Margret Sauter

Plant cyclins : a unified nomenclature for plant A-, B- and D-type cyclins based on sequence organization

The comparative analysis of a large number of plant cyclins of the A/B family has recently revealed that plants possess two distinct B-type groups and three distinct A-type groups of cyclins [1]. Despite earlier uncertainties, this large-scale comparative analysis has allowed an unequivocal definition of plant cyclins into either A or B classes. We present here the most important results obtained in this study, and extend them to the case of plant D-type cyclins, in which three groups are identified. For each of the plant cyclin groups, consensus sequences have been established and a new, rational, plant-wide naming system is proposed in accordance with the guidelines of the Commission on Plant Gene Nomenclature. This nomenclature is based on the animal system indicating cyclin classes by an upper-case roman letter, and distinct groups within these classes by an arabic numeral suffix. The naming of plant cyclin classes is chosen to indicate homology to their closest animal class. The revised nomenclature of all described plant cyclins is presented, with their classification into groups CycA1, CycA2, CycA3, CycB1, CycB2, CycD1, CycD2 and CycD3.

Arabidopsis RAP2.2: An Ethylene Response Transcription Factor That Is Important for Hypoxia Survival

Arabidopsis (Arabidopsis thaliana) RAP2.2 (At3g14230) is an APETALA2/ethylene response factor-type transcription factor that belongs to the same subfamily as the rice (Oryza sativa) submergence tolerance gene SUB1A. RAP2.2 is expressed at constitutively high levels in the roots and at lower levels in the shoots, where it is induced by darkness. Effector studies and analysis of ethylene signal transduction mutants indicate that RAP2.2 is induced in shoots by ethylene and functions in an ethylene-controlled signal transduction pathway. Overexpression of RAP2.2 resulted in improved plant survival under hypoxia (low-oxygen) stress, whereas lines containing T-DNA knockouts of the gene had poorer survival rates than the wild type. This indicates that RAP2.2 is important in a plants ability to resist hypoxia stress. Observation of the expression pattern of 32 low-oxygen and ethylene-associated genes showed that RAP2.2 affects only part of the low-oxygen response, particularly the induction of genes encoding sugar metabolism and fermentation pathway enzymes, as well as ethylene biosynthesis genes. Our results provide a new insight on the regulation of gene expression under low-oxygen conditions. Lighting plays an important regulatory role and is intertwined with hypoxia conditions; both stimuli may act collaboratively to regulate the hypoxic response.

Adventitious Root Growth and Cell-Cycle Induction in Deepwater Rice

Deepwater rice (Oryza sativa) is adapted to survive conditions of severe flooding over extended periods of time. During such periods adventitious roots develop to provide water, nutrients, and anchorage. In the present study the growth of adventitious roots was induced by treatment with ethylene but not auxin, cytokinin, or gibberellin. Root elongation was enhanced between 8 and 10 h after submergence. The population of cells in the S phase and expression of the S-phase-specific histone H3 gene increased within 4 to 6 h. Within 6 to 8 h the G2-phase population increased. Cell-cycle activation was accompanied by sequential induction of a cdc2-activating kinase homolog, R2, of two cdc2 genes, cdc2Os-1 and cdc2Os-2, and of three cyclin genes, cycA1;3, cycB2;1, and cycB2;2, but only induction of the R2 gene expression preceded the induction of the S phase, possibly contributing to cell-cycle regulation in the G1 phase. Both cdc2 genes were expressed at slightly higher levels during DNA replication. Transcripts of the A-type cyclin accumulated during the S and G2 phases, and transcripts of the B-type cyclins accumulated during the G2 phase. Cyclin expression was induced at all nodes with a similar time course, suggesting that ethylene acts systemically and that root primordia respond to ethylene at an early developmental stage.

Interactions between ethylene, gibberellin and abscisic acid regulate emergence and growth rate of adventitious roots in deepwater rice.

