Difference in chilling-induced flavonoid profiles, antioxidant activity and chilling tolerance between soybean near-isogenic lines for the pubescence color gene

Cellular dynamics in the maize leaf growth zone during recovery from chilling depends on the leaf developmental stage

KEY MESSAGE

A novel non-steady-state kinematic analysis shows differences in cell division and expansion determining a better recovery from a 3-day cold spell in emerged compared to non-emerged maize leaves. Zea mays is highly sensitive to chilling which frequently occurs during its seedling stage. Although the direct effect of chilling is well studied, the mechanisms determining the subsequent recovery are still unknown. Our goal is to determine the cellular basis of the leaf growth response to chilling and during recovery of leaves exposed before or after their emergence. We first studied the effect of a 3-day cold spell on leaf growth at the plant level. Then, we performed a kinematic analysis to analyse the dynamics of cell division and elongation during recovery of the 4th leaf after exposure to cold before or after emergence. Our results demonstrated cold more strongly reduced the final length of non-emerged than emerged leaves (- 13 vs. - 18%). This was not related to growth differences during cold, but a faster and more complete recovery of the growth of emerged leaves. This difference was due to a higher cell division rate on the 1st and a higher cell elongation rate on the 2nd day of recovery, respectively. The dynamics of cell division and expansion during recovery determines developmental stage-specific differences in cold tolerance of maize leaves.

Chilling tolerance in rice: Past and present

Rice is generally sensitive to chilling stress, which seriously affects growth and yield. Since early in the last century, considerable efforts have been made to understand the physiological and molecular mechanisms underlying the response to chilling stress and improve rice chilling tolerance. Here, we review the research trends and advances in this field. The phenotypic and biochemical changes caused by cold stress and the physiological explanations are briefly summarized. Using published data from the past 20 years, we reviewed the past progress and important techniques in the identification of quantitative trait loci (QTL), novel genes, and cellular pathways involved in rice chilling tolerance. The advent of novel technologies has significantly advanced studies of cold tolerance, and the characterization of QTLs, key genes, and molecular modules have sped up molecular design breeding for cold tolerance in rice varieties. In addition to gene function studies based on overexpression or artificially generated mutants, elucidating natural allelic variation in specific backgrounds is emerging as a novel approach for the study of cold tolerance in rice, and the superior alleles identified using this approach can directly facilitate breeding.

A pubescence color gene enhances tolerance to cold-induced seed cracking in yellow soybean

In yellow soybean, severe cold weather causes seed cracking on the dorsal side. Yellow soybeans carry the I or iⁱ allele of the I locus and have a yellow (I) or pigmented (iⁱ ) hilum. We previously isolated an additional allele, designated as Ic, of the I locus, and reported that yellow soybeans with the IcIc genotype may be tolerant to cold-induced seed cracking. The Ic allele by itself, however, does not confer high tolerance. The association of a pubescence color gene (T) with suppression of low-temperature-induced seed coat deterioration has been previously reported. In the present study, we tested whether T is effective for the suppression of cold-induced seed cracking using two pairs of near-isogenic lines for the T locus in the iⁱiⁱ or IcIc background. In both backgrounds, the cracked seed rate of the near-isogenic line with the TT genotype was significantly lower than that with the tt genotype, which indicates that T has an inhibitory effect on cold-induced seed cracking. Furthermore, we also showed that gene pyramiding of Ic and T can improve tolerance to cold-induced seed cracking. Our findings should aid the development of highly SC-tolerant cultivars in soybean breeding programs.

Enhancement of plant cold tolerance by soybean RCC1 family gene GmTCF1a

BACKGROUND

Low temperature severely limits the growth, yield, and geographic distributions of soybean. Soybean plants respond to cold stress by reprogramming the expression of a series of cold-responsive genes. However, the intrinsic mechanism underlying cold-stress tolerance in soybean remains unclear. A. thaliana tolerant to chilling and freezing 1 (AtTCF1) is a regulator of chromosome condensation 1 (RCC1) family protein and regulates freezing tolerance through an independent C-repeat binding transcription factor (CBF) signaling pathway.

