Local, Efflux-Dependent Auxin Gradients as a Common Module for Plant Organ Formation

https://www.cell.com/trends/plant-science/pdf/S1360-1385(04)00296-1.pdf

NAC transcription factors: structurally distinct, functionally diverse

Seed dormancy provides a mechanism for plants to delay germina- tion until conditions are optimal for survival of the next generation. Dormancy release is regulated by a combination of environmental and endogenous signals with both synergistic and competing effects. Molecular studies of dormancy have correlated changes in transcrip- tomes, proteomes, and hormone levels with dormancy states ranging from deep primary or secondary dormancy to varying degrees of re- lease. The balance of abscisic acid (ABA):gibberellin (GA) levels and sensitivity is a major, but not the sole, regulator of dormancy status. ABA promotes dormancy induction and maintenance, whereas GA promotes progression from release through germination; environ- mental signals regulate this balance by modifying the expression of biosynthetic and catabolic enzymes. Mediators of environmental and hormonal response include both positive and negative regulators, many of which are feedback-regulated to enhance or attenuate the response. The net result is a slightly heterogeneous response, thereby providing more temporal options for successful germination.

Molecular Aspects of Seed Dormancy

https://www.cell.com/current-biology/pdf/S0960-9822(05)01213-3.pdf

Patterns of Auxin Transport and Gene Expression during Primordium Development Revealed by Live Imaging of the Arabidopsis Inflorescence Meristem

Land plants evolved xylem vessels to conduct water and nutrients, and to support the plant. Microarray analysis with a newly established Arabidopsis in vitro xylem vessel element formation system and promoter analysis revealed the possible involvement of some plant-specific NAC-domain transcription factors in xylem formation. VASCULAR-RELATED NAC-DOMAIN6 (VND6) and VND7 can induce transdifferentiation of various cells into metaxylem- and protoxylem-like vessel elements, respectively, in Arabidopsis and poplar. A dominant repression of VND6 and VND7 specifically inhibits metaxylem and protoxylem vessel formation in roots, respectively. These findings suggest that these genes are transcription switches for plant metaxylem and protoxylem vessel formation.

/pdf/transcription-switches-for-protoxylem-and-metaxylem-vessel-59w94bmn0u.pdf

Transcription switches for protoxylem and metaxylem vessel formation

From an enhancer trap screen for genes expressed in Arabidopsis embryos, we identified a gene expressed from the octant stage onward in the boundary between the two presumptive cotyledons and in a variety of postembryonic organ and meristem boundaries. This gene, CUP-SHAPED COTYLEDON3 (CUC3), encodes a putative NAC-domain transcription factor that is homologous with CUC1 and CUC2. Analysis of a CUC3 hypomorph and a putative cuc3 null mutant indicates that CUC3 function is partially redundant with that of CUC1 and CUC2 in the establishment of the cotyledon boundary and the shoot meristem, thus revealing an even higher degree of redundancy in this class of genes than was thought previously. The CUC3 expression pattern, the cuc3 phenotypes, and CUC3 expression in a series of shoot meristem mutants and transgenes suggest a primary role for CUC3 in the establishment of boundaries that contain cells with low proliferation and/or differentiation rates. The CUC-mediated establishment of such boundaries may be essential for the initiation of shoot meristems.

The CUP-SHAPED COTYLEDON3 Gene Is Required for Boundary and Shoot Meristem Formation in Arabidopsis

Embryogenesis in plants can commence from cells other than the fertilized egg cell. Embryogenesis initiated from somatic cells in vitro is an attractive system for studying early embryonic stages when they are accessible to experimental manipulation. Somatic embryogenesis in Arabidopsis offers the additional advantage that many zygotic embryo mutants can be studied under in vitro conditions. Two systems are available. The first employs immature zygotic embryos as starting material, yielding continuously growing embryogenic cultures in liquid medium. This is possible in at least 11 ecotypes. A second, more efficient and reproducible system, employing the primordia timing mutant (pt allelic to hpt, cop2, and amp1), was established. A significant advantage of the pt mutant is that intact seeds, germinated in 2,4-dichlorophenoxyacetic acid (2,4-D) containing liquid medium, give rise to stable embryonic cell cultures, circumventing tedious hand dissection of immature zygotic embryos. pt zygotic embryos are first distinguishable from wild type at early heart stage by a broader embryonic shoot apical meristem (SAM). In culture, embryogenic clusters originate from the enlarged SAMs. pt somatic embryos had all characteristic embryo pattern elements seen in zygotic embryos, but with higher and more variable numbers of cells. Embryogenic cell cultures were also established from seedling, of other mutants with enlarged SAMs, such as clavata (clv). pt clv double mutants showed additive effects on SAM size and an even higher frequency of seedlings producing embryogenic cell lines. pt clv double mutant plants had very short fasciated inflorescence stems and additive effects on the number of rosette leaves. This suggests that the PT and CLV genes act in independent pathways that control SAM size. An increased population of noncommitted SAM cells may be responsible for facilitated establishment of somatic embryogenesis in Arabidopsis.