Growth of adventitious roots is induced in deepwater rice (Oryza sativa L.) when plants become submerged. Ethylene which accumulates in flooded plant parts is responsible for root growth induction. Gibberellin (GA) is ineffective on its own but acts in a synergistic manner together with ethylene to promote the number of penetrating roots and the growth rate of emerged roots. Studies with the GA biosynthesis inhibitor paclobutrazol revealed that root emergence was dependent on GA activity. Abscisic acid (ABA) acted as a competitive inhibitor of GA activity. Root growth rate on the other hand was dependent on GA concentration and ABA acted as a potent inhibitor possibly of GA but also of ethylene signaling. The results indicated that root emergence and elongation are distinct phases of adventitious root growth that are regulated through different networking between ethylene, GA and ABA signaling pathways. Adventitious root emergence must be coordinated with programmed death of epidermal cells which cover root primordia. Epidermal cell death is also controlled by ethylene, GA and ABA albeit with cell-type specific cross-talk. Different interactions between the same hormones may be a means to ensure proper timing of cell death and root emergence and to adjust the growth rate of emerged adventitious roots.

Epidermal Cell Death in Rice Is Confined to Cells with a Distinct Molecular Identity and Is Mediated by Ethylene and H2O2 through an Autoamplified Signal Pathway

Rice (Oryza sativa) forms adventitious root primordia at stem nodes during normal development. Root emergence is preceded by ethylene-induced, H2O2-mediated local death of epidermal cells. Exogenous H2O2 or enhancement of endogenous H2O2 promoted epidermal cell death in a dose-dependent manner. Inhibition of NADPH oxidase lowered ethylene-induced cell death rates. Inhibition of ethylene perception by 1-methylcyclopropene did not abolish H2O2-induced cell death, indicating that H2O2 acts downstream of ethylene. Microarray studies of epidermal cells that undergo cell death identified 61 genes coregulated by the ethylene-releasing compound ethephon and by H2O2, supporting a joint signaling pathway. Regulation of the ethylene biosynthetic genes 1-Aminocyclopropane-1-Carboxylate Oxidase1 and Ethylene Overproducer-Like1 and downregulation of Metallothionein2b (MT2b), which encodes a reactive oxygen scavenger, indicated mutual enhancement of ethylene and H2O2 signaling. Analysis of MT2b knockdown mutants showed that cell death rates were inversely related to MT2b transcript abundance. Epidermal cells above adventitious roots have a morphological and molecular identity distinct from other epidermal cells. Pro-death signals regulated several transcription factor genes with a proposed function in cell type specification. It is hypothesized that induction of cell death is dependent on epidermal cell identity.

Gibberellin-induced growth and regulation of the cell division cycle in deepwater rice

Excised stem sections of deepwater rice (Oryza sativa L., cv. Pin Gaew 56) containing the highest internode were used to study induction of rapid internodal elongation by gibberellin. It has been shown previously that this growth response is based on an increased cell production rate in the intercalary meristem and on increased cell elongation. Our investigations were aimed at establishing the temporal sequence of these GA-regulated processes. Cell sizes were determined by scanning electron microscopy, the phases of the cell cycle by flow cytometry, and DNA synthesis by [3H]thymidine incorporation. The lag time for the onset of gibberellic acid (GA3)-induced growth was 40 min. Treatment with GA3 promoted cell elongation in the intercalary meristem within 2 h. After 4 h of treatment with GA3, the fraction of meristematic cells in the G2 phase had declined, indicating that cells in the G2 phase had entered mitosis. Subsequent activation of DNA replication led to an overall increase in the cell-production rate. This was evident from an increase in the percentage of cells in the S phase and from enhanced incorporation of [3H]thymidine into DNA between 4 and 7 h of GA3 treatment. An increase in the final cell length contributed to the growth response after 7 h of GA3 application. Our results are consistent with the hypothesis that gibberellin first promotes cell elongation in the intercalary meristem and that cell division is stimulated as a result of cell-growth.

