Plant salt tolerance

Agricultural productivity worldwide is subject to increasing environmental constraints, particularly to drought and salinity due to their high magnitude of impact and wide distribution. Traditional breeding programs trying to improve abiotic stress tolerance have had some success, but are limited by the multigenic nature of the trait. Tolerant plants such as Craterostigma plantagenium, Mesembryanthemum crystallinum, Thellungiella halophila and other hardy plants could be valuable tools to dissect the extreme tolerance nature. In the last decade, Arabidopsis thaliana, a genetic model plant, has been extensively used for unravelling the molecular basis of stress tolerance. Arabidopsis also proved to be extremely important for assessing functions for individual stress-associated genes due to the availability of knock-out mutants and its amenability for genetic transformation. In this review, the responses of plants to salt and water stress are described, the regulatory circuits which allow plants to cope wit...

Drought and Salt Tolerance in Plants

Auxin: regulation, action, and interaction.

Abscisic acid (ABA) regulates many agronomically important aspects of plant development, including the synthesis of seed storage proteins and lipids, the promotion of seed desiccation tolerance and dormancy, and the inhibition of the phase transitions from embryonic to germinative growth and from

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Abscisic Acid Signaling in Seeds and Seedlings

The regular arrangement of leaves around a plant's stem, called phyllotaxis, has for centuries attracted the attention of philosophers, mathematicians and natural scientists; however, to date, studies of phyllotaxis have been largely theoretical. Leaves and flowers are formed from the shoot apical meristem, triggered by the plant hormone auxin. Auxin is transported through plant tissues by specific cellular influx and efflux carrier proteins. Here we show that proteins involved in auxin transport regulate phyllotaxis. Our data indicate that auxin is transported upwards into the meristem through the epidermis and the outermost meristem cell layer. Existing leaf primordia act as sinks, redistributing auxin and creating its heterogeneous distribution in the meristem. Auxin accumulation occurs only at certain minimal distances from existing primordia, defining the position of future primordia. This model for phyllotaxis accounts for its reiterative nature, as well as its regularity and stability.

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Regulation of phyllotaxis by polar auxin transport.

To assess the role of auxin flows in plant vascular patterning, the development of vascular systems under conditions of inhibited auxin transport was analyzed. In Arabidopsis, nearly identical responses evoked by three auxin transport inhibitor substances revealed an enormous plasticity of the vascular pattern and suggest an involvement of auxin flows in determining the sites of vascular differentiation and in promoting vascular tissue continuity. Organs formed under conditions of reduced auxin transport contained increased numbers of vascular strands and cells within those strands were improperly aligned. In leaves, vascular tissues became progressively confined towards the leaf margin as the concentration of auxin transport inhibitor was increased, suggesting that the leaf vascular system depends on inductive signals from the margin of the leaf. Staged application of auxin transport inhibitor demonstrated that primary, secondary and tertiary veins became unresponsive to further modulations of auxin transport at successive stages of early leaf development. Correlation of these stages to anatomical features in early leaf primordia indicated that the pattern of primary and secondary strands becomes fixed at the onset of lamina expansion. Similar alterations in the leaf vascular responses of alyssum, snapdragon and tobacco plants suggest common functions of auxin flows in vascular patterning in dicots, while two types of vascular pattern alterations in Arabidopsis auxin transport mutants suggest that at least two distinct primary defects can result in impaired auxin flow. We discuss these observations with regard to the relative contributions of auxin transport, auxin sensitivity and the cellular organisation of the developing organ on the vascular pattern.

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Responses of plant vascular systems to auxin transport inhibition

In the embryo of Arabidopsis thaliana (L.) Heynh., formation of the hypocotyl/root axis is initiated at the early-globular stage, recognizable as oriented expansion of formerly isodiametric cells. The process depends on the activity of the gene MONOPTEROS (MP); mp mutant embryos fail to produce hypocotyl and radicle. We have analyzed the morphology and anatomy of mp mutant plants throughout the Arabidopsis life cycle. Mutants form largely normal rosettes and root systems, but inflorescences either fail to form lateral flowers or these flowers are greatly reduced. Furthermore, the auxin transport capacity of inflorescence axes is impaired and the vascular strands in all analyzed organs are distorted. These features of the mutant phenotype suggest that the MP gene promotes cell axialization and cell file formation at multiple stages of plant development.

