Differential expression of four genes encoding 1-aminocyclopropane-1-caroboxylate synthase in Lupinus albus during germination, and in response to indole-3-acetic acid and wounding

Regulation of the turnover of ACC synthases by phytohormones and heterodimerization in Arabidopsis

Ethylene influences many aspects of plant growth and development. The biosynthesis of ethylene is highly regulated by a variety of internal and external cues. A key target of this regulation is 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACS), generally the rate-limiting step in ethylene biosynthesis, which is regulated both transcriptionally and post-transcriptionally. Prior studies have demonstrated that cytokinin and brassinosteroid (BR) act as regulatory inputs to elevate ethylene biosynthesis by increasing the stability of ACS proteins. Here, we demonstrate that several additional phytohormones also regulate ACS protein turnover. Abscisic acid, auxin, gibberellic acid, methyl jasmonic acid, and salicylic acid differentially regulate the stability of ACS proteins, with distinct effects on various isoforms. In addition, we demonstrate that heterodimerization influences the stability of ACS proteins. Heterodimerization between ACS isoforms from distinct subclades results in increased stability of the shorter-lived partner. Together, our study provides a comprehensive understanding of the roles of various phytohormones on ACS protein stability, which brings new insights into crosstalk between ethylene and other phytohormones, and a novel regulatory mechanism that controls ACS protein stability through a heterodimerization of ACS isoforms.

Functional characterization of two almond C-repeat-binding factors involved in cold response

Low temperature plays a crucial role in seasonal development of woody plants and may directly impact crop production, more particularly in temperate fruit trees. Given its high genetic variability and adaptability to different climatic conditions, almond (Prunus dulcis Mill.) is an interesting model to understand the mechanisms regulating low temperature sensing in fruit trees. In this paper, we report the cloning and characterization of two genes (PdCBF1 and PdCBF2) belonging to the C-repeat-binding factor (CBF) family of transcription factors. Southern blotting analysis showed that this family is composed of at least five members. In almond shoots propagated in vitro, transcription of these genes was rapidly induced by low temperature, suggesting an involvement in cold acclimation. Transactivation assays showed that PdCBF1 and PdCBF2 could bind to dehydration responsive element/C-repeat containing sequences, as activators of gene expression. In addition, induction of both PdCBFs by cold was higher towards the end of the day, which agreed with the expression pattern of PdDehydrin1, a predicted CBF target gene. Furthermore, PdCBF1 and PdCBF2 were also transiently induced by abscisic acid and drought treatments. Considering the bin mapping analysis that correlated PdCBFs and PdDHN1 (respectively in linkage groups 5 and 7) with two different quantitative trait locicontrolling blooming time, it is relevant to perform further association studies that may validate their effect on this trait.

Growing in darkness: The etiolated lupin hypocotyls

Epigeal germination of a dicot, like lupin (Lupinus albus L.), produces a seedling with a characteristic hypocotyl, which grows in darkness showing a steep growth gradient with an elongation zone just below the apex. The role of phytohormones, such as auxin and ethylene, in etiolated hypocotyl growth has been the object of our research for some time. The recent cloning and expression of three genes of influx and efflux carriers for polar auxin transport (LaAUX1, LaPIN1 and LaPIN3) reinforces a previous model proposed to explain the accumulation of auxin in the upper growth zone of the hypocotyl.

The involvement of auxin in the ripening of climacteric fruits comes of age: the hormone plays a role of its own and has an intense interplay with ethylene in ripening peaches

Ethylene has long been regarded as the main regulator of ripening in climacteric fruits. The characterization of a few tomato mutants, unable to produce climacteric ethylene and to ripen their fruits even following treatments with exogenous ethylene, has shown that other factors also play an important role in the control of climacteric fruit ripening. In climacteric peach and tomato fruits it has been shown that, concomitant with ethylene production, increases in the amount of auxin can also be measured. In this work a genomic approach has been used in order to understand if such an auxin increase is functional to an independent role played by the hormone during ripening of the climacteric peach fruits. Besides the already known indirect activity on ripening due to its up-regulation of climacteric ethylene synthesis, it has been possible to show that auxin plays a role of its own during ripening of peaches. In fact, the hormone has shown the ability to regulate the expression of a number of different genes. Moreover, many genes involved in biosynthesis and transport and, in particular, the signalling (receptors, Auxin Response Factors and Aux/IAA) of auxin had increased expression in the mesocarp during ripening, thus strengthening the idea that this hormone is actively involved in the ripening of peaches. This study has also demonstrated the existence of an important cross-talk between auxin and ethylene, with genes in the auxin domain regulated by ethylene and genes in the ethylene domain regulated by auxin.

