Phytohormones are integral to the regulation of fruit development and maturation. This review expands upon current understanding of the relationship between hormone signaling and fruit development, emphasizing fleshy fruit and highlighting recent work in the model crop tomato (Solanum lycopersicum) and additional species. Fruit development comprises fruit set initiation, growth, and maturation and ripening. Fruit set transpires after fertilization and is associated with auxin and gibberellic acid (GA) signaling. Interaction between auxin and GAs, as well as other phytohormones, is mediated by auxin-responsive Aux/IAA and ARF proteins. Fruit growth consists of cell division and expansion, the former shown to be influenced by auxin signaling. While regulation of cell expansion is less thoroughly understood, evidence indicates synergistic regulation via both auxin and GAs, with input from additional hormones. Fruit maturation, a transitional phase that precipitates ripening, occurs when auxin and GA levels subside with a concurrent rise in abscisic acid (ABA) and ethylene. During fruit ripening, ethylene plays a clear role in climacteric fruits, whereas non-climacteric ripening is generally associated with ABA. Recent evidence indicates varying requirements for both hormones within both ripening physiologies, suggesting rebalancing and specification of roles for common regulators rather than reliance upon one. Numerous recent discoveries pertaining to the molecular basis of hormonal activity and crosstalk are discussed, while we also note that many questions remain such as the molecular basis of additional hormonal activities, the role of epigenome changes, and how prior discoveries translate to the plethora of angiosperm species.

Phytohormones in fruit development and maturation

The early maize (Zea mays) seed undergoes several developmental stages after double fertilization to become fully differentiated within a short period of time, but the genetic control of this highly dynamic and complex developmental process remains largely unknown. Here, we report a high temporal-resolution investigation of transcriptomes using 31 samples collected at an interval of 4 or 6 h within the first six days of seed development. These time-course transcriptomes were clearly separated into four distinct groups corresponding to the stages of double fertilization, coenocyte formation, cellularization, and differentiation. A total of 22,790 expressed genes including 1415 transcription factors (TFs) were detected in early stages of maize seed development. In particular, 1093 genes including 110 TFs were specifically expressed in the seed and displayed high temporal specificity by expressing only in particular period of early seed development. There were 160, 22, 112, and 569 seed-specific genes predominantly expressed in the first 16 h after pollination, coenocyte formation, cellularization, and differentiation stage, respectively. In addition, network analysis predicted 31,256 interactions among 1317 TFs and 14,540 genes. The high temporal-resolution transcriptome atlas reported here provides an important resource for future functional study to unravel the genetic control of seed development.

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High Temporal-Resolution Transcriptome Landscape of Early Maize Seed Development.

Embryophytes (land plants) can be found in almost any habitat on the Earth's surface. All of this ecologically diverse embryophytic flora arose from algae through a singular evolutionary event. Traits that were, by their nature, indispensable for the singular conquest of land by plants were those that are key for overcoming terrestrial stressors. Not surprisingly, the biology of land plant cells is shaped by a core signaling network that connects environmental cues, such as stressors, to the appropriate responses-which, thus, modulate growth and physiology. When did this network emerge? Was it already present when plant terrestrialization was in its infancy? A comparative approach between land plants and their algal relatives, the streptophyte algae, allows us to tackle such questions and resolve parts of the biology of the earliest land plants. Exploring the biology of the earliest land plants might shed light on exactly how they overcame the challenges of terrestrialization. Here, we outline the approaches and rationale underlying comparative analyses towards inferring the genetic toolkit for the stress response that aided the earliest land plants in their conquest of land.

Evo-physio: on stress responses and the earliest land plants.

GRAS are plant-specific transcription factors that play important roles in plant growth and development. Although the GRAS gene family has been studied in many plants, there has been little research on the GRAS genes of Tartary buckwheat (Fagopyrum tataricum), which is an important crop rich in rutin. The recently published whole genome sequence of Tartary buckwheat allows us to study the characteristics and expression patterns of the GRAS gene family in Tartary buckwheat at the genome-wide level. In this study, 47 GRAS genes of Tartary buckwheat were identified and divided into 10 subfamilies: LISCL, HAM, DELLA, SCR, PAT1, SCL4/7, LAS, SHR, SCL3, and DLT. FtGRAS genes were unevenly distributed on 8 chromosomes, and members of the same subfamily contained similar gene structures and motif compositions. Some FtGRAS genes may have been produced by gene duplications; tandem duplication contributed more to the expansion of the GRAS gene family in Tartary buckwheat. Real-time PCR showed that the transcription levels of FtGRAS were significantly different in different tissues and fruit development stages, implying that FtGRAS might have different functions. Furthermore, an increase in fruit weight was induced by exogenous paclobutrazol, and the transcription level of the DELLA subfamily member FtGRAS22 was significantly upregulated during the whole fruit development stage. Therefore, FtGRAS22 may be a potential target for molecular breeding or genetic editing. Collectively, this systematic analysis lays a foundation for further study of the functional characteristics of GRAS genes and for the improvement of Tartary buckwheat crops.

