Ectopic divisions in vascular and ground tissues of Arabidopsis thaliana result in distinct leaf venation defects.

Leaf venation patterns vary considerably between species and between leaves within a species. A mechanism based on canalization of auxin transport has been suggested as the means by which plastic yet organized venation patterns are generated. This study assessed the plasticity of Arabidopsis thaliana leaf venation in response to ectopic ground or procambial cell divisions and auxin transport inhibition (ATI). Ectopic ground cell divisions resulted in vascular fragments between major veins, whereas ectopic procambial cell divisions resulted in additional, abnormal vessels along major veins, with more severely perturbed lines forming incomplete secondary and higher-order venation. These responses imply limited vascular plasticity in response to unscheduled cell divisions. Surprisingly, a combination of ectopic ground cell divisions and ATI resulted in massive vascular overgrowth. It is hypothesized that the vascular overproduction in auxin transport-inhibited wild-type leaves is limited by simultaneous differentiation of ground cells into mesophyll cells. Ectopic ground cell divisions may negate this effect by providing undifferentiated ground cells that respond to accumulated auxin by differentiation into vascular cells.

Impaired growth in transgenic plants over-expressing an expansin isoform.

Expansins are cell wall proteins characterised by their ability to stimulate wall loosening during cell expansion. The expression of some expansin isoforms is clearly correlated with growth and the external application of expansins can stimulate cell expansion in vivo in several systems. We report here the expression of a heterologous expansin coding sequence in transgenic tomato plants (Lycopersicon esculentum Mill.) under the control of a constitutive promoter. In some transgenic lines with high levels of expansin activity extractable from cell walls, we observed alterations of growth: mature plants were stunted, with shorter leaves and internodes, and dark-grown seedlings had shorter and wider hypocotyls than their wild-type counterparts. Examination of hypocotyl sections revealed similar differences at the cellular level: cortical and epidermal cells were shorter and wider than those from wild-type seedlings. The observed stimulation of radial expansion did not compensate for the decreased elongation, and overall growth was reduced in the transgenics. As this observation can seem paradoxical given the known effect of expansins on isolated cell walls, we examined the mechanical behaviour of transgenic tissue. We measured a decrease in hypocotyl elongation in response to acidic pH in the transformants. This result may account for the alterations in cell expansion, and could itself be explained by a reduced susceptibility of transgenic cell walls to expansin action.

Cell division patterns of the protoderm and root cap in the "closed" root apical meristem of Arabidopsis thaliana.

The peripheral root cap and protoderm in Arabidopsis thaliana are organized into modular packets of cells derived from formative T-divisions of the root cap/protoderm (RCP) initials and subsequent proliferative divisions of their daughter cells. Each module consists of protoderm and peripheral root cap packets derived from the same periclinal T-division event of an RCP initial. Anatomical analyses are used to interpret the history of extensively coordinated cell divisions producing this modular construction. Within a given layer of root cap, the columella and RCP initials divided in a centrifugal sequence from the innermost columella initials toward the RCP initials. All RCP initials in the lineages around the circumference of the root divided nearly simultaneously in "waves" to form one module prior to the next wave of initial divisions forming a younger module. The peripheral root cap and protoderm packets within each module completed four rounds of proliferative divisions in the axial plane to produce, on average, 16 cells per packet in the basalmost modules in axial view. Peripheral root cap and protoderm cells predominantly in the T-type (trichoblast) lineages also underwent radial divisions as they were displaced basipetally. The regularity in the cellular pattern within the modules suggests a timing mechanism controlling highly coordinated cell division in the initials and their daughter cells.

Modular construction of the protoderm and peripheral root cap in the "open" root apical meristem of Trifolium repens cv. Ladino.

