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Among alternative splicing events in the human transcriptome, tandem NAGNAG acceptor splice sites represent an appreciable proportion. Both proximal and distal NAG can be used to produce two splicing isoforms differing by three nucleotides. In some cases, the upstream exon can be alternatively spliced as well, which further increases the number of possible transcripts. In this study, we showed that NAG choice in tandem splice site depends considerably not only on the concerned acceptor, but also on the upstream donor splice site sequence. Using an extensive set of experiments with systematically modified two-exonic minigene systems of AFAP1L2 or CSTD gene, we recognized the third and fifth intronic upstream donor splice site position and the tandem acceptor splice site region spanning from −10 to +2, including NAGNAG itself, as the main drivers. In addition, competition between different branch points and their composition were also shown to play a significant role in NAG choice. All these nucleotide effects appeared almost additive, which explained the high variability in proximal versus distal NAG usage.

Acceptor splice site recognition (3&prime, splice site: 3&prime, ss) is a fundamental step in precursor messenger RNA (pre-mRNA) splicing. Generally, the U2 small nuclear ribonucleoprotein (snRNP) auxiliary factor (U2AF) heterodimer recognizes the 3&prime, ss, of which U2AF35 has a dual function: (i) It binds to the intron&ndash, exon border of some 3&prime, ss and (ii) mediates enhancer-binding splicing activators&rsquo, interactions with the spliceosome. Alternative mechanisms for 3&prime, ss recognition have been suggested, yet they are still not thoroughly understood. Here, we analyzed 3&prime, ss recognition where the intron&ndash, exon border is bound by a ubiquitous splicing regulator SRSF1. Using the minigene analysis of two model exons and their mutants, BRCA2 exon 12 and VARS2 exon 17, we showed that the exon inclusion correlated much better with the predicted SRSF1 affinity than 3&prime, ss quality, which were assessed using the Catalog of Inferred Sequence Binding Preferences of RNA binding proteins (CISBP-RNA) database and maximum entropy algorithm (MaxEnt) predictor and the U2AF35 consensus matrix, respectively. RNA affinity purification proved SRSF1 binding to the model 3&prime, ss. On the other hand, knockdown experiments revealed that U2AF35 also plays a role in these exons&rsquo, inclusion. Most probably, both factors stochastically bind the 3&prime, ss, supporting exon recognition, more apparently in VARS2 exon 17. Identifying splicing activators as 3&prime, ss recognition factors is crucial for both a basic understanding of splicing regulation and human genetic diagnostics when assessing variants&rsquo, effects on splicing.

Three-amino-acid-loop-extension homeodomain proteins affect various aspects during root development, such as root skewing or lateral root initiation and growth. Crossed lines of the BP overexpressor in Arabidopsis thaliana (L.) Heynh. were used to elucidate the role of BREVIPEDICELLUS (BP) in root development. The potential heterodimerization partner of BP, BEL1-like homeodomain protein 6 (BLH6), could be an important modulator of BP function. BLH6 is a member of BEL-like family of homeodomain proteins consisting of all conserved domains and motifs. We identified a new BLH-specific TERM motif at the C terminus of BLH proteins. BLH6 was expressed in different root tissues, including the meristematic zone and bases of lateral roots, and overlap the BP expression zone, enabling coordinated function of these transcription factors. To elucidate potential interactions of BP and BLH6, crossed lines of BP overexpressor were analyzed. While other tested BLH mutations do not affect BP overexpression phenotype, knock-out mutation blh6-1 weakened 35S-BP influence on shoot and root development. We demonstrated that growth of lateral roots was modulated by BP overexpression and auxins, and that this effect was dependent on lateral root position along the primary root. In the upper part of the primary root, BP inhibited lateral root growth despite indol-3-acetic acid treatment and this effect correlated with predicted activity of BLH6 in crossed lines of 35S-BP and blh6 mutants.

Recent progress in plant hormone and macronutrient research is largely associated with characterising the molecular mechanisms of their actions. Quantitative reverse transcription-PCR is a powerful technique that enables elucidation of regulatory relationships through quantifying transcript levels, but its reliability requires precise normalization. We compared eight Arabidopsis thaliana housekeeping genes for expression stability after treatment with cytokinins, auxins, ethylene and after macronutrient level manipulation. The expression stability of housekeeping genes in cytokinin series was studied in wide spectrum of plants treated with different adenine- and phenylurea-type cytokinins as well as in plants with controlled overexpression of agrobacterial isopentenyltransferase. NormFinder and geNorm software were used to predict a unique set of genes as best for particular hormone groups. Generally, 18S rRNA, GAPC, and ACT were confirmed as less stable; and UBQ10, UBC, and EF-1α exhibited increased stability. Based on geNorm pairwise variation analysis, we confirmed accurate normalization required at least three reference genes.

Pre-mRNA splicing is an essential step in gene expression, when introns are removed and exons joined by the complex of proteins called spliceosome. Correct splicing requires a precise exon/intron junction definition, which is determined by a consensual donor and acceptor splice site at the 5′ and 3′ end, respectively. An acceptor splice site (3′ss) consists of highly conserved AG nucleotides in positions E−2 and E−1. These nucleotides can appear in tandem, located 3 bp from each other. Then they are referred to as NAGNAG or tandem 3′ss, which can be alternatively spliced. NAG/TAG 3′ss motif abundance is extremely low and cannot be easily explained by just a nucleotide preference in this position. We tested artificial NAG/TAG motif’s potential negative effect on exon recognition using a minigene assay. Introducing the NAG/TAG motif into seven different exons revealed no general negative effect on exon recognition. The only observed effect was the partial use of the newly formed distal 3′ss. We can conclude that this motif’s extremely low preference in a natural 3′ss is not a consequence of the NAG/TAG motif’s negative effect on exon recognition, but more likely the result of other RNA processing aspects, such as an alternative 3′ss choice, decreased 3′ss strength, or incorporating an amber stop codon.

