Some time ago, Dr. Jemima Brinton and Dr. Crisobal Uauy of the JIC published a review entitled "A reductionist approach to dissecting grain weight and yield in wheat" in the Journal of Integrative Plant Biology. This review discusses the influencing factors of wheat grain regraviation process from multiple levels, which is worth reading in depth by small partners who are interested in wheat grain development. The original text is longer, and we will introduce it in detail in three phases:
Introduction
Increasing global food production is an urgent need, and wheat, the main food crop, provides humans with more than 20% calorific and 23% protein. Increasing wheat production will have a significant impact on global food and nutrition security. However, the current trend of increasing wheat production will not be sufficient to meet future demand, and the situation will be even more critical given the future climate change.
The final harvest yield of a plant is the final result of plant growth and development, so most genes contribute directly or indirectly to the yield. The long-term accumulation of breeding knowledge of wheat shows that we need to improve both the "source" and the "library". While several strategies can be used to achieve this improvement, the theory (rationale) behind it is fundamentally different.
Some current studies have elucidated the final yield by studying individual components of wheat yield, such as panicle number/square meter, panicle number/panicle number, and grain weight. Given the complexity of biological systems, we need an integrated approach, i.e. the final food yields need to be studied within the framework of the entire system, rather than by looking at individual components.
We believe that a simplified approach is needed as an entry point to understand the gene networks and regulatory mechanisms between yield factors. These factors are themselves highly complex and polygenic traits. This requires profiling beyond the traditional yield components (panicle number/square meter, panicle number/panicle and grain weight). By understanding the precise mechanisms by which individual genes regulate individual yield composition, we will better understand the negative correlation between yield compositions and possibly decouple.
This paper focuses on grain weight, an important yield component, which is stable and hereditary, and is itself composed of multiple sub-components, including carpel size, grain morphological parameters (length, width, height), and grain filling rate. Throughout the review, we focused primarily on wheat studies, but also looked at insights from other grains and model species, including barley (Hordeum vulgare), rice (Oryza sativa), and Arabidopsis thaliana. It is important to note that there are fundamental differences between seeds from cereals (e.g., wheat, barley, rice) and other model species (e.g., Arabidopsis thaliana). All seeds are surrounded by a layer of cells called the seed coat, derived from the mother beads. However, in the case of grains, grains are a fruit rather than a true seed. This fruit is called the fruit, and its seed coat is fused with the peel, which is the maternal tissue that comes from the ovary wall. The cereal fruit is a dried, single-seed fruit formed from a single carpel and is unchasped, meaning it does not crack and releases true seeds when ripe. Arabidopsis seeds, on the other hand, are the true seeds contained in the cracked fruit, which is made up of two carpels with many separate seeds inside.
So while some processes and mechanisms are conserved between different species, they also differ in the way grain and seed development is regulated. In this review, we will use the term 'grain' to explicitly refer to grain fruit and 'seed' to refer to Arabidopsis seeds. When it is necessary to mention both words, we will use the term 'seed'.
Panicle grain weight
Each wheat plant will produce multiple tillers (inflorescences), often called spikes, each consisting of flowers arranged into specialized branches called spikelets, which are connected to a spindle (rachis) and alternately distributed. A typical wheat panicle has 15 to 20 spikelets, each consisting of two glumes and 4 to 6 fertile florets. Each floret has two sheathed structures in turn, the outer lemma and the inner palea, which surround the carpel and the three stamens. It is in this flower structure, which surrounds the outer and inner grains, that the wheat grain begins to develop after the heart is fertilized.
The final granular weight is influenced by a variety of factors, including genetics and the environment. In fact, within a single genotype, or even on a single wheat panicle, there is a large degree of difference in grain weight. Not all panicle-like inflorescences and spikelets occur and develop simultaneously, and there may be differences of days or weeks between the first spikelets starting to developing to the last one. The primordials on a single panicle grow and develop at different rates, which means that flowering will occur within a few days. In single panicles, the differentiation and development of spikelets begins in the middle of the panicle and extends bidirectionally towards the top and bottom. Therefore, some studies have shown that the grain weight of the central spikelet is higher than that of the top and bottom spikelets (Figure 1B-d).
The development of florets within a single spikelet is also sequential, but is one-way, starting at the bottom of flowerlet 1 and developing upward on either side of the panicle meristem. In general, small flowers that begin later produce smaller grains. The two most basal florets (florets 1 and 2) begin to form particles of similar size at approximately the same time, however, the largest particles are always produced by floret 2 (Figure 1B-d).