https://www.genetics.org/content/genetics/149/2/549.full.pdf

Somatic Embryogenesis in Arabidopsis thaliana Is Facilitated by Mutations in Genes Repressing Meristematic Cell Divisions

A seed marks the transition between two developmental states; a plant is an embryo during seed formation, whereas it is a seedling after emergence from the seed. Two factors have been identified in Arabidopsis that play a role in establishment of repression of the embryonic state: PKL (PICKLE), which codes for a putative CHD3 chromatin remodeling factor, and gibberellin (GA), a plant growth regulator. Previous observations have also suggested that PKL mediates some aspects of GA responsiveness in the adult plant. To investigate possible mechanisms by which PKL and GA might act to repress the embryonic state, we further characterized the ability of PKL and GA to repress embryonic traits and reexamined the role of PKL in mediating GA-dependent responses. We found that PKL acts throughout the seedling to repress expression of embryonic traits. Although the ability of pkl seedlings to express embryonic traits is strongly induced by inhibiting GA biosynthesis, it is only marginally responsive to abscisic acid and SPY (SPINDLY), factors that have previously been demonstrated to inhibit GA-dependent responses during germination. We also observed that pkl plants exhibit the phenotypic hallmarks of a mutation in a positive regulator of a GA response pathway including reduced GA responsiveness and increased synthesis of bioactive GAs. These observations indicate that PKL may mediate a subset of GA-dependent responses during shoot development.

PICKLE acts throughout the plant to repress expression of embryonic traits and may play a role in gibberellin-dependent responses.

Zygotic embryos of three Arabidopsis thaliana (L.) Heynh. mutants lacking an embryonic shoot apical meristem (SAM), shoot meristemless (stm), wuschel (wus) and zwille/pinhead (zll/pnh) were used as explants to establish embryogenic cell cultures. Somatic embryos of all three mutants showed the same mutant phenotypes as their zygotic equivalents. These results provide genetic evidence that the developmental program of somatic and zygotic embryos is indistinguishable. They also suggest that a functional SAM is not required for somatic embryogenic cell formation in Arabidopsis.

Somatic embryogenesis from Arabidopsis shoot apical meristem mutants

By means of co-culture in growth regulator-free medium we analysed whether factors secreted into the medium of Daucus carota (carrot) somatic embryo cultures would be able to overcome the developmental arrest of globular Arabidopsis thaliana somatic embryos. Instead of Arabidopsis embryogenesis being promoted the development of carrot somatic embryos was inhibited at the globular stage in the presence of Arabidopsis suspension culture aggregates with attached globular embryos. Several experiments showed that this was due to the release of previously accumulated 2,4-D by the Arabidopsis cultures. (1) In addition to arresting carrot embryogenesis, co-culture with Arabidopsis cell suspensions also induced callus formation on Arabidopsis root segments. (2) Both effects only occurred with Arabidopsis suspensions grown in the presence of 2,4-D and not with those grown in the presence of NAA, demonstrating that Arabidopsis is not segregating a “general” inhibiting factor. (3) Both effects could be prevented by either binding 2,4-D to active charcoal or by washing it away by changing the medium daily. (4) Uptake of 2,4-D into Arabidopsis cells during culture in 2,4-D containing medium and subsequent release of 2,4-D after transfer to growth regulator-free medium was measured. (5) These low levels of released 2,4-D (0.2– 0.5 μm) could mimic the observed effects. Taken together these data suggest that the high intracellular 2,4-D content of Arabidopsis cultures may interfere with Arabidopsis somatic embryo development beyond the globular stage.

Andreas P. Mordhorst

Papers

The CUP-SHAPED COTYLEDON3 Gene Is Required for Boundary and Shoot Meristem Formation in Arabidopsis

Somatic Embryogenesis in Arabidopsis thaliana Is Facilitated by Mutations in Genes Repressing Meristematic Cell Divisions

PICKLE acts throughout the plant to repress expression of embryonic traits and may play a role in gibberellin-dependent responses.

Somatic embryogenesis from Arabidopsis shoot apical meristem mutants

Co-culture with Daucus carota somatic embryos reveals high 2,4-D uptake and release rates of Arabidopsis thaliana cultured cells