Drought Stress Memory At The Plant Cycle Level: A Review Part 1

Abstract: 

Plants are sessile organisms whose survival depends on their strategy to cope with dynamic, stressful conditions. It is urgent to improve the ability of crops to adapt to recurrent stresses to alleviate the negative impacts on their productivity.

The relationship between stressful conditions and memory is very close. The right amount of stress can improve our memory, while excessive stress can weaken our memory.

First, the right amount of stress can stimulate our memory. When we are under a certain degree of stress, we will focus more and think positively to cope with various challenges and pressures. In this case, our memory will be exercised and improved.

Second, excessive stress can greatly affect our memory. When we are under unbearable stress, our bodies secrete a hormone called cortisol, which can harm our brain health, slowing down the metabolism of our brain cells and inhibiting memory formation. This makes us unable to remember things very well in this situation.

Therefore, to maintain a good memory, we need to pay attention to moderate stress regulation and enhance our stress resistance. We can use effective methods to relieve stress, such as moderate exercise, listening to music, adjusting our mentality, etc. Through these methods, we can put ourselves in a moderate state of stress and better use our memory. It can be seen that we need to improve memory, and Cistanche deserticola can significantly improve memory because Cistanche deserticola is a traditional Chinese medicinal material that has many unique effects, one of which is to improve memory. The efficacy of Cistanche deserticola comes from the many active ingredients it contains, including tannic acid, polysaccharides, flavonoid glycosides, etc. These ingredients can promote brain health through a variety of pathways.

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Although our knowledge of plant adaptation to drought has been extensively enhanced during the last decades, recent studies have tackled plant responses to recurrent stresses. The present review synthesizes the major findings from studies addressing plant responses to multiple drought events and demonstrates the ability of plants to memorize drought stress. 

Stress memory is described as a priming effect allowing a different response to reiterated stress when compared to a single stress event. Here, by specifically focusing on water stress memory at the plant cycle level, we describe the different underlying processes at the molecular, physiological, and morphological levels in crops as well as in the model species Arabidopsis thaliana. 

Moreover, a conceptual analysis framework is proposed to study drought stress memory. Finally, the essential role of interactions between plants and soil microorganisms is emphasized during reiterated stresses because their plasticity can play a key role in supporting overall plant resilience.

Keywords: water stress; resilience; plant-microbe interplay; priming; memory genes; soil legacy.

1. Introduction

The world's population should reach around 9.1 billion in 2050. An important increase in food demand is already being observed [1]. At the same time, agricultural production is facing the threat of climate change, which is characterized by more severe and frequent stressful conditions that hamper plant growth. 

Population increase and climate change are creating an unprecedented challenge in breeding plants that are more resilient to climate fluctuations to feed the world population. In this context, drought has been identified as the most important and harmful stress to plant production worldwide, affecting yield at several crucial moments during the crop cycle. 

While numerous studies have characterized the effects of various drought intensities occurring at different plant developmental stages [2,3], fewer have investigated the impact of recurrent drought periods on plant development and growth [4–6]. 

Plants can "remember" a stress event and modify their behavior in response to subsequent stress [6]. This so-called memory is defined as "an ability to access experience so that new responses incorporate relevant information from the past", and "information storage of previous signaling, with the ability to retrieve the information at a much later time." [7]. 

Lämke and Bäurle [8] defined three different types of stress memory: (i) somatic stress memory, mitotically heritable that lasts only during the lifespan of an organism; (ii) intergenerational stress memory that is observable only in the first stress-free offspring generation and (iii) transgenerational memory that is meiotically heritable and observable after more than two stress-free offspring generations. 

Somatic stress memory allows plants that have experienced a stress event to benefit from stored information for days, weeks, or months and to adapt their response when facing further stress. For example, this mechanism has been well characterized in cold hardening [9]. 

Moreover, the information derived from previous stress can be passed on from parents to offspring through intergenerational and transgenerational stress memory [10,11]. This aspect is not discussed in the present review which focuses only on memory within the plant life cycle.
Plant stress memory was first observed in the 1990s when researchers noted that some plants developed an acquired systemic resistance to further infections after being exposed to a pathogen attack [12–14]. 

Since then, it has been found that plant memory allows plants to respond faster or stronger to subsequent stress and may provide enhanced protection when compared to naïve plants that have never encountered any stress. The first studies exploring the topic led to major advances in improving the understanding of priming on abiotic constraints by documenting physiological, proteomic, transcriptional, and epigenetic modifications leading to a stress imprint crucial for plant memory establishment [6]. 

In the present review, we illustrate how these mechanisms are interconnected during recurrent drought events and can help the plant to be more resilient. 

Moreover, because plants strongly interact with the soil and its components, we consider interactions between the plant and soil micro-organisms as another possible piece of the puzzle leading to plant drought memory. 

Indeed, soil microbial community composition and activity, as well as soil physicochemical properties, are shaped by soil legacy effects and could influence plant responses to subsequent stress [15,16].

2. Water Stress Memory: From the Plant Side

Water stress memory has been explored in different crop species with specific focuses ranging from molecular to physiological underlying processes (Table 1). Because water stress events are likely to occur more frequently with climate change, plants may mobilize water stress memory from early stages in their life cycle to minimize or alleviate the negative impact of subsequent stresses on growth and production. 

In some cases, plant priming resulted in a higher and faster response to subsequent stress (Table 1). To date, the general understanding of the mechanism is the following. First, a stress imprint is established during the first stress event, which involves different physiological and molecular mechanisms such as the accumulation of stress-responsive osmoregulating metabolites or the synthesis of protective proteins [9]. 

Second, during the post-stress recovery period, this stored information allows the plant to switch into a permissive state, that allows a faster or stronger response to a subsequent stress. This information storage implies (i) an accumulation of proteins in an inactive conformation [17,18] and of metabolites and phytohormones, and (ii) epigenetic modifications through DNA methylation, histone modification, or chromatin remodeling [9,17,19–21]. Chromatine plasticity, whether meiotically inherited or not, has a crucial role both during immediate stress response and in long-term adaptation [22,23]. 

Third, during the subsequent stress, the prior recruitment of these different compounds reduces the time for their synthesis in large amounts, thus allowing a faster response [24,25]. Pioneer's work in Arabidopsis thaliana and Zea mays [25–27] showed that plants display a transcriptional stress memory in response to multiple exposures to drought, revealing the existence of memory genes. These genes are defined as producing different levels of transcripts in response to the first and the second stress, but basal levels of transcripts are similar to those of the non-primed plants during the recovery period [25]. 

Following this concept, Figure 1 summarizes the classification of "memory genes" and "non-memory genes" into four categories according to the regulation of their expression during the second stress period, when compared to the first period. The expression of the [+/+] (or [−/−]) memory genes (Figure 1a) is induced (or repressed) during both the first and the second stresses when compared to the control, with priming increasing differential expression in the subsequent stress. Some memory genes can also display the opposite regulation in response to the first and second stresses. 

This is the case (i) for the [−/+] memory genes (Figure 1b), the expression of which is down-regulated during the first stress but up-regulated during the second stress, and (ii) for the [+/−] memory genes, the expression of which is up-regulated during the first stress but down-regulated during the subsequent stress. 

On the other hand, genes producing similar levels of transcripts in response to each stress are considered "non-memory genes" and are annotated as [+/=] or [−/=] genes (Figure 1d) [26]. This succession of transcriptional events is translated into physiological changes [18], which are detailed below and summarized in Figure 2.

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