RESULTS

In this study, we identified a homologous gene of AtTCF1 in soybean (named GmTCF1a), which mediates plant tolerance to low temperature. Like AtTCF1, GmTCF1a contains five RCC1 domains and is located in the nucleus. GmTCF1a is strongly and specifically induced by cold stress. Interestingly, ectopic overexpression of GmTCF1a in Arabidopsis greatly increased plant survival rate and decreased electrolyte leakage under freezing stress. A cold-responsive gene, COR15a, was highly induced in the GmTCF1a-overexpressing transgenic lines.

CONCLUSIONS

GmTCF1a responded specifically to cold stress, and ectopic expression of GmTCF1a enhanced cold tolerance and upregulated COR15a levels. These results indicate that GmTCF1a positively regulates cold tolerance in soybean and may provide novel insights into genetic improvement of cold tolerance in crops.

Seasonal variations in group leaf characteristics in species with red young leaves

The leaves of many plants are red during particular stages of their lives, but the adaptive significance of leaf colouration is not yet clearly understood. In order to reveal whether anthocyanins play a similar role (i.e. antioxidants) in different seasonal contexts, this study investigated species with red young leaves in the subtropical forest of Dinghushan biosphere reserve (South China) during summer and winter and compared group leaf characteristics between the two seasons. Of 62 total species, 33 exhibited red young leaves in summer only, 6 in winter only, and 23 in both seasons. The anthocyanins extracted from most of these species had an absorption peak at ~530 nm. Frequency distribution analysis showed that the species containing anthocyanins at levels ranging from 0.02 to 0.04 μmol cm-2 occurred most frequently in summer or winter. Based on conditional grouping of the species, no significant variation was observed in the average anthocyanin contents and antioxidant abilities between summer and winter; the flavonoid content in summer was 2-fold that in winter, whereas the anthocyanin:flavonoid ratio in summer was only half that in winter. Moreover, a positive correlation between anthocyanins and flavonoids was found in summer. Therefore, it is less likely for anthocyanins to serve as antioxidants in summer than winter, because such a function in summer leaves is readily replaced by other flavonoids.

Anthocyanins and flavonols are responsible for purple color of Lablab purpureus (L.) sweet pods

Lablab pods, as dietary vegetable, have high nutritional values similar to most of edible legumes. Moreover, our studies confirmed that purple lablab pods contain the natural pigments of anthocyanins and flavonols. Compared to green pods, five kinds of anthocyanins (malvidin, delphinidin and petunidin derivatives) were found in purple pods by HPLC-ESI-MS/MS and the major contents were delphinidin derivatives. Besides, nine kinds of polyphenol derivatives (quercetin, myricetin, kaempferol and apigenin derivatives) were detected by UPLC-ESI-MS/MS and the major components were quercetin and myricetin derivatives. In order to discover their molecular mechanism, expression patterns of biosynthesis and regulatory gens of anthocyanins and flavonols were investigated. Experimental results showed that LpPAL, LpF3H, LpF3'H, LpDFR, LpANS and LpPAP1 expressions were significantly induced in purple pods compared to green ones. Meanwhile, transcripts of LpFLS were more abundant in purple pods than green or yellow ones, suggestind that co-pigments of anthocyanins and flavonols are accumulated in purple pods. Under continuously dark condition, no anthocyanin accumulation was detected in purple pods and transcripts of LpCHS, LpANS, LpFLS and LpPAP1 were remarkably repressed, indicating that anthocyanins and flavonols biosynthesis in purple pods was regulated in light-dependent manner. These results indicate that co-pigments of anthocyanins and flavonols contribute to purple pigmentations of pods.

Identification of chilling-responsive microRNAs and their targets in vegetable soybean (Glycine max L.)

Chilling stress is a major factor limiting the yield and quality of vegetable soybean (Glycine max L.) on a global scale. In the present study, systematic identification and functional analysis of miRNAs under chilling stress were carried out to clarify the molecular mechanism of chilling resistance. Two independent small RNA libraries from leaves of soybean were constructed and sequenced with the high-throughput Illumina Solexa system. A total of 434 known miRNAs and 3 novel miRNAs were identified. Thirty-five miRNAs were verified by qRT-PCR analysis. Furthermore, their gene targets were identified via high-throughput degradome sequencing. A total of 898 transcripts were targeted by 54 miRNA families attributed to five categories. More importantly, we identified 51 miRNAs differentially expressed between chilling stress and control conditions. The targets of these miRNAs were enriched in oxidation-reduction, signal transduction, and metabolic process functional categories. Our qRT-PCR analysis confirmed a negative relationship among the miRNAs and their targets under chilling stress. Our work thus provides comprehensive molecular evidence supporting the involvement of miRNAs in chilling-stress responses in vegetable soybean.