Expression of ipt gene controlled by an ethylene and auxin responsive fragment of the LEACO1 promoter increases flower number in transgenic Nicotiana tabacum

Cytokinins play important roles in regulating plant growth and development. A new genetic construct for regulating cytokinin content in plant cells was cloned and tested. The gene coding for isopentenyl transferase (ipt) was placed under the control of a 0.821 kb fragment of the 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase gene promoter from Lycopersicon esculentum (LEACO1) and introduced into Nicotiana tabacum (cv. Havana). Some LEACO1(0.821) (kb)-ipt transgenic plant lines displayed normal shoot morphology but with a dramatic increase in the number of flower buds compared to nontransgenic plants. Other transgenic lines produced excessive lateral branch development but no change in flower bud number. Isolated leaves of transgenic tobacco plants showed a significantly prolonged retention of chlorophyll under dark incubation (25 degrees C for 20 days). Leaves of nontransformed plants senesced gradually under the same conditions. Experiments with LEACO1(0.821) (kb)-gus transgenic tobacco plants suggested auxin and ethylene involvement in induction of LEACO1(0.821) (kb) promoter activity. Multiple copies of nucleotide base sequences associated with either ethylene or auxin response elements were identified in the LEACO1(0.821) (kb) promoter fragment. The LEACO1(0.821) (kb)-ipt fusion gene appears to have potential utility for improving certain ornamental and agricultural crop species by increasing flower bud initiation and altering branching habit.

The female-specific Cs-ACS1G gene of cucumber. A case of gene duplication and recombination between the non-sex-specific 1-aminocyclopropane-1-carboxylate synthase gene and a branched-chain amino acid transaminase gene

Cucumber (Cucumis sativus L.) is a monoecious plant in which female sex expression (gynoecy) is controlled by the Female (F) locus that can be modified by other sex-determining genes as well as by environmental and hormonal factors. As in many other cucurbits, ethylene is the major plant hormone regulating female sex expression. Previously we isolated the Cs-ACS1 (ACS, 1-aminocyclopropane-1-carboxylate synthase) gene that encodes the rate-limiting enzyme in the ethylene biosynthetic pathway. We proposed that Cs-ACS1 is present in a single copy in monoecious (ffMM) plants whereas gynoecious plants (FFMM) contain an additional copy Cs-ACS1G that was mapped to the F locus. To study the origin of Cs-ACS1G, we cloned and analyzed both the gynoecious-specific Cs-ACS1G gene and the non-sex-specific Cs-ACS1 gene. Our results indicate that Cs-ACS1G is the result of a relatively recent gene duplication and recombination, between Cs-ACS1 and a branched-chain amino acid transaminase (BCAT) gene. Taking into consideration that the Cs-ACS1G gene was mapped to the F locus, we propose that this duplication event gave rise to the F locus and to gynoecious cucumber plants. Computer analysis of the 1 kb region upstream of the transcription initiation site revealed several putative cis-acting regulatory elements that can potentially confer the responsiveness of Cs-ACS1G to developmental and hormonal factors and thereby control female sex determination in cucumber. These findings lead us to a model explaining the action of Cs-ACS1 and Cs-ACS1G in cucumber floral sex determination.

Importance of methionine biosynthesis for Arabidopsis seed germination and seedling growth

Proteomics of Arabidopsis seeds revealed the differential accumulation during germination of two housekeeping enzymes. The first corresponded to methionine synthase that catalyses the last step in the plant methionine biosynthetic pathway. This protein was present at low level in dry mature seeds, and its level was increased strongly at 1-day imbibition, prior to radicle emergence. Its level was not increased further at 2-day imbibition, coincident with radicle emergence. However, its level in 1-day imbibed seeds strongly decreased upon subsequent drying of the imbibed seeds back to the original water content of the dry mature seeds. The second enzyme corresponded to S-adenosylmethionine synthetase that catalyses the synthesis of S-adenosylmethionine from methionine and ATP. In this case, this enzyme was detected in the form of two isozymes with different pI and Mr. Both proteins were absent in dry mature seeds and in 1-day imbibed seeds, but specifically accumulated at the moment of radicle protrusion. Arabidopsis seed germination was strongly delayed in the presence of dl-propargylglycine, a specific inhibitor of methionine synthesis. Furthermore, this compound totally inhibited seedling growth. These phenotypic effects were largely alleviated upon methionine supplementation in the germination medium. The results indicated that methionine synthase and S-adenosylmethionine synthetase are fundamental components controlling metabolism in the transition from a quiescent to a highly active state during seed germination. Moreover, the observed temporal patterns of accumulation of these proteins are consistent with an essential role of endogenous ethylene in Arabidopsis only after radicle protrusion.

Auxin cross-talk: integration of signalling pathways to control plant development

Plants sense and respond to endogenous signals and environmental cues to ensure optimal growth and development. Plant cells must integrate the myriad transduction events into a comprehensive network of signalling pathways and responses. The phytohormone auxin occupies a central place within this transduction network, frequently acting in conjunction with other signals, to co-ordinately regulate cellular processes such as division, elongation and differentiation. As a non-cell autonomous signal, auxin also interacts with other signalling pathways to regulate inter-cellular developmental processes. As part of this especially themed edition of Plant Molecular Biology, we will review examples of 'cross-talk' between auxin and other signalling pathways. Given the current state of knowledge, we have deliberately focused our efforts reviewing auxin interactions with other phytohormone and light signalling pathways. We conclude by discussing how new genomic approaches and the Arabidopsis genome sequence are likely to impact this area of research in the future.