling of day and night, these dynamics have shaped the evolutionary trajectories of all life on Earth. While evolution elegantly describes the fluid and reflexive way populations of organisms can respond to challenges at a species level, individual organisms respond to environmental pressures by mounting temporary stress responses. One way that plants have enhanced their ability to deal with periodic stresses, such as droughts or heatwaves, is to store information of an antecedent stress and use this ‘memory’ to respond more rapidly or with greater intensity the next time this stress is perceived. This adaptive strategy is commonly known as priming, and the mechanisms by which these stress memories are stored, retrieved and acted upon are growing topics of importance, particularly in valuable crop species such as rice (for an excellent review, see Crisp et al. 2016).

In light of this, do Amaral et al. (2020) analysed the physiological, biochemical, and epigenetic responses of two rice genotypes (one sensitive to salinity, the other tolerant) in response to 48 hours of salt stress administered at either (1) an early vegetative developmental stage, (2) a later reproductive stage or (3) in response to repeated salt stress at both early and late developmental stages (Fig. 1). Interestingly, the researchers found that in both single and recurring salt treatments, the roots of the tolerant rice genotype actually had consistently higher Na⁺/K⁺ ratios than the roots of the sensitive genotype, while Na⁺/K⁺ ratios in the leaves remained consistently low in both genotypes and all conditions, indicating that the tolerant genotype is less successful at excluding salt from the root and that priming had little effect in this instance. In contrast, most of the physiological markers of acute salt stress in the leaves revealed the benefit of priming in both genotypes, with the positive effects (elevated stomatal conductance and chlorophyll content, reduced electrolyte leakage) being more pronounced in the tolerant genotype. Similarly, measurements of the content of reactive oxygen species (ROS) and the activity of key enzymes tasked with detoxifying these harmful compounds revealed that in both genotypes, priming positively influenced the outcome of treatment with salinity stress. Although these biochemical alterations are symptomatic of stress memory in plants, they do not explain the underlying mechanisms at work.

To answer this question, the researchers catalogued the tell‐tale signs of epigenetic modifications in the two genotypes across the sampling time course. Epigenetics is the study of heritable phenotypic changes that are not solely attributable to alterations in the genome's underlying genetic code, and instead derive from stable and/or inheritable changes to gene expression dynamics. Major drivers of epigenetic regulation include covalent modification (e.g. methylation or acetylation) of DNA and the proteins DNA associates with structurally (histones). These modifications can lead to alterations in the way DNA is packaged and thus influence the accessibility of specific genes. In this study, do Amaral et al. (2020) demonstrated that in the sensitive rice genotype, global levels of DNA methylation were only altered (significantly elevated) when the salt stress was administered at the reproductive stage, contrasting sharply with the tolerant genotype, which displayed huge variation in global DNA methylation across the different treatment time points and a strong relaxation of DNA methylation following the stress treatment (recovery). The researchers then looked at the correlation of these global methylation dynamics with the expression of key genes encoding proteins associated with DNA methylation and demethylation. In the sensitive genotype, they identified a suite of demethylases negatively correlated with overall DNA methylation levels during stress administration but not during the subsequent recovery period, suggesting that active removal of methyl regulatory groups occurs while the stress is ongoing and passive processes likely takeover during recovery. In contrast, the tolerant genotype exhibited less evidence for active regulation of global DNA methylation during the stress and slightly enhanced regulation during recovery.

The study of stress memory in plants has exciting potential, enabling the strategic treatment of plants to bolster their tolerance to future challenges; a powerful tool in the face of a changing climate. These priming effects can be highly durable and persist not only for the lifespan of the affected plant but be passed on to their progeny in a form of generational memory. However, just as in humans, not all memories are positive, with some maladaptive stress memories actually hindering plant recovery and ultimately reducing crop yield. Thus, future priming strategies will need to delineate the beneficial memories that aid a plant's survival from those that are best forgotten, while unpacking the mechanisms by which these processes are regulated.