Staying Alive: Molecular Aspects of Seed Longevity

Mature seeds are an ultimate physiological status that enables plants to endure extreme conditions such as high and low temperature, freezing and desiccation. Seed longevity, the period over which seed remains viable, is an important trait not only for plant adaptation to changing environments, but also, for example, for agriculture and conservation of biodiversity. Reduction of seed longevity is often associated with oxidation of cellular macromolecules such as nucleic acids, proteins and lipids. Seeds possess two main strategies to combat these stressful conditions: protection and repair. The protective mechanism includes the formation of glassy cytoplasm to reduce cellular metabolic activities and the production of antioxidants that prevent accumulation of oxidized macromolecules during seed storage. The repair system removes damage accumulated in DNA, RNA and proteins upon seed imbibition through enzymes such as DNA glycosylase and methionine sulfoxide reductase. In addition to longevity, dormancy is also an important adaptive trait that contributes to seed lifespan. Studies in Arabidopsis have shown that the seed-specific transcription factor ABSCISIC ACID-INSENSITIVE3 (ABI3) plays a central role in ABA-mediated seed dormancy and longevity. Seed longevity largely relies on the viability of embryos. Nevertheless, characterization of mutants with altered seed coat structure and constituents has demonstrated that although the maternally derived cell layers surrounding the embryos are dead, they have a significant impact on longevity.

Linkage mapping, molecular cloning and functional analysis of soybean gene Fg2 encoding flavonol 3-O-glucoside (1 → 6) rhamnosyltransferase

There are substantial genotypic differences in the levels of flavonol glycosides (FGs) in soybean leaves. The first objective of this study was to identify and locate genes responsible for FG biosynthesis in the soybean genome. The second objective was to clone and verify the function of these candidate genes. Recombinant inbred lines (RILs) were developed by crossing the Kitakomachi and Koganejiro cultivars. The FGs were separated by high performance liquid chromatography (HPLC) and identified. The FGs of Koganejiro had rhamnose at the 6″-position of the glucose or galactose bound to the 3-position of kaempferol, whereas FGs of Kitakomachi were devoid of rhamnose. Among the 94 RILs, 53 RILs had HPLC peaks classified as Koganejiro type, and 41 RILs had peaks classified as Kitakomachi type. The segregation fitted a 1:1 ratio, suggesting that a single gene controls FG composition. SSR analysis, linkage mapping and genome database survey revealed a candidate gene in the molecular linkage group O (chromosome 10). The coding region of the gene from Koganejiro, designated as GmF3G6″Rt-a, is 1,392 bp long and encodes 464 amino acids, whereas the gene of Kitakomachi, GmF3G6″Rt-b, has a two-base deletion resulting in a truncated polypeptide consisting of 314 amino acids. The recombinant GmF3G6″Rt-a protein converted kaempferol 3-O-glucoside to kaempferol 3-O-rutinoside and utilized 3-O-glucosylated/galactosylated flavonols and UDP-rhamnose as substrates. GmF3G6″Rt-b protein had no activity. These results indicate that GmF3G6″Rt encodes a flavonol 3-O-glucoside (1 → 6) rhamnosyltransferase and it probably corresponds to the Fg2 gene. GmF3G6″Rt was designated as UGT79A6 by the UGT Nomenclature Committee.

Dynamic changes in catechin levels and catechin biosynthesis-related gene expression in albino tea plants (Camellia sinensis L.)

