Identification of gibberellin A1 in the embryo suspensor of Phaseolus coccineus

The plant cell cycle--15 years on

The basic components of the plant cell cycle are G1 (postmitotic interphase), S-phase (DNA synthesis phase), G2 (premitotic interphase) and mitosis/cytokinesis. Proliferating cells are phosphoregulated by cyclin-dependent protein kinases (CDKs). Plant D-type cyclins are sensors of the G0 to G1 transition, and are also important for G2/M. At G1/S, the S-phase transcription factor, E2F, is released from inhibitory retinoblastoma protein. Negative regulation of G1 events is through KRPs (Kip-related proteins). Plant S-phase genes are similar to animal ones, but timing of expression can be different (e.g. CDC6 at the start of S-phase) and functional evidence is limited. At G2/M, A-type and the unique B-type CDKs when bound to A, B and D cyclins, drive cells into division; they are negatively regulated by ICK1/2 and perhaps also by WEE1 kinase. In Arabidopsis, a putative CDC25 lacks a regulatory domain. Mitosis depends on correct temporal activity of CDKs, Aurora kinases and anaphase promotion complex; CDK-cyclin B activity beyond metaphase is catastrophic. Endoreduplication (re-replication of DNA in the absence of mitosis) is characterized by E2F expression and down-regulation of mitotic cyclins. Some cell size data support, whilst others negate, the idea of cell size having an impact on development.

The cell cycle in plant development

The aim of this review is to discuss the molecular controls of the cell cycle in relation to higher plant development. An analysis is made of the current models of the cell cycle based on the biochemistry and genetics of the budding yeast, Saccharomyces cerevisiae and the fission yeast, Schizosaccharomyces pombe. What emerges are universal mechanisms observed in a wide range of taxonomic groups involving a group of protein kinases which regulate the transition from both post-synthetic interphase (G2) to mitosis and from pre-synthetic interphase (G1) to DNA synthetic-(S) phase. The data are consistent in showing the activity of protein kinase complexes operating in conjunction with at least one dephosphorylating enzyme. The natural substrate(s) for the key cell division cycle gene product, p34^{cdc 2} , has yet to be resolved although the nuclear lamins and microtubular apparatus are strong candidates. These models serve as a basis for assessing the cell cycle in higher plants. Mitosis and various stages of nuclear DNA replication are considered in relation to the presumed initiation and termination factors that regulate these events. In order to make a link between the cell cycle and plant development special consideration has been given to plant meristems. In particular, the activity of the cell cycle in cells that have the capacity to regenerate whole tissue systems ('stem cells') within the meristem is discussed. In the root meristem, the quiescent centre cells conform to a stem cell population; a non-cycling stem cell may be immune to the morphogenetic signals that cause cycling cells to arrest and differentiate. The pericycle may act as a vestigial stem cell population. The shoot apex is also discussed in relation to both vegetative and floral growth. Although gradients of cell division exist in shoot meristems it is far less obvious where 'stem', or founder, cells reside in the apex. The way in which the cell cycle shortens on transition to floral growth is considered critical for identifying when the meristem becomes florally determined. Temperature and toxic metals are given special attention where it is emphasized that G1 phase becomes protracted when plants are stressed. Species that can tolerate stressful environments may have meristems in which a greater number of cells are competent for division. Finally, the cell cycle in vitro is discussed in relation to rapid changes in gene expression which are linked to the transition from G1 to S phase. The latter emerges as a key cell cycle transition for plant meristems both in vivo and in vitro. CONTENTS Summary 1 I. Introduction 2 II. The cell division cycle (cdc) genes 2 III. The plant cell cycle 5 IV. Meristems 9 V. Effects of external stress on the cell cycle in plant meristems 14 VI. The plant cell cycle in vitro 15 VII. Conclusions 15 Acknowledgements 16 References 16.

Gibberellin-like activity in suspensors of Tropaeolum majus L. and Cytisus laburnum L

Gibberellins in the embryo-suspensor system have been considered so far only in Phaseolus coccineus. We present in this report the localization of gibberellin-like substances in the suspensors of Tropaeolum majus L. and Cytisus laburnum L. The total gibberellin activity (expressed as gibberellic-acid equivalent in the α-amylase bioassay) in 2000 suspensors (106 mg fresh weight; FW) of C. laburnum and in 600 suspensors (236 mg FW) of T. majus were 50.9 μg g(-1) FW and 8.9 μg g(-1) FW respectively.

Quantitation of gibberellins and the metabolism of [(3)H]gibberellin A 1 during somatic embryogenesis in carrot and anise cell cultures

In a carrot (Daucus carota L.) cell line lacking the ability to undergo somatic embryogenasis, and in carrot and anise (Pimpinella anisum L.) cell lines in which embryogenesis could be regulated by presence or absence of 2,4-dichlorophen-oxyacetic acid (2,4-D), in the medium (+2,4-D=no embryogenesis,-2,4-D=embryo differentiation and development), the levels of endogenous gibberellin(s) (GA) were determined by the dwarfrice bioassay, and the metabolism of [(3)H]GA1 was followed. Embryos harvested after 14 d of subculture in-2,4-D had low levels (0.2-0.3 μg g(-1) dry weight) of polar GA (e.g. GA1-like), but much (3-22 times) higher levels of less-polar GA (GA4/7-like); GA1, GA4 and GA7 are native to these cultures. Conversely, the undifferentiated cells in a non-embryogenic strain, and proembryos of an embryogenic strain (+2,4-D) showed very high levels of polar GA (2.9-4.4 μg g(-1)), and somewhat reduced levels of less-polar GA. Cultures of anise undergoing somatic embryo development (-2,4-D) metabolized [(3)H]GA1 very quickly, whereas proembryo cultures of anise (+2,4-D) metabolized [(3)H]GA1 slowly. The major metabolites of [(3)H]GA1 in anise were tentatively identified as GA8-glucoside (24%), GA8 (15%), GA1-glucoside (8%) and the Δ(1(10))GA1-counterpart (2%). Thus, high levels of a GA1-like substance and a reduced ability to metabolize GA1 are correlated with the absence of embryo development, while lowered levels of GA1-like substance and a rapid metabolism of GA1 into GA8 and GA-conjugates are correlated with continued embryo development. Exogenous application of GA3 is known to reduce somatic embryogenesis in carrot cultures; GA4 was found to have the same effect in anise cultures. Thus, a role (albeit negative) in somatic embryogenesis for a polar, biologically active GA is implied.