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Genome-wide identification, expression analysis and functional study of the GRAS gene family in Tartary buckwheat (Fagopyrum tataricum)

Differentially expressed bZIP transcription factors confer multi-tolerances in Gossypium hirsutum L.

Cotton is a major fiber and oil crop worldwide. Cotton production, however, is often threatened by abiotic environmental stresses. GRAS family proteins are among the most abundant transcription factors in plants and play important roles in regulating root and shoot development, which can improve plant resistance to abiotic stresses. However, few studies on the GRAS family have been conducted in cotton. Recently, the G. hirsutum genome sequences have been released, which provide us an opportunity to analyze the GRAS family in G. hirsutum. In total, 150 GRAS proteins from G. hirsutum were identified. Phylogenetic analysis showed that these GRAS protins could be classified into 14 subfamilies including SCR, DLT, OS19, LAS, SCL4/7, OS4, OS43, DELLA, PAT1, SHR, HAM, SCL3, LISCL and G_GRAS. The gene structure and motif distribution analysis of the GRAS members in G. hirsutum revealed that many genes of the SHR subfamily have more than one intron, which maybe a kind of form in the evolution of plant by obtaining or losing introns. Chromosomal location and duplication analysis revealed that segment and tandem duplication maybe the reasons of the expension of the GRAS family in cotton. Gene expression analysis confirmed the expression level of GRAS members were up-regulated under different abiotic stresses, suggesting that their possible roles in response to stresses. What’s more, higher expression level in root, stem, leaf and pistil also indicated these genes may have effect on the development and breeding of cotton. This study firstly shows the comprehensive analysis of GRAS members in G. hirsutum. Our results provide important information about GRAS family and a framework for stress-resistant breeding in G. hirsutum.

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Genome-wide analysis of GRAS transcription factor gene family in Gossypium hirsutum L.

Atractylodes lancea Thunb. DC (cangzhu) is a traditional Chinese medicinal plant (Cai et al., 2020). In June 2020, leaf spots were observed in A. lancea plants at the Chongqing Institute of Medicinal Plant Cultivation located in Nanchuan District, Chongqing, China (29°8'26.46″ N, 107°13'23'21″ E). Approximately 75% of the plants displayed leaf spot, partial leaf wilting, and stunted growth, and some plants died. To determine the cause of this disease, five typical leaf spots were cut into small pieces. The pieces were successively surface-disinfected with 0.5% NaClO for 1 min and 75% ethanol for 30 s, washed thrice with sterile water, and placed on potato dextrose agar (PDA) to incubate at 25 ℃. These isolates initially formed abundant white aerial mycelium, then gradually developed a rose pigmentation with a brownish color in the center and grayish rose at the periphery of the colony (Li et al. 2019). Mycelial tips were picked and placed on carnation leaf agar (CLA) and inoculated for 7 days. The macroconidia of the isolates were slender, distinctively curved in the bottom half of the apical cell, and sickle-shaped, with 3-4 septa. They ranged in size from 16.68-26.49 × 1.48-2.34 μm (n=50). The microconidia were fusiform with or without one septum. Their size ranged from 6.19-11.02 × 1.25-1.43 μm (n=50) (Li et al. 2019). The morphological characteristics of the isolates were consistent with those of Fusarium spp. PCR amplification and DNA sequencing of the internal transcribed spacer (ITS) region and β-tubulin (TUB2) gene were performed using the primers ITS1/ITS4 (White et al. 1990) and Bt-2a/Bt-2b (Robideau et al. 2011), respectively. BLASTn analysis revealed that the ITS sequences of the isolates were 100% identical to those of the F. acuminatum isolates from the Fusarium MLST database (http://isolate.fusariumdb.org/guide.php). Further analysis revealed that the TUB2 sequences were 99.14% identical to those of the F. acuminatum strain S16 isolates (MF662644) from the GeneBank database of the NCBI server. Based on the morphology and sequence analyses, the isolates were identified as F. acuminatum. Pathogenicity tests were conducted on 1.5-year-old A. lancea plants by inoculating spore suspensions under greenhouse conditions (25°C). For this, wound were made on leaves by piercing with sterilized toothpicks. 30 μl of spore suspension containing 2 × 106 conidia/ml was placed on each wound. Wounds on the leaves of control plants were inoculated with 10 μl of sterile distilled water. There were three plants for each treatment. After incubation at 25 °C for 5 days in a greenhouse, the leaves of the treated plants all showed partial wilting, consistent with the field observations. No symptoms were observed in controlled plants. The fungi were again isolated from the symptomatic tissues and were identical to the original isolate. The experiment was repeated twice with similar results. Pathogenicity symptoms were similar to what was first observed in the field and the isolated fungi were verified based on morphological characteristics, thus fulfilling Koch's postulate. To the best of our knowledge, this is the first time that A. lancea leaf spot caused by F. acuminatum has been discovered in China. The leaf spot caused by F. acuminatum on A. lancea has serious yield loss, and proper control measures should be applied.

Xue Y. Zhang

Papers

Genome-wide analysis of GRAS transcription factor gene family in Gossypium hirsutum L.

First report of Atractylodes lancea leaf spot caused by Fusarium acuminatum in China.