Tea (Camellia sinensis (L.) O. Kuntze) leaves are a major source of flavonoids that mainly belong to the flavan-3-ols or catechins and are implicated in a wide range of health benefits. Although the catechins in tea leaves were identified long ago, the regulatory mechanisms governing catechin biosynthesis remain unclear. In the present work, the dynamic changes of catechin levels and the expression profiles of catechin-related genes in albino tea plants were intensively examined. The amounts of most catechins decreased to their lowest levels in the albino phase, when epigallocatechingallate was the highest of the catechins compared to all catechins, and catechin the lowest. Enzyme assays indicated that phenylalanine ammonia-lyase (PAL) activity was positively correlated with the concentration of catechins (r = 0.673). Gene expression profiling by quantitative real-time reverse transcription-polymerase chain reaction showed that the transcript abundance of flavonoid biosynthetic genes followed a tightly regulated biphasic pattern, and was affected by albinism. These genes (PAL, C4H, 4CL, CHS, CHI, F3H, FLS, F3'H, F3'5'H, DFR, LAR, ANS and ANR) encode enzymes in flavonoid biosynthesis. The expression levels of PAL, F3H and FLS were correlated with the concentration of catechins and the correlation coefficients were -0.683, 0.687 and -0.602, respectively. Therefore, these results indicate that PAL might be a core regulator in the control of catechin biosynthesis in albino tea plants.

Evaluation of candidate reference genes for normalization of quantitative RT-PCR in soybean tissues under various abiotic stress conditions

Quantitative RT-PCR can be a very sensitive and powerful technique for measuring differential gene expression. Changes in gene expression induced by abiotic stresses are complex and multifaceted, which make determining stably expressed genes for data normalization difficult. To identify the most suitable reference genes for abiotic stress studies in soybean, 13 candidate genes collected from literature were evaluated for stability of expression under dehydration, high salinity, cold and ABA (abscisic acid) treatments using delta CT and geNorm approaches. Validation of reference genes indicated that the best reference genes are tissue- and stress-dependent. With respect to dehydration treatment, the Fbox/ABC, Fbox/60s gene pairs were found to have the highest expression stability in the root and shoot tissues of soybean seedlings, respectively. Fbox and 60s genes are the most suitable reference genes across dehydrated root and shoot tissues. Under salt stress the ELF1b/IDE and Fbox/ELF1b are the most stably expressed gene pairs in roots and shoots, respectively, while 60s/Fbox is the best gene pair in both tissues. For studying cold stress in roots or shoots, IDE/60s and Fbox/Act27 are good reference gene pairs, respectively. With regard to gene expression analysis under ABA treatment in either roots, shoots or across these tissues, 60s/ELF1b, ELF1b/Fbox and 60s/ELF1b are the most suitable reference genes, respectively. The expression of ELF1b/60s, 60s/Fbox and 60s/Fbox genes was most stable in roots, shoots and both tissues, respectively, under various stresses studied. Among the genes tested, 60s was found to be the best reference gene in different tissues and under various stress conditions. The highly ranked reference genes identified from this study were proved to be capable of detecting subtle differences in expression rates that otherwise would be missed if a less stable reference gene was used.

The soybean F3'H protein is localized to the tonoplast in the seed coat hilum

We previously isolated a soybean (Glycine max (L.) Merr.) flavonoid 3'-hydroxylase (F3'H) gene (sf3'h1) corresponding to the T locus, which controls pubescence and seed coat color, from two near-isogenic lines (NILs), To7B (TT) and To7G (tt). The T allele is also associated with chilling tolerance. Here, Western-blot analysis shows that the sf3'h1 protein was predominantly detected in the hilum and funiculus of the immature seed coat in To7B, whereas sf3'h1 was not detected in To7G. A truncated sf3'h1 protein isolated from To7G was detected only upon enrichment by immunoprecipitation. An analysis using diphenylboric acid 2-aminoethyl ester (DBPA) staining revealed that flavonoids accumulated in the hilum and the funiculus in both To7B and To7G. Further, the scavenging activity of the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical in methanol extracts from the funiculus and hilum of To7B was higher than that of To7G. Moreover, the enzymatic activity of F3'H was detected using microsomal fractions from yeast transformed with sf3'h1 from To7B, but not from To7G. These results indicate that sf3'h1 is involved in flavonoid biosynthesis in the seed coat and affects the antioxidant properties of those tissues. As shown by immunofluorescence microscopy, the sf3'h1 protein was detected primarily around the vacuole in the parenchymatic cells of the hilum in To7B. Further immunoelectron microscopy detected sf3'h1 protein on the membranous structure of the vacuole. Based on these observations, we conclude that F3'H, which is a cytochrome P450 monooxygenase and has been found to be localized to the ER in other plant systems, is localized in the tonoplast in soybean.