植物MYB转录因子功能及调控机制研究进展

MYB转录因子是植物中数量最大、功能最多样的转录因子之一,在众多生命过程中扮演重要的角色,已成为当前植物基因功能及表达网络调控研究的热点。结合最新研究进展,综述了植物MYB转录因子家族的进化,并着重阐述了生物学功能及表达调控,为进一步分析功能未知的植物MYB转录因子提供参考。     

TT8 controls its own expression in a feedback regulation involving TTG1 and homologous MYB and bHLH factors, allowing a strong and cell-specific accumulation of flavonoids in Arabidopsis thaliana

The control of TT8 expression was investigated in this study, and it was demonstrated that it constitutes a major regulatory step in the specific activation of the expression of flavonoid structural genes. First, the GUS activity generated in planta from a TT8::uidA construct revealed cell-specific activation of the TT8 promoter consistent with the known involvement of the TT8 bHLH factor in proanthocyanidin, anthocyanin and mucilage biosynthesis. Moreover, the activity of this reporter construct was strongly affected in ttg1, TT2 overexpressers (OE), and PAP1-OE, suggesting interplay between TT2, PAP1, TTG1 and the activation of the TT8 promoter in planta. To further investigate the mechanisms involved, we used 35S::TT2-GR and 35S::TTG1-GR transgenic plants (expressing fusion proteins with the glucocorticoid receptor), as well as one-hybrid experiments, to determine the direct effect of these factors on TT8 expression. Interestingly, in vivo binding of TT2 and PAP1 to the TT8 promoter was dependent on the simultaneous expression of TT8 or the homologous bHLH factors GL3 and EGL3. Consistent with these results, the activity of the TT8::uidA reporter was strongly affected in the seed endothelium of a tt8 mutant. Similarly, a strong decrease in the level of TT8 mRNA was detected in the siliques of a gl3 x egl3 mutant and in plants that express a dominant negative form of the PAP1 protein, suggesting that TT8 expression is controlled by different combinations of MYB and bHLH factors in planta. The importance of this positive feedback mechanism in the strong and specific induction of proanthocyanidin biosynthesis in the seed coat of Arabidopsis thaliana is discussed.     

TT2, TT8, and TTG1 synergistically specify the expression of BANYULS and proanthocyanidin biosynthesis in Arabidopsis thaliana

Genetic analyses have demonstrated that together with TTG1, a WD-repeat (WDR) protein, TT2 (MYB), and TT8 (bHLH) are necessary for the correct expression of BANYULS (BAN). This gene codes for the core enzyme of proanthocyanidin biosynthesis in Arabidopsis thaliana seed coat. The interplays of TT2, TT8, and their closest MYB/bHLH relatives, with TTG1 and the BAN promoter have been investigated using a combination of genetic and molecular approaches, both in yeast and in planta. The results obtained using glucocorticoid receptor fusion proteins in planta strongly suggest that TT2, TT8, and TTG1 can directly activate BAN expression. Experiments using yeast two- and three-hybrid clearly demonstrated that TT2, TT8, and TTG1 can form a stable ternary complex. Furthermore, although TT2 and TT8 were able to bind to the BAN promoter when simultaneously expressed in yeast, the activity of the complex correlated with the level of TTG1 expression in A. thaliana protoplasts. In addition, transient expression experiments revealed that TTG1 acts mainly through the bHLH partner (i.e. TT8 or related proteins) and that TT2 cannot be replaced by any other related A. thaliana MYB proteins to activate BAN. Finally and consistent with these results, the ectopic expression of TT2 was sufficient to trigger BAN activation in vegetative parts, but only where TTG1 was expressed. Taken together, these results indicate that TT2, TT8, and TTG1 can form a ternary complex directly regulating BAN expression in planta.     

Arabidopsis mutants in the C-S lyase of glucosinolate biosynthesis establish a critical role for indole-3-acetaldoxime in auxin homeostasis

We report characterization of SUPERROOT1 (SUR1) as the C-S lyase in glucosinolate biosynthesis. This is evidenced by selective metabolite profiling of sur1, which is completely devoid of aliphatic and indole glucosinolates. Furthermore, following in vivo feeding with radiolabeled p-hydroxyphenylacetaldoxime to the sur1 mutant, the corresponding C-S lyase substrate accumulated. C-S lyase activity of recombinant SUR1 heterologously expressed in Escherichia coli was demonstrated using the C-S lyase substrate djenkolic acid. The abolishment of glucosinolates in sur1 indicates that the SUR1 function is not redundant and thus SUR1 constitutes a single gene family. This suggests that the "high-auxin" phenotype of sur1 is caused by accumulation of endogenous C-S lyase substrates as well as aldoximes, including indole-3-acetaldoxime (IAOx) that is channeled into the main auxin indole-3-acetic acid (IAA). Thereby, the cause of the "high-auxin" phenotype of sur1 mutant resembles that of two other "high-auxin" mutants, superroot2 (sur2) and yucca1. Our findings provide important insight to the critical role IAOx plays in auxin homeostasis as a key branching point between primary and secondary metabolism, and define a framework for further dissection of auxin biosynthesis.     

