Biomedical subjects

Alon Samach

Publications and source records attributed to Alon Samach.

9 recordsLinked to original sources

PubMed2006

CONSTANS and the CCAAT box binding complex share a functionally important domain and interact to regulate flowering of Arabidopsis.

The CCT (for CONSTANS, CONSTANS-LIKE, TOC1) domain is found in 45 Arabidopsis thaliana proteins involved in processes such as photoperiodic flowering, light signaling, and regulation of circadian rhythms. We show that this domain exhibits similarities to yeast HEME ACTIVATOR PROTEIN2 (HAP2), which is a subunit of the HAP2/HAP3/HAP5 trimeric complex that binds to CCAAT boxes in eukaryotic promoters. Moreover, we demonstrate that CONSTANS (CO), which promotes Arabidopsis flowering, interacts with At HAP3 and At HAP5 in yeast, in vitro, and in planta. Mutations in CO that delay flowering affect residues highly conserved between CCT and the DNA binding domain of HAP2. Taken together, these data suggest that CO might replace At HAP2 in the HAP complex to form a trimeric CO/At HAP3/At HAP5 complex. Flowering was delayed by overexpression of At HAP2 or At HAP3 throughout the plant or in phloem companion cells, where CO is expressed. This phenotype was correlated with reduced abundance of FLOWERING LOCUS T (FT) mRNA and no change in CO mRNA levels. At HAP2 or At HAP3 overexpression may therefore impair formation of a CO/At HAP3/At HAP5 complex leading to reduced expression of FT. During plant evolution, the number of genes encoding HAP proteins was greatly amplified, and these proteins may have acquired novel functions, such as mediating the effect of CCT domain proteins on gene expression.

Amino Acid Sequence↗

PubMed2006

Postharvest heat and conditioning treatments activate different molecular responses and reduce chilling injuries in grapefruit.

A combination of hot water (a rinse at 62 degrees C for 20 s) and conditioning (pre-storage at 16 degrees C for 7 d) treatments synergistically reduced chilling injury development in grapefruit (Citrus paradisi, cv. "Star Ruby") during cold storage at 2 degrees C, suggesting that the treatments may activate different chilling tolerance responses. To study the molecular mechanisms involved, chilling- and conditioning-responsive genes were isolated by polymerase chain reaction (PCR) cDNA subtraction, cDNA libraries were constructed from hot water- and conditioning-treated fruit, and cDNA sequencing was used to identify putative stress-responsive and chilling tolerance genes. PCR cDNA subtraction revealed the identification of 17 chilling-responsive and heat- and conditioning-induced genes, and the expression patterns of 11 additional stress-related genes, antioxidant defensive genes, and genes encoding enzymes involved in membrane lipid modifications were characterized. It was found that hot water and conditioning treatments had little effect on gene expression by themselves, but rather had a priming effect, and enabled the fruit to activate their defence responses after subsequent exposure to chilling. RNA gel blot hybridizations revealed that the expression patterns of eight genes, including HSP19-I, HSP19-II, dehydrin, universal stress protein (USP), EIN2, 1,3;4-beta-D-glucanase, and superoxide dismutase (SOD), were specifically regulated by the heat treatment, and four genes, including fatty acid desaturase2 (FAD2) and lipid transfer protein (LTP), were specifically regulated by the conditioning treatment. Furthermore, four more genes were identified, including a translation initiation factor (SUI1), a chaperonin, and alcohol dehydrogenase (ADH), that were commonly regulated by both heat and conditioning treatments. According to these data, it is suggested that pre-storage heat and conditioning treatments may enhance fruit chilling tolerance by activating different molecular mechanisms. The hot water treatment activates mainly the expression of various stress-related genes, whereas the conditioning treatment activates mainly the expression of lipid membrane modification enzymes.

Acclimatization↗

PubMed2006

Novel roles for GIGANTEA revealed under environmental conditions that modify its expression in Arabidopsis and Medicago truncatula.