Although each spikelet will produce 8 to 12 floret primordials, usually only the first 4 to 6 florets are likely to be fertile and produce a carpel (including ovary, peduncle, and stigma). A strong correlation between carpel size and final grain weight has been observed, suggesting that grain weight can be determined by the mother, even before the beginning of the grain development itself.
< h1 toutiao-origin="h2" > overview of grain development</h1>
Grain development in wheat and barley begins with ovary fertilization (the ovary is a maternal structure containing ovules) and ends with mature grains consisting of three main tissues: embryo, endosperm, and maternal outer layer. These tissues contain large amounts of starch, protein, and other nutrients that accumulate during grain development. We discuss the processes that underlie food development below and summarize their dynamics in Figure 2.
These data are from multiple independent studies, and most of the reported values are based on the number of days after flowering as the time point (DPA). This makes direct comparisons difficult because genetic differences between different varieties and different environmental conditions (e.g. temperature) affect the speed of these processes. Our purpose here is to provide an overview diagram of the general relationship between these processes, so we'll merge the data into Figure 2. However, this suggests that a more standardized data is needed to describe the development of the data, such as the use of thermal time after anthesis in degree days.
The ovules of wheat and barley consist of a haploid blastocyst surrounded by diploid bead-centered tissue and two beads. During pollen release, the blastocyst undergoes a "double fertilization" phenomenon, forming a triploid endosperm nucleus (a single pollen nucleus fuses with two polar nuclei in the blastocyst) and a diploid embryo (the second pollen nucleus fuses with the egg nucleus). These structures are surrounded by several layers of maternal tissue, including seed coats from bead indumentum and peels from the ovary wall.
In the first few days, the increase in the size of the grain does not change much in appearance relative to the ovary, and the overall shape remains similar ("a blunt inverted cone"). During this period, in the absence of cytoplasmic division or cell wall formation, the endosperm nucleus undergoes several rounds of mitosis. This process produces endosperm cavity cells, a multinucleated cell with large vacuoles. At this point, the cells in the outer layer of the grain are proliferating actively. Cytokinin levels in grains are at their highest levels in the early stages (0-7dpa) and may initiate rapid cell division. Similarly, the sugar-signaling metabolite trehalose-6-phosphate (T6P), in the early stages of grain development and grain grouting, regulates growth and development in maternal and offspring tissues to cope with the availability of anabolics, at its highest value.
After the initial stage, the length of the developing grain increases significantly, reaching a maximum length around 15 years after flowering. 6 days after flowering, endosperm is cytositized, after which the endosperm expands rapidly due to cell division and expansion. Instead, cell proliferation in the outer layer of the grain decreases, and subsequently, the growth of these tissues is mainly due to cell expansion.
At the same time as endosperm cytositization, endosperm also differentiates into the main cell types: starch endosperm, paste powder layer, and transfer cells (Figure 1F). Dicotyledonous plants, including Arabidopsis thaliana, retain only one major cell type in mature seed endosperm, and the presence of multiple different cell types is one of the main differences between dicotyledonous plants and grain crop endosperm.
After the basic structure of the grain is formed, the grain filling period begins. This involves the accumulation of multiple stored ingredients, including starch and proteins such as gliadin and gluten, as well as micronutrients such as iron, zinc and calcium. When the grain filling rate reaches the highest value (∼14ー28 dpa), the dry weight of the grain is approximately doubled, and the grain volume continues to increase, but not in the longitudinal direction.
Grain abscisic acid (ABA) content is positively correlated with the grain filling rate, peaking at the maximum grain filling rate and may play an important role in grain nutrient transport and grain filling processes such as starch accumulation. During this time, the moisture content of the grain also reaches its maximum and is maintained until the grain reaches its maximum dry weight around 40 dpa. Shortly after the grain reaches its maximum dry weight and volume, that is, during physiological maturity, the grain begins to dehydrate and mature.
In addition to cell division and dilation, cell programmed death (PCD) also plays an important role throughout grain development, depending on the tissue type. Shortly after fertilization, most of the bead hearts undergo PCD, and the rest of the tissue differentiates into nucellar projections, a key tissue that transports nutrients from the mother to the endosperm by transferring cells (Figure 1F). PCD is also found in the peel, and as early as 4 dpa, the tissue is made up of many different layers. At about 15 dpa, the peel consists of fewer layers, and the thickness is significantly reduced due to PCD. Conversely, PCD occurs slightly later in the endosperm, in some areas above 16 dpa, and in the entire endosperm at about 30 dpa. Some tissues in the grain remain alive when mature, such as the paste layer and embryo.
(To be continued.) ）