很难将记忆能力与思维行为分开。确实，记忆被认为是一种基本的认知行为，以至于没有大脑（更不用说神经系统）存储和作用于记忆的有机体的概念是极其荒谬的。尽管如此，植物科学领域的研究仍在继续揭示植物与环境相互作用的复杂方式。辨别敌人，将信息传达给附近的植物，甚至根据过去的苦难记忆重新校准未来的压力反应。在本期《植物志》中，Amaral等人进行了研究。（2020年 ）研究了稻米可以回忆起早期应激事件以更好地应对未来困难的机制；诗人WS Merwin的名言引起了共鸣：“您所记得的东西会拯救您”。

所有生物都存在于变化的世界中。从地质过程的缓慢进展到昼夜的快速循环，这些动力学已经塑造了地球上所有生命的进化轨迹。尽管进化论优雅地描述了生物种群在物种层面上应对挑战的流畅而自反的方式，但单个生物却通过施加临时性压力响应来应对环境压力。植物增强其应对周期性胁迫（例如干旱或热浪）的能力的一种方式是存储先前胁迫的信息，并在下次感知到这种胁迫时使用此“内存”更快或更强地响应。这种自适应策略通常称为启动（priming），存储这些压力记忆的机制2016）。

有鉴于此，请做Amaral等。（2020）分析了两种水稻基因型的生理，生化和表观遗传响应（一种对盐度敏感，另一种对水稻耐性）响应在以下任一条件下施用48小时的盐胁迫：（1）处于早期营养生长阶段；（2） （3）在发育早期和晚期对盐胁迫的反复响应（图1）。有趣的是，研究人员发现，在单一和反复盐处理中，耐性水稻基因型的根实际上一直比敏感基因型的根具有更高的Na ⁺ / K ⁺比，而Na ⁺ / K ⁺在两种基因型和所有条件下，叶片中的比率始终保持较低，这表明耐受型基因型在根部排除盐分方面不太成功，并且在这种情况下，引发作用几乎没有。相反，大多数急性盐胁迫的叶片生理指标显示了两种基因型均能引发的好处，在耐性基因型中，积极作用（气孔导度和叶绿素含量增加，电解质渗漏减少）更为明显。类似地，对活性氧（ROS）含量和负责对这些有害化合物进行解毒的关键酶活性的测量结果表明，在两种基因型中，启动引发均对盐度胁迫的治疗结果产生积极影响。

为了回答这个问题，研究人员在整个采样时间过程中，对两种基因型的表观遗传修饰的迹象进行了分类。表观遗传学是对可遗传表型变化的研究，这种变化不仅仅归因于基因组基础遗传密码的改变，而是源于基因表达动态的稳定和/或可遗传的变化。表观遗传调控的主要驱动力包括DNA的共价修饰（例如甲基化或乙酰化）以及DNA与结构上缔合的蛋白质（组蛋白）。这些修饰会导致DNA封装方式的改变，从而影响特定基因的可及性。在这项研究中，请做Amaral等。（2020年）表明，在敏感的水稻基因型中，仅在生殖阶段施用盐胁迫时，全球DNA甲基化水平才发生改变（显着升高），这与耐性基因型形成鲜明对比，而耐性基因型则显示了不同物种之间全球DNA甲基化的巨大差异处理时间点和应力处理（恢复）后DNA甲基化的强烈松弛。然后，研究人员研究了这些总体甲基化动力学与编码与DNA甲基化和去甲基化相关的蛋白质的关键基因的表达之间的相关性。在敏感的基因型中，他们鉴定出一组脱甲基酶，它们在压力管理期间与总体DNA甲基化水平呈负相关，但在随后的恢复期却没有，这表明在压力持续进行的同时会主动除去甲基调节基团，并且在恢复过程中可能会接管被动过程。相反，耐性基因型显示较少的证据表明在胁迫过程中主动调节总体DNA甲基化，而在恢复过程中调节程度略有提高。

植物中的压力记忆研究具有令人兴奋的潜力，能够对植物进行战略性处理以增强其对未来挑战的耐受性；面对气候变化的强大工具。这些启动作用可以高度持久，不仅在受影响植物的生命周期内持续存在，而且还可以世代记忆的形式传递给它们的后代。但是，就像人类一样，并非所有记忆都是积极的，有些适应不良的压力记忆实际上阻碍了植物的恢复并最终降低了农作物的产量。因此，未来的启动策略将需要划定有益的记忆，以帮助植物从最好被遗忘的记忆中解脱出来，同时解开调节这些过程的机制。