After its initial definition in 1999, the TCP family of transcription factors has become the focus of a multiplicity of studies related with plant development at the cellular, organ, and tissue levels. Evidence has accumulated indicating that TCP transcription factors are the main regulators of plant form and architecture and constitute a tool through which evolution shapes plant diversity. The TCP transcription factors act in a multiplicity of pathways related with cell proliferation and hormone responses. In recent years, the molecular pathways of TCP protein action and biochemical studies on their mode of interaction with DNA have begun to shed light on their mechanism of action. However, the available information is fragmented and a unifying view of TCP protein action is lacking, as well as detailed structural studies of the TCP-DNA complex. Also important, the possible role of TCP proteins as integrators of plant developmental responses to the environment has deserved little attention. In this review, we summarize the current knowledge about the structure and functions of TCP transcription factors and analyze future perspectives for the study of the role of these proteins and their use to modify plant development.

Mitochondrial respiration is an energy producing process that involves the coordinated action of several protein complexes embedded in the inner membrane to finally produce ATP. Complex IV or Cytochrome c Oxidase (COX) is the last electron acceptor of the respiratory chain, involved in the reduction of O₂ to H₂O. COX is a multimeric complex formed by multiple structural subunits encoded in two different genomes, prosthetic groups (heme a and heme a₃), and metallic centers (Cu_A and Cu_B). Tens of accessory proteins are required for mitochondrial RNA processing, synthesis and delivery of prosthetic groups and metallic centers, and for the final assembly of subunits to build a functional complex. In this review, we perform a comparative analysis of COX composition and biogenesis factors in yeast, mammals and plants. We also describe possible external and internal factors controlling the expression of structural proteins and assembly factors at the transcriptional and post-translational levels, and the effect of deficiencies in different steps of COX biogenesis to infer the role of COX in different aspects of plant development. We conclude that COX assembly in plants has conserved and specific features, probably due to the incorporation of a different set of subunits during evolution.

TCP proteins integrate a family of transcription factors involved in the regulation of developmental processes and hormone responses. It has been shown that most members of class I, one of the two classes in which the TCP family is divided, contain a conserved Cys that leads to inhibition of DNA binding when oxidized. In this work, we describe that the class-I TCP protein TCP15 inhibits anthocyanin accumulation during exposure of plants to high light intensity by modulating the expression of transcription factors involved in the induction of anthocyanin biosynthesis genes, as suggested by the study of plants that express TCP15 from the 35SCaMV promoter and mutants in TCP15 and the related gene TCP14. In addition, the effect of TCP15 on anthocyanin accumulation is lost after prolonged incubation under high light intensity conditions. We provide evidence that this is due to inactivation of TCP15 by oxidation of Cys-20 of the TCP domain. Thus, redox modulation of TCP15 activity in vivo by high light intensity may serve to adjust anthocyanin accumulation to the duration of exposure to high irradiation conditions.

The function of the class I TCP transcription factor TCP15 from Arabidopsis thaliana has been studied through the analysis of plants that express a fusion of this protein to the EAR repressor domain. Constitutive expression of TCP15-EAR produces growth arrest at the seedling stage, before leaf emergence. Expression of the repressor fusion from the AtTCP15 promoter produces small plants with leaves whose margins progressively curve upwards, starting from the basal part of the lamina. Leaves contain smaller and less differentiated cells, both on the adaxial and abaxial sides. The abaxial domain is relatively enlarged, with disorganized cells separated by empty spaces. TCP15-EAR also affects the growth of leaf petioles, flower pedicels, and anther filaments. Flowers show reduced elongation of the three outer whorls and altered gynoecia with irregular carpel surfaces and enlarged repla. Ectopic stigma-like structures develop from medial and basal parts of the replum. TCP15-EAR produces an increase in expression of the boundary-specific genes LOB, CUC1, and CUC2. Changes in CUC1 and CUC2 expression can be explained by the existence of lower levels of miR164 in leaves and the repression of IAA3/SHY2 and the SAUR-like gene At1g29460 in leaves and flowers. TCP15 binds to the promoter regions of IAA3/SHY2 and At1g29460, suggesting that these genes may be direct targets of the transcription factor. The results indicate that TCP15 regulates the expression of boundary-specific genes through a pathway that affects auxin homeostasis and partially overlaps with the one modulated by class II CIN-like TCP proteins.

The promoters of the Arabidopsis (Arabidopsis thaliana) cytochrome c genes, Cytc-1 and Cytc-2, were analyzed using plants transformed with fusions to the β-glucuronidase coding sequence. Histochemical staining of plants indicated that the Cytc-1 promoter directs preferential expression in root and shoot meristems and in anthers. In turn, plants transformed with the Cytc-2 promoter fusions showed preferential expression in vascular tissues of cotyledons, leaves, roots, and hypocotyls, and also in anthers. Quantitative measurements in extracts prepared from different organs suggested that expression of Cytc-1 is higher in flowers, while that of Cytc-2 is higher in leaves. The analysis of a set of deletions and site-directed mutants of the Cytc-1 promoter indicated that a segment located between −147 and −156 from the translation start site is required for expression and that site II elements (TGGGCC/T) located in this region, coupled with a downstream internal telomeric repeat (AAACCCTAA), are responsible for the expression pattern of this gene. Proteins present in cauliflower nuclear extracts, as well as a recombinant protein from the TCP-domain family, were able to specifically bind to the region required for expression. We propose that expression of the Cytc-1 gene is linked to cell proliferation through the elements described above. The fact that closely located site II motifs are present in similar locations in several genes encoding proteins involved in cytochrome c-dependent respiration suggests that these elements may be the target of factors that coordinate the expression of nuclear genes encoding components of this part of the mitochondrial respiratory chain.