Studies on the role of the Arabidopsis gene MONOPTEROS in vascular development and plant cell axialization.

A number of observations have implicated auxin in the formation of vascular tissues in plant organs. These include vascular strand formation in response to local auxin application, the effects of impaired auxin transport on vascular patterns and suggestive phenotypes of Arabidopsis auxin response mutants. In this study, we have used molecular markers to visualize auxin response patterns in developing Arabidopsis leaves as well as Arabidopsis mutants and transgenic plants to trace pathways of auxin signal transduction controlling the expression of early procambial genes. We show that in young Arabidopsis leaf primordia, molecular auxin response patterns presage sites of procambial differentiation. This is the case not only in normal development but also upon experimental manipulation of auxin transport suggesting that local auxin signals are instrumental in patterning Arabidopsis leaf vasculature. We further found that the activity of the Arabidopsis gene MONOPTEROS, which is required for proper vascular differentiation, is also essential in a spectrum of auxin responses, which include the regulation of rapidly auxin-inducible AUX/IAA genes, and discovered the tissue-specific vascular expression profile of the class I homeodomain-leucine zipper gene, AtHB20. Interestingly, MONOPTEROS activity is a limiting factor in the expression of AtHB8 and AtHB20, two genes encoding transcriptional regulators expressed early in procambial development. Our observations connect general auxin signaling with early controls of vascular differentiation and suggest molecular mechanisms for auxin signaling in patterned cell differentiation.

Auxin Signaling in Arabidopsis Leaf Vascular Development

Homeodomain-leucine zipper (HD-Zip) proteins are putative transcription factors encoded by a class of recently discovered homeobox genes as yet found only in plants. This paper reports on the characterization of one of these genes, ATHB-7, in Arabidopsis thaliana. ATHB-7 transcripts were present in all organs of the plant at low levels, but expression was induced several-fold by water deficit, osmotic stress as well as by exogenous treatment with abscisic acid (ABA), a response being detectable at 10(-8) M and reaching a maximum at 10(-6) M ABA. The ATHB-7 transcript was detected within 30 min after treatment with ABA and the transcript level was rapidly reduced after removal of the hormone. The induction of ATHB-7 was shown to be mediated strictly via ABA, since no induction of ATHB-7 was detectable in the ABA-deficient mutant aba-3 subjected to drought treatment. Induction levels in two ABA-insensitive mutants abi2 and abi3 were similar to the wild-type response. In the abi1 mutant, however, induction was impaired as 100-fold higher concentrations of ABA were required for a maximum induction as compared with wild-type. In this mutant the ATHB-7 response was reduced also after drought and osmotic stress treatments. These results indicate that ATHB-7 is transcriptionally regulated in an ABA-dependent manner and may act in a signal transduction pathway which mediates a drought response and also includes ABI1.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-313X.1996.10020375.x

The Arabidopsis homeobox gene ATHB‐7 is induced by water deficit and by abscisic acid

Genetic evidence links the Arabidopsis MONOPTEROS (MP) and PIN-FORMED1 (PIN1) genes to the patterning of leaf veins. To elucidate their potential functions and interactions in this process, we have assessed the dynamics of MP and PIN1 expression during vascular patterning in Arabidopsis leaf primordia. Both genes undergo a dynamic process of gradual refinement of expression into files one to two cells wide before overt vascular differentiation. The subcellular distribution of PIN1 is also gradually refined from a non-polar distribution in isodiametric cells to strongly polarized in elongated procambial cells and provides an indication of overall directions of auxin flow. We found evidence that MP expression can be activated by auxin exposure and that PIN1 as well as DR5::GUS expression is defective in mp mutant leaves. Taken together the results suggest a feedback regulatory loop that involves auxin, MP and PIN1 and provide novel experimental support for the canalization-of-auxin-flow hypothesis.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-313X.2006.02977.x

Jim Mattsson

Papers

Responses of plant vascular systems to auxin transport inhibition

Studies on the role of the Arabidopsis gene MONOPTEROS in vascular development and plant cell axialization.

Auxin Signaling in Arabidopsis Leaf Vascular Development

The Arabidopsis homeobox gene ATHB‐7 is induced by water deficit and by abscisic acid

Dynamics of MONOPTEROS and PIN-FORMED1 expression during leaf vein pattern formation in Arabidopsis thaliana.