Arabidopsis RETINOBLASTOMA RELATED directly regulates DNA damage responses through functions beyond cell cycle control.

The rapidly proliferating cells in plant meristems must be protected from genome damage. Here, we show that the regulatory role of the Arabidopsis RETINOBLASTOMA RELATED (RBR) in cell proliferation can be separated from a novel function in safeguarding genome integrity. Upon DNA damage, RBR and its binding partner E2FA are recruited to heterochromatic γH2AX-labelled DNA damage foci in an ATM- and ATR-dependent manner. These γH2AX-labelled DNA lesions are more dispersedly occupied by the conserved repair protein, AtBRCA1, which can also co-localise with RBR foci. RBR and AtBRCA1 physically interact in vitro and in planta Genetic interaction between the RBR-silenced amiRBR and Atbrca1 mutants suggests that RBR and AtBRCA1 may function together in maintaining genome integrity. Together with E2FA, RBR is directly involved in the transcriptional DNA damage response as well as in the cell death pathway that is independent of SOG1, the plant functional analogue of p53. Thus, plant homologs and analogues of major mammalian tumour suppressor proteins form a regulatory network that coordinates cell proliferation with cell and genome integrity.

ErbB-3 BINDING PROTEIN 1 Regulates Translation and Counteracts RETINOBLASTOMA RELATED to Maintain the Root Meristem.

The ErbB-3 BINDING PROTEIN 1 (EBP1) drives growth, but the mechanism of how it acts in plants is little understood. Here, we show that EBP1 expression and protein abundance in Arabidopsis (Arabidopsis thaliana) are predominantly confined to meristematic cells and are induced by sucrose and partially dependent on TARGET OF RAPAMYCIN (TOR) kinase activity. Consistent with being downstream of TOR, silencing of EBP1 restrains, while overexpression promotes, root growth, mostly under sucrose-limiting conditions. Inducible overexpression of RETINOBLASTOMA RELATED (RBR), a sugar-dependent transcriptional repressor of cell proliferation, depletes meristematic activity and causes precocious differentiation, which is attenuated by EBP1. To understand the molecular mechanism, we searched for EBP1- and RBR-interacting proteins by affinity purification and mass spectrometry. In line with the double-stranded RNA-binding activity of EBP1 in human (Homo sapiens) cells, the overwhelming majority of EBP1 interactors are part of ribonucleoprotein complexes regulating many aspects of protein synthesis, including ribosome biogenesis and mRNA translation. We confirmed that EBP1 associates with ribosomes and that EBP1 silencing hinders ribosomal RNA processing. We revealed that RBR also interacts with a set of EBP1-associated nucleolar proteins as well as factors that function in protein translation. This suggests EBP1 and RBR act antagonistically on common processes that determine the capacity for translation to tune meristematic activity in relation to available resources.

Naturally occurring allele diversity allows potato cultivation in northern latitudes.

Potato (Solanum tuberosum L.) originates from the Andes and evolved short-day-dependent tuber formation as a vegetative propagation strategy. Here we describe the identification of a central regulator underlying a major-effect quantitative trait locus for plant maturity and initiation of tuber development. We show that this gene belongs to the family of DOF (DNA-binding with one finger) transcription factors and regulates tuberization and plant life cycle length, by acting as a mediator between the circadian clock and the StSP6A mobile tuberization signal. We also show that natural allelic variants evade post-translational light regulation, allowing cultivation outside the geographical centre of origin of potato. Potato is a member of the Solanaceae family and is one of the world's most important food crops. This annual plant originates from the Andean regions of South America. Potato develops tubers from underground stems called stolons. Its equatorial origin makes potato essentially short-day dependent for tuberization and potato will not make tubers in the long-day conditions of spring and summer in the northern latitudes. When introduced in temperate zones, wild material will form tubers in the course of the autumnal shortening of day-length. Thus, one of the first selected traits in potato leading to a European potato type is likely to have been long-day acclimation for tuberization. Potato breeders can exploit the naturally occurring variation in tuberization onset and life cycle length, allowing varietal breeding for different latitudes, harvest times and markets.

