Temporal small RNA transcriptome profiling unraveled partitioned miRNA expression in developing maize endosperms between reciprocal crosses

MicroRNA transcriptomic analysis of the sixth leaf of maize (Zea mays L.) revealed a regulatory mechanism of jointing stage heterosis

BACKGROUND

Zhengdan 958 (Zheng 58 × Chang 7-2), a commercial hybrid that is produced in a large area in China, is the result of the successful use of the heterotic pattern of Reid × Tang-SPT. The jointing stage of maize is the key period from vegetative to reproductive growth, which determines development at later stages and heterosis to a certain degree. MicroRNAs (miRNAs) play vital roles in the regulation of plant development, but how they function in the sixth leaf at the six-leaf (V6) stage to influence jointing stage heterosis is still unclear.

RESULT

Our objective was to study miRNAs in four hybrid combinations developed in accordance with the Reid × Tang-SPT pattern, Zhengdan 958, Anyu 5 (Ye 478 × Chang 7-2), Ye 478 × Huangzaosi, Zheng 58 × Huangzaosi, and their parental inbred lines to explore the mechanism related to heterosis. A total of 234 miRNAs were identified in the sixth leaf at the V6 stage, and 85 miRNAs were differentially expressed between the hybrid combinations and their parental inbred lines. Most of the differentially expressed miRNAs were non-additively expressed, which indicates that miRNAs may participate in heterosis at the jointing stage. miR164, miR1432 and miR528 families were repressed in the four hybrid combinations, and some miRNAs, such as miR156, miR399, and miR395 families, exhibited different expression trends in different hybrid combinations, which may result in varying effects on the heterosis regulatory mechanism.

CONCLUSIONS

The potential targets of the identified miRNAs are related to photosynthesis, the response to plant hormones, and nutrient use. Different hybrid combinations employ different mature miRNAs of the same miRNA family and exhibit different expression trends that may result in enhanced or repressed gene expression to regulate heterosis. Taken together, our results reveal a miRNA-mediated network that plays a key role in jointing stage heterosis via posttranscriptional regulation.

Integrated transcriptome, small RNA, and degradome analysis reveals the complex network regulating starch biosynthesis in maize

Background

Starch biosynthesis in endosperm is a key process influencing grain yield and quality in maize. Although a number of starch biosynthetic genes have been well characterized, the mechanisms by which the expression of these genes is regulated, especially in regard to microRNAs (miRNAs), remain largely unclear.

Results

Sequence data for small RNAs, degradome, and transcriptome of maize endosperm at 15 and 25 d after pollination (DAP) from inbred lines Mo17 and Ji419, which exhibit distinct starch content and starch granule structure, revealed the mediation of starch biosynthetic pathways by miRNAs. Transcriptome analysis of these two lines indicated that 33 of 40 starch biosynthetic genes were differentially expressed, of which 12 were up-regulated in Ji419 at 15 DAP, one was up-regulated in Ji419 at 25 DAP, 14 were up-regulated in Ji419 at both 15 and 25 DAP, one was down-regulated in Ji419 at 15 DAP, two were down-regulated in Ji419 at 25 DAP, and three were up-regulated in Ji419 at 15 DAP and down-regulated in Ji419 at 25 DAP, compared with Mo17. Through combined analyses of small RNA and degradome sequences, 22 differentially expressed miRNAs were identified, including 14 known and eight previously unknown miRNAs that could target 35 genes. Furthermore, a complex co-expression regulatory network was constructed, in which 19 miRNAs could modulate starch biosynthesis in endosperm by tuning the expression of 19 target genes. Moreover, the potential operation of four miRNA-mediated pathways involving transcription factors, miR169a-NF-YA1-GBSSI/SSIIIa and miR169o-GATA9-SSIIIa/SBEIIb, was validated via analyses of expression pattern, transient transformation assays, and transactivation assays.

Conclusion

Our results suggest that miRNAs play a critical role in starch biosynthesis in endosperm, and that miRNA-mediated networks could modulate starch biosynthesis in this tissue. These results have provided important insights into the molecular mechanism of starch biosynthesis in developing maize endosperm.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5945-1) contains supplementary material, which is available to authorized users.

Heterotic patterns of primary and secondary metabolites in the oilseed crop Brassica juncea

Heterosis refers to the superior performance of F1 hybrids over their respective parental inbred lines. Although the genetic and expression basis of heterosis have been previously investigated, the metabolic basis for this phenomenon is poorly understood. In a preliminary morphological study in Brassica juncea, we observed significant heterosis at the 50% flowering stage, wherein both the growth and reproduction of F1 reciprocal hybrids were greater than that of their parents. To identify the possible metabolic causes or consequences of this heterosis, we carried out targeted LC-MS analysis of 48 primary (amino acids and sugars) and secondary metabolites (phytohormones, glucosinolates, flavonoids, and phenolic esters) in five developmental tissues at 50% flowering in hybrids and inbred parents. Principal component analysis (PCA) of metabolites clearly separated inbred lines from their hybrids, particularly in the bud tissues. In general, secondary metabolites displayed more negative heterosis values in comparison to primary metabolites. The tested primary and secondary metabolites displayed both additive and non-additive modes of inheritance in F1 hybrids, wherein the number of metabolites showing an additive mode of inheritance were higher in buds and siliques (52.77-97.14%) compared to leaf tissues (47.37-80%). Partial least regression (PLS) analysis further showed that primary metabolites, in general, displayed higher association with morphological parameters in F1 hybrids. Overall, our results are consistent with a resource-cost model for heterosis in B. juncea, where metabolite allocation in hybrids appears to favor growth, at the expense of secondary metabolism.