Epidermal Cell Death in Rice Is Regulated by Ethylene, Gibberellin, and Abscisic Acid

Programmed cell death (PCD) of epidermal cells that cover adventitious root primordia in deepwater rice (Oryza sativa) is induced by submergence. Early suicide of epidermal cells may prevent injury to the growing root that emerges under flooding conditions. Induction of PCD is dependent on ethylene signaling and is further promoted by gibberellin (GA). Ethylene and GA act in a synergistic manner, indicating converging signaling pathways. Treatment of plants with GA alone did not promote PCD. Treatment with the GA biosynthesis inhibitor paclobutrazol resulted in increased PCD in response to ethylene and GA presumably due to an increased sensitivity of epidermal cells to GA. Abscisic acid (ABA) was shown to efficiently delay ethylene-induced as well as GA-promoted cell death. The results point to ethylene signaling as a target of ABA inhibition of PCD. Accumulation of ethylene and GA and a decreased ABA level in the rice internode thus favor induction of epidermal cell death and ensure that PCD is initiated as an early response that precedes adventitious root growth.

Root responses to flooding

Soil water-logging and submergence pose a severe threat to plants. Roots are most prone to flooding and the first to suffer from oxygen shortage. Roots are vital for plant function, however, and maintenance of a functional root system upon flooding is essential. Flooding-resistant plants possess a number of adaptations that help maintain oxygen supply to the root. Plants are also capable of initiating organogenesis to replace their original root system with adventitious roots if oxygen supply becomes impossible. This review summarizes current findings on root development and de novo root genesis in response to flooding.

Rice in deep water: "how to take heed against a sea of troubles".

Abstract Plants are aerobic organisms for which oxygen shortage poses a severe problem. Waterlogging and flooding are the main causes of anaerobiosis and can lead to damage or even death of the plant. Rice is well adapted to semi-aquatic conditions. It is the only cereal that can be grown in flooded areas such as the great river deltas of Asia. In rice, two major strategies have evolved to cope with conditions of flooding. One is to escape submergence and thereby avoid anaerobiosis as much as possible. This is achieved through elongation growth and through extensive aeration of submerged plant parts by way of internal and external air spaces. The second adaptation is a metabolic one which includes the efficient use of carbohydrate resources and maintenance of energy charge when the cells do become anaerobic. The mainly ethanolic fermentation pathway found in anaerobic rice avoids acidification of the cytoplasm and thereby contributes to the maintenance of cell integrity. Genetic analysis indicates that the submergence tolerance trait, which is based on metabolic changes, is encoded by only one or a few as yet unidentified gene(s). Identifying these genes is a major goal in anaerobic stress research.

Internodal elongation and orientation of cellulose microfibrils and microtubules in deepwater rice

Excised stem sections of deepwater rice (Oryza sativa L.) containing the highest internode were used to study the induction of rapid internodal elongation by gibberellin (GA). It has been shown before that this growth response is based on enhanced cell division in the intercalary meristem and on increased cell elongation. In both GA-treated and control stem sections, the basal 5-mm region of the highest internode grows at the fastest rate. During 24 h of GA treatment, the internodal elongation zone expands from 15 to 35 mm. Gibberellin does not promote elongation of internodes from which the intercalary meristem has been excised. The orientation of cellulose microfibrils (CMFs) is a determining factor in cell growth. Elongation is favored when CMFs are oriented transversely to the direction of growth while elongation is limited when CMFs are oriented in the oblique or longitudinal direction. The orientation of CMFs in parenchymal cells of GA-treated and control internodes is transverse throughout the internode, indicating that CMFs do not restrict elongation of these cells. Changes in CMF orientation were observed in epidermal cells, however. In the basal 5-mm zone of the internode, which includes the intercalary meristem, CMFs of the epidermal cell walls are transversely oriented in both GA-treated and control stem sections. In slowly growing control internodes, CMF orientation changes to the oblique as cells are displaced from this basal 5-mm zone to the region above it. In GA-treated rapidly growing internodes, the reorientation of CMFs from the transverse to the oblique is more gradual and extends over the 35-mm length of the elongation zone. The CMFs of older epidermal cells are obliquely oriented in control and GA-treated internodes. The orientation of the CMFs parallels that of the cortical microtubules. This is consistent with the hypothesis that cortical microtubules determine the direction of CMF deposition. We conclude that GA acts on cells that have transversely oriented CMFs but does not promote growth of cells whose CMFs are already obliquely oriented at the start of GA treatment.