EBP1 regulates organ size through cell growth and proliferation in plants.

Plant organ size shows remarkable uniformity within species indicating strong endogenous control. We have identified a plant growth regulatory gene, functionally and structurally homologous to human EBP1. Plant EBP1 levels are tightly regulated; gene expression is highest in developing organs and correlates with genes involved in ribosome biogenesis and function. EBP1 protein is stabilised by auxin. Elevating or decreasing EBP1 levels in transgenic plants results in a dose-dependent increase or reduction in organ growth, respectively. During early stages of organ development, EBP1 promotes cell proliferation, influences cell-size threshold for division and shortens the period of meristematic activity. In postmitotic cells, it enhances cell expansion. EBP1 is required for expression of cell cycle genes; CyclinD3;1, ribonucleotide reductase 2 and the cyclin-dependent kinase B1;1. The regulation of these genes by EBP1 is dose and auxin dependent and might rely on the effect of EBP1 to reduce RBR1 protein level. We argue that EBP1 is a conserved, dose-dependent regulator of cell growth that is connected to meristematic competence and cell proliferation via regulation of RBR1 level.

Isolation of a gene encoding a copper chaperone for the copper/zinc superoxide dismutase and characterization of its promoter in potato.

Gene expression during the potato (Solanum tuberosum) tuber lifecycle was monitored by cDNA-amplified fragment-length polymorphism, and several differentially expressed transcript-derived fragments were isolated. One fragment, named TDFL431, showed high homology to a copper (Cu) chaperone for Cu/zinc superoxide dismutase (CCS). The Ccs protein is responsible for the delivery of Cu to the Cu/zinc superoxide dismutase enzyme. The potato CCS (StCCS) full-length gene was isolated, and its sequence was compared with CCSs from other species. The promoter region of this gene was isolated, fused to the firefly luciferase coding sequence, and used for transformation of potato plants. The highest level of StCCS-luciferase expression was detected in the cortex of stem (like) tissues, such as stem nodes, stolons, and tubers; lower levels were detected in roots and flowers. The StCCS promoter contains regions highly homologous to several plant cis-acting elements. Three of them are related to auxin response, whereas four others are related to response to various stresses. Induction of the StCCS promoter was analyzed on 18 media, differing in hormone, sugar, and Cu content. StCCS expression was induced by auxin, gibberellins (GA4 + 7), fructose, sucrose, and glucose and was inhibited by relatively high concentrations of Cu.

Expression analysis of a family of nsLTP genes tissue specifically expressed throughout the plant and during potato tuber life cycle.

Non-specific lipid-transfer proteins (nsLTPs) are capable of binding lipid compounds in plant tissues and are coded by the nsLTP genes. Here, we present the analysis of expression of a family of potato (Solanum tuberosum) nsLTP genes that express throughout the developing plant in a highly tissue-specific manner. Three transcript-derived fragments were isolated using an amplified restriction fragment polymorphism-derived technique for RNA fingerprinting that show homology to plant nsLTP genes. These transcript-derived fragments displayed modulated expression profiles related to the development of new tissues, with a peak of transcription around the time of tuberization and just prior to sprout development, at dormancy breakage. In addition, a homologous family of expressed sequence tags was identified whose individual members could be classified according to their tissue specificity. Two subgroups of expressed sequence tags were found to express during tuber life cycle. To study the regulation of potato nsLTP genes, two putative potato nsLTP promoters were isolated and their expression was studied using promoter-marker-gene fusions. The results showed that one of the two promoters directed a highly specific pattern of expression detected in the phloem surrounding the nodes of young plants and in the same tissue of tuber related organs, whereas the second putative promoter showed little tissue or organ specificity. This difference in expression is likely due to a 331-bp insertion present in the tissue-specific promoter.