Identification and expression profiles of putative leaf growth related microRNAs in maize (Zea mays L.) hybrid ADA313

Throughout the plant life cycle, growth of new leaves is governed by cell division and cell expansion. During steady-state growth of the maize leaf, these processes are spatially separated between the meristem zone, consisting of dividing cells at the leaf base, the elongation zone, consisting of expanding cells moving upwards from the meristem, and the mature zone containing differentiated mature cells. Increased leaf size can be achieved through increasing cell number or cell size, for example by manipulating the genes controlling the transition between those zones. In this study, microRNA (miRNA) genes, which are a class of endogenous small, non-coding gene regulatory RNAs, were investigated in the growth zones, to gain insight into their role in the transition between cell division and cell expansion. A genome-wide survey was conducted using a miRNA-microarray and 59 miRNA genes were detected to be differentially expressed between the growth zones. miR160, miR166, miR168, miR172, miR319 and miR390 families were significantly up-regulated in the meristem relative to the elongation and mature zones. In contrast, expression of the miR167 and miR396 families was lower in the meristem and higher in the mature zone. Therefore, these were considered to be candidate growth-regulated miRNAs that control cell division processes indirectly by repressing target genes. The miR156, miR166, miR167, miR399, miR408 and miR2275 families were expressed most highly in the elongation zone, and so were classified as elongation-specific, with possible roles in switching from cell division to cell elongation during leaf differentiation. In silico target prediction analysis showed that these miRNAs target several transcription factors and metabolic genes, and a reciprocal relationship between the expression levels of miR319 and miR396 and their targets was confirmed by qRT-PCR. Furthermore, 12 candidate novel miRNAs were identified from the microarray data and computationally verified. Three out of twelve were also validated by qRT-PCR. These findings provide important information regarding the regulatory functions of miRNAs in controlling progression of growth mechanisms.

The pivotal role of small non-coding RNAs in the regulation of seed development

Seeds represent a crucial stage of the seed plants life cycle. It is during seed development that the foundations of the future plant body, and the ability to give rise to a new plant capable of growing under sometimes adverse environmental conditions, are established. Small non-coding RNAs are major regulators of gene expression both at the post-transcriptional and transcriptional levels and, not surprisingly, these elements play major roles in seed development and germination. We review here the current knowledge about small RNA expression and functions in seed development, going from the morphogenesis phase comprehending embryo development and patterning, to the several steps of the maturation phase, ending in the transition to the germination. A special focus is given to the small RNAs for which functional studies have been conducted and their participation in regulatory networks operating in seeds. Many challenges remain ahead for dissecting the complex small RNA landscape in seeds, but this is a highly relevant issue in plant biology and advances in this area will most certainly impact plant breeding.

Development of Incompletely Fused Carpels in Maize Ovary Revealed by miRNA, Target Gene and Phytohormone Analysis

Although the molecular basis of carpel fusion in maize ovary development remains largely unknown, increasing evidence suggests a critical role of microRNAs (miRNAs). In this study, a combination of miRNA sequencing, degradome and physiological analyses was used to characterize carpel fusion development in maize ovaries showing incompletely (IFC) and completely fused carpels (CFC). A total of 162 known miRNAs distributed across 33 families were identified, of which 20 were differentially expressed. In addition, 53 miRNA candidates were identified, of which 10 were differentially expressed in the IFC and CFC ovaries. In degradome analysis, a total of 113 and 11 target genes were predicted for the known and novel miRNAs, respectively. Moreover, 24 (60%) target genes of the differentially expressed known miRNAs were found to code transcription factors, including auxin response factor (ARF), TB1-CYC-PCFs (TCP), APETALA2 (AP2), growth regulating factor (GRF), MYB, NAC, and NF-YA, all of which have been shown to play a role in carpel fusion development. Correlation analysis of these differentially expressed known miRNAs and their targets with phytohormone signals revealed significant correlations with at least one phytohormone signal, the main regulator of carpel fusion development. These results suggest that incomplete carpel fusion is partly the result of differential expression of certain miRNAs and their targets. Overall, these findings improve our knowledge of the effect of miRNA regulation on target expression, providing a useful resource for further analysis of the interactions between miRNAs, target genes and phytohormones during carpel fusion development in maize.

Identification and characterization of microRNAs in maize endosperm response to exogenous sucrose using small RNA sequencing

Sucrose acts as a signaling molecule for genes critical to starch biosynthesis in maize endosperm. Previously, we showed that sucrose could regulate starch biosynthesis in maize via transcription factors. To better understand the complex regulation of starch biosynthesis, the 10days after pollination endosperm from Zea mays L. B73 inbred line was collected and treated with sucrose for small RNA sequencing. The sequencing results revealed that 24 known miRNAs and 190 novel miRNAs were significantly differentially expressed in response to sucrose. In addition, most of target mRNAs were characterized as transcription factors, mainly including, MYB, ARF, NAC, AP2/ERF, WRKY, and GRAS, which play important roles in starch biosynthesis and seed development in maize endosperm. The expression profiles of miR398a/b and miR159b/j/k followed opposite expression trends to their target genes when analyzed by qPCR. In conclusion, these results show that sucrose regulates the expression of starch synthetic genes through miRNAs.