Tumour development in Arabidopsis thaliana involves the Shaker-like K+ channels AKT1 and AKT2/3

After completion of the Arabidopsis genome-sequencing programme, crown galls induced by Agrobacterium tumefaciens may become a model system to study plant tumour development. The molecular mechanisms of nutrient supply to support tumour growth and development are still unknown. In this study, we have identified a unique profile of Shaker-like potassium channels in agrobacteria-induced Arabidopsis tumours. Comparing the gene expression pattern of rapidly growing tumours with that of non-infected tissues, we found the suppression of shoot in favour of root-specific K+ channels. Among these, the upregulation of AKT1 and AtKC1 and the suppression of AKT2/3 and GORK were most pronounced. As a consequence, K+ uptake and accumulation were elevated in the tumour (163 mm) compared to control tissues (92 mm). Patch clamp studies on tumour protoplasts identified a population expressing the electrical properties of the AKT1 K+ channel. Furthermore, plants lacking a functional AKT1 or the AKT2/3 phloem K+ channel gene did not support tumour growth. This indicates that the delivery of potassium by AKT1 and the direction of assimilates, triggered by AKT2/3, are essential for tumour growth.     

TRANSPARENT TESTA1 interacts with R2R3-MYB factors and affects early and late steps of flavonoid biosynthesis in the endothelium of Arabidopsis thaliana seeds

Wild type seed coats of Arabidopsis thaliana are brown due to the accumulation of proanthocyanidin pigments (PAs). The pigmentation requires activation of phenylpropanoid biosynthesis genes and mutations in some of these genes cause a yellow appearance of seeds, termed transparent testa (tt) phenotype. The TT1 gene encodes a WIP‐type zinc finger protein and is expressed in the seed coat endothelium where most of the PAs accumulate in wild type plants. In this study we show that TT1 is not only required for correct expression of PA‐specific genes in the seed coat, but also affects CHS, encoding the first enzyme of flavonoid biosynthesis. Many steps of this pathway are controlled by complexes of MYB and BHLH proteins with the WD40 factor TTG1. We demonstrate that TT1 can interact with the R2R3 MYB protein TT2 and that ectopic expression of TT2 can partially restore the lack in PA production in tt1. Reduced seed coat pigmentation was obtained using a TT1 variant lacking nuclear localisation signals. Based on our results we propose that the TT2/TT8/TTG1 regulon may also comprise early genes like CHS and discuss steps to further unravel the regulatory network controlling flavonoid accumulation in endothelium cells during A. thaliana seed development.     

Role of the penetration‐resistance genes PEN1, PEN2 and PEN3 in the hypersensitive response and race‐specific resistance in Arabidopsis thaliana

Plants are highly capable of recognizing and defending themselves against invading microbes. Adapted plant pathogens secrete effector molecules to suppress the host's immune system. These molecules may be recognized by host‐encoded resistance proteins, which then trigger defense in the form of the hypersensitive response (HR) leading to programmed cell death of the host tissue at the infection site. The three proteins PEN1, PEN2 and PEN3 have been found to act as central components in cell wall‐based defense against the non‐adapted powdery mildew Blumeria graminis fsp. hordei (Bgh). We found that loss of function mutations in any of the three PEN genes cause decreased hypersensitive cell death triggered by recognition of effectors from oomycete and bacterial pathogens in Arabidopsis. There were considerable additive effects of the mutations. The HR induced by recognition of AvrRpm1 was almost completely abolished in the pen2 pen3 and pen1 pen3 double mutants and the loss of cell death could be linked to indole glucosinolate breakdown products. However, the loss of the HR in pen double mutants did not affect the plants' ability to restrict bacterial growth, whereas resistance to avirulent isolates of the oomycete Hyaloperonospora arabidopsidis was strongly compromised. In contrast, the double and triple mutants demonstrated varying degrees of run‐away cell death in response to Bgh. Taken together, our results indicate that the three genes PEN1, PEN2 and PEN3 extend in functionality beyond their previously recognized functions in cell wall‐based defense against non‐host pathogens.