Here we present an optimized procedure to generate amino acid variations at specific site(s) of proteins, followed by a simple one-step screen for mutants with the desired β-glucosidase activity. The procedure was evaluated by introducing sequence variation into a codon specifying a non-functional variant of the catalytic nucleophile (E401) of the maize β-glucosidase Zm-p60.1. Observed and theoretically expected frequencies of the four possible variants of the codon and the two possible phenotypes (functional and non-functional) were investigated. Deviations in codon and phenotype frequencies were expressed as a coefficient. This coefficient was then used to estimate the extent of oversampling, of the mutant library, which would be necessary to compensate for the underrepresentation of some sequences. This evaluation of the overall performance of the method allows experimentally derived parameters to be incorporated into mutant library design. This method combines the application of a well-defined distribution of variability with a reliable screening process. Thus, it facilitates the production of novel functional variants of β-glucosidases for either fundamental studies or potential biotechnological applications.

Plant hormones control plant development by modulating the expression of regulatory genes, including homeobox-containing KNOXI genes. However, much remains to be elucidated about the interactions involved. Therefore, hormonal regulation of KNOXI gene expression was investigated using hormone applications and an inducible transgenic ipt expression system to increase endogenous cytokinin (CK) levels. Treatments with auxin, abscisic acid (ABA), cytokinins, ethylene, and gibberellin (GA) did not result in ectopic expression of the BP (BREVIPEDICELLUS) gene. However, BP expression was strongly reduced by ABA, increased by auxin treatment (correlating with the initiation of lateral root meristems, which strongly express BP), and did not significantly respond to short-term treatments with the other hormones in whole seedlings. Following short-term ipt activation, organ-specific differential regulation of KNOXI gene expression was observed. While several KNOXI genes were transiently up-regulated to low levels, STM was selectively repressed (especially at low light) in hypocotyls. In cotyledons, activation of CK-responsive genes preceded ipt induction, suggesting that CKs are transported more rapidly than the inducing agent (dexamethasone). Long-term increases in CK levels induced raised levels of several KNOXI transcripts in hypocotyls, correlating with the radial expansion of vascular tissues, the main domains of KNOXI gene expression, suggesting that CKs had little effect on KNOXI promoter activity. No alterations in hormone sensitivity were observed in a bp null mutant. Constitutive BP overexpression caused reductions in the length and number of lateral roots, while the primary root remained unaffected. The transgenic seedlings displayed weak, but significant, alterations in sensitivity to ABA, CK, and 1-amino-cyclopropane-1-carboxylic acid.

The development and activity of the procambium and cambium, which ensure vascular tissue formation, is critical for overall plant architecture and growth. However, little is known about the molecular factors affecting the activity of vascular meristems and vascular tissue formation. Here, we show that the His kinase CYTOKININ-INDEPENDENT1 (CKI1) and the cytokinin receptors ARABIOPSIS HISTIDINE KINASE2 (AHK2) and AHK3 are important regulators of vascular tissue development in Arabidopsis thaliana shoots. Genetic modifications of CKI1 activity in Arabidopsis cause dysfunction of the two-component signaling pathway and defects in procambial cell maintenance. CKI1 overexpression in protoplasts leads to cytokinin-independent activation of the two-component phosphorelay, and intracellular domains are responsible for the cytokinin-independent activity of CKI1. CKI1 expression is observed in vascular tissues of inflorescence stems, and CKI1 forms homodimers both in vitro and in planta. Loss-of-function ahk2 and ahk3 mutants and plants with reduced levels of endogenous cytokinins show defects in procambium proliferation and an absence of secondary growth. CKI1 overexpression partially rescues ahk2 ahk3 phenotypes in vascular tissue, while the negative mutation CKI1H405Q further accentuates mutant phenotypes. These results indicate that the cytokinin-independent activity of CKI1 and cytokinin-induced AHK2 and AHK3 are important for vascular bundle formation in Arabidopsis.

Postembryonic de novo organogenesis represents an important competence evolved in plants that allows their physiological and developmental adaptation to changing environmental conditions. The phytohormones auxin and cytokinin (CK) are important regulators of the developmental fate of pluripotent plant cells. However, the molecular nature of their interaction(s) in control of plant organogenesis is largely unknown. Here, we show that CK modulates auxin-induced organogenesis (AIO) via regulation of the efflux-dependent intercellular auxin distribution. We used the hypocotyl explants-based in vitro system to study the mechanism underlying de novo organogenesis. We show that auxin, but not CK, is capable of triggering organogenesis in hypocotyl explants. The AIO is accompanied by endogenous CK production and tissue-specific activation of CK signaling. CK affects differential auxin distribution, and the CK-mediated modulation of organogenesis is simulated by inhibition of polar auxin transport. CK reduces auxin efflux from cultured tobacco cells and regulates expression of auxin efflux carriers from the PIN family in hypocotyl explants. Moreover, endogenous CK levels influence PIN transcription and are necessary to maintain intercellular auxin distribution in planta . Based on these findings, we propose a model in which auxin acts as a trigger of the organogenic processes, whose output is modulated by the endogenously produced CKs. We propose that an important mechanism of this CK action is its effect on auxin distribution via regulation of expression of auxin efflux carriers.