GIGANTEA (GI) is a large nuclear protein which is involved in circadian-clock function, red-light signaling and photoperiodic flowering. Accumulation of GI transcript displays a strong diurnal pattern, and is under circadian-clock control, as demonstrated in several diverse species. Clock entrainment and compensation, as well as flowering time, are largely responsive to changes in the environment. We asked if part of this response is mediated through modification of GI expression. We identified a strong response of GI expression to changes in temperature and light, in both Arabidopsis and the model legume Medicago truncatula. Extreme temperatures resulted in increased GI trough levels. Light increased GI expression near dawn and the response to light appeared to be gated by the circadian clock. We provide evidence that the GI response to blue and far-red light requires CRYPTOCHROME function in Arabidopsis. Unknown roles for GI in both blue-light deetiolation and precocious flowering under warm short days were revealed. Plants seem to respond to changes in the environment partly through environmentally induced modifications of a basal clock-regulated pattern of GI transcript accumulation.

Arabidopsis↗

PubMed2006

The CCAAT binding factor can mediate interactions between CONSTANS-like proteins and DNA.

CONSTANS-Like (COL) proteins are plant-specific nuclear regulators of gene expression but do not contain a known DNA-binding motif. We tested whether a common DNA-binding protein can deliver these proteins to specific cis-acting elements. We screened for proteins that interact with two members of a subgroup of COL proteins. These COL proteins were Tomato COL1 (TCOL1), which does not seem to be involved in the control of flowering time, and the Arabidopsis thaliana CONSTANS (AtCO) protein which mediates photoperiodic induction of flowering. We show that the C-terminal plant-specific CCT (CO, CO-like, TIMING OF CAB EXPRESSION 1) domain of both proteins binds the trimeric CCAAT binding factor (CBF) via its HAP5/NF-YC component. Chromatin immunoprecipitation demonstrated that TCOL is recruited to the CCAAT motifs of the yeast CYC1 and HEM1 promoters by HAP5. In Arabidopsis, each of the three CBF components is encoded by several different genes that are highly transcribed. Under warm long days, high levels of expression of a tomato HAP5 (THAP5a) gene can reduce the flowering time of Arabidopsis. A mutation in the CCT domain of TCOL1 disrupts the interaction with THAP5 and the analogous mutation in AtCO impairs its function and delays flowering. CBFs are therefore likely to recruit COL proteins to their DNA target motifs in planta.

Amino Acid Motifs↗

PubMed2005

The flowering integrator FT regulates SEPALLATA3 and FRUITFULL accumulation in Arabidopsis leaves.

The transition to flowering involves major changes in the shoot apical meristem and in the fate of existing leaf primordia. Transcripts of the Arabidopsis thaliana flowering-promoting gene FLOWERING LOCUS T (FT) are present in leaf tissue but can also promote flowering when artificially introduced into the meristem. FT may normally act in the leaf and/or the meristem, initiating or constituting a mobile flower-promoting signal. We studied FT-dependent events in the rosette leaf, some of which might precede or mimic events in the meristem and its primordia. We show FT-dependent transcript accumulation of the MADS box transcription factors FRUITFULL (FUL) and SEPALLATA3 (SEP3) in leaves. Abnormally high levels of FT further increase the expression of these genes, leading to morphological changes in the leaves. Loss of the flowering-time gene FD, as well as environmental conditions that delay flowering, reduce FT's effect on leaves via reduced activation of its targets. FUL, SEP3, and APETALA1 accumulation in the meristem is associated with and contributes to the transition to flowering. We propose that FT functions through partner-dependent transcriptional activation of these and as-yet-unknown genes and that this occurs at several sites. Organ fate may depend on both degree of activation and the developmental stage reached by the organ before activation occurs.

Arabidopsis↗

PubMed2005

Arabidopsis KNOXI proteins activate cytokinin biosynthesis.

Plant architecture is shaped through the continuous formation of organs by meristems. Class I KNOTTED1-like homeobox (KNOXI) genes are expressed in the shoot apical meristem (SAM) and are required for SAM maintenance. KNOXI proteins and cytokinin, a plant hormone intimately associated with the regulation of cell division, share overlapping roles, such as meristem maintenance and repression of senescence, but their mechanistic and hierarchical relationship have yet to be defined. Here, we show that activation of three different KNOXI proteins using an inducible system resulted in a rapid increase in mRNA levels of the cytokinin biosynthesis gene isopentenyl transferase 7 (AtIPT7) and in the activation of ARR5, a cytokinin response factor. We further demonstrate a rapid and dramatic increase in cytokinin levels following activation of the KNOXI protein SHOOT MERISTEMLESS (STM). Application of exogenous cytokinin or expression of a cytokinin biosynthesis gene through the STM promoter partially rescued the stm mutant. We conclude that activation of cytokinin biosynthesis mediates KNOXI function in meristem maintenance. KNOXI proteins emerge as central regulators of hormone levels in plant meristems.

Arabidopsis↗

PubMed2005

Ambient temperature perception in plants.

Since plants are sessile they must be able to sense and rapidly respond to changes in ambient temperature. Key aspects of plant development, including the transition to flowering and the circadian clock, have important inputs from ambient temperature. In the model system Arabidopsis thaliana, molecular candidates for mediating these roles have recently been uncovered, which will be critical for obtaining an understanding of the mechanisms involved.

Biological Clocks↗

PubMed2004

Photoreceptor regulation of CONSTANS protein in photoperiodic flowering.

Many plants flower in response to seasonal fluctuations in day length. The CONSTANS (CO) gene of Arabidopsis promotes flowering in long days. Flowering is induced when CO messenger RNA expression coincides with the exposure of plants to light. However, how this promotes CO activity is unknown. We show that light stabilizes nuclear CO protein in the evening, whereas in the morning or in darkness the protein is degraded by the proteasome. Photoreceptors regulate CO stability and act antagonistically to generate daily rhythms in CO abundance. This layer of regulation refines the circadian rhythm in CO messenger RNA and is central to the mechanism by which day length controls flowering.

Arabidopsis↗

PubMed2004

Isolation of a dehydrin cDNA from orange and grapefruit citrus fruit that is specifically induced by the combination of heat followed by chilling temperatures.

Dehydrins (DHNs; late embryogenesis abundant D-11) are a family of plant proteins induced in response to environmental stresses such as water stress, salinity and freezing or which occur during the late stages of embryogenesis. Previously, it was reported that citrus contains a small gene family encoding a unique class of dehydrins that differs from most other plant dehydrins in various respects, such as having an unusual K-segment similar to that of gymnosperms. In the present study, we identified by cDNA differential display analysis a 'Navel' orange 202-bp polymerase chain reaction (PCR) fragment, which encoded the typical plant angiosperm-type K-segment consensus sequence, and of which the expression was down-regulated by exposure to low oxygen levels. The full-length cDNA sequence of the orange DHN, designated csDHN (for Citrus sinensis DHN), was further isolated by 5'-and 3'-RACE; it had a total length of 933 bp and encoded a predicted polypeptide of 235 amino acids. In addition, the same 202-bp 'Navel' dehydrin PCR fragment was used to screen a 'Star Ruby' grapefruit flavedo cDNA library, and its full-length grapefruit homologue, designated cpDHN (for C. paradisi DHN) was isolated and found to have a total length of 1024 bp and to encode a predicted polypeptide of 234 amino acids. The defined orange and grapefruit DHN proteins were completely identical in the 196 amino acids of their N-terminus but differed in their C-terminus region. Overall, the csDHN and cpDHN proteins share 84% identity and contain the conserved dehydrin serine cluster (S-segment) and a putative nuclear localization signal, but csDHN has one conserved dehydrin K-segment consensus sequence, whereas cpDHN contains two dehydrin K-segments. Both csDHN and cpDHN represent single copy genes, in 'Navel' orange and 'Star Ruby' grapefruit genomes, respectively. We found that the cpDHN gene was consistently expressed in the fruit peel tissue at harvest, but that its message levels dramatically decreased during storage at either ambient or low temperatures. However, a pre-storage hot water treatment, given to enhance fruit-chilling tolerance, increased cpDHN mRNA levels during the first 3 weeks of cold storage at 2 degrees C, and enabled the message levels to be retained for up to a further 8 weeks of cold storage at 2 degrees C. The hot water treatment by itself had no inductive effect on cpDHN gene expression when the fruits were held at non-chilling temperatures. Other stresses applied to the fruit, such as wounding, UV irradiation, water stress, low oxygen and exposure to the stress hormone ethylene decreased DHN mRNA levels, whereas abscisic acid had no effect at all.

Journal Article↗