Roots with open apical organization are defined by not having specific tiers of initial cells in the root apical meristem; those with closed apical organization have specific initial tiers to which all cell files can be traced. An example of the clear organization of closed roots is the development protocol of the root cap and protoderm. The key event in differentiating these tissues is the T-division, a periclinal division of the root cap/protoderm (RCP) initial that establishes a module. Each module comprises two packets, the protoderm and peripheral root cap. Consecutive T-divisions of the same RCP initial produce up to five modules on average in a lineage of cells in white clover (Trifolium repens cv. Ladino), with all lineages around the circumference of the root dividing in "waves" to form one module prior to the next. On average, clover has approximately 32 axial protoderm and peripheral root cap cells in each module, and 32 RCP lineages. The occurrence of RCP T-divisions in white clover, a root with open apical organization, and the subsequent modular construction of the root cap and protoderm, provides a link between open and closed roots and suggests a common developmental feature that most roots of seed plants may share independent of their root meristem organization type. The open apical organization of the white clover root varies from roots with closed apical organization in that the RCP initials occur in staggered positions instead of connected to discrete tiers, and the peripheral root cap and columella daughter cells form additional layers of cells. White clover also forms root hairs on all protoderm cells irrespective of their position relative to the underlying cortical cells.

Gibberellin-induced changes in growth anisotropy precede gibberellin-dependent changes in cortical microtubule orientation in developing epidermal cells of barley leaves. Kinematic and cytological studies on a gibberellin-responsive dwarf mutant, M489.

We conducted kinematic and cytological studies on "between vein" epidermal cells of the gibberellin (GA)-deficient M489 dwarf mutant of barley (Hordeum vulgare L. Himalaya). GAs affect radial and axial components of cell expansion and cortical microtubule orientation. Adaxial cells in particular expand radially after leaving the elongation zone (EZ), probably as part of leaf unrolling. Exogenous gibberellic acid corrects the mutant's short, wide blades, short EZ, and slow elongation rate. Cell production rates increase more on the adaxial than on the abaxial surface. Cells spend equal periods of time elongating in dwarf and tall plants, but relative elemental growth rates start to decline sooner in the dwarf. GA increased the rate at which longitudinal wall area increased because the increased axial growth more than compensated for reduced radial growth. In dwarf leaves, increased radial expansion was detected in basal parts of the EZ before cortical microtubules lost transverse orientation in the distal elongation zone. We conclude that loss of microtubule orientation is not required for low GA levels to reduce growth anisotropy.

Phosphate-solubilizing bacteria associated with proteoid roots of seedlings of waratah [Telopea speciosissima (Sm.) R.Br.].

Bacteria isolated from proteoid roots and non-proteoid lateral roots of Telopea speciosissima (Sm.) R.Br. seedlings were able to acidify the medium and solubilize calcium phosphates when grown in culture in the presence of ammonium salts and an appropriate carbon source. In general this activity was not detected when NO3 - was substituted for NH4 + , and it is proposed that protons were secreted in exchange for ammonium ions. Cation exchange between these protons and calcium in the medium is a possible cause of the calcium phosphate solubilizing activity. The relevance of these data to phosphate-solubilizing activity in natural environments is discussed.

Salmeterol: a novel, long-acting beta 2-agonist.

OBJECTIVE: The clinical pharmacology, pharmacokinetics, clinical efficacy, and adverse effects of the long-acting beta 2-agonist salmeterol are reviewed. DATA SOURCES: A MEDLINE search was performed to identify English-language publications pertaining to salmeterol. STUDY SELECTION: Open and controlled trials were reviewed in assessing clinical efficacy. Only the results of controlled, randomized trials were considered in the effectiveness evaluation. DATA EXTRACTION: The primary measures of effectiveness in the clinical trials were bronchodilator activity and reduction of hyperresponsiveness that may reflect antiinflammatory activity. Bronchodilator activity was measured as changes in pulmonary function; reduction of hyperresponsiveness was evaluated using respiratory challenge with methacholine, histamine, allergen, or cold air. Secondary measures included symptom scores, need for rescue doses, and patient preference. DATA SYNTHESIS: Salmeterol is a selective, beta 2-agonist that has been studied in the treatment of exercise-induced, nocturnal, and allergen-induced asthma. Salmeterol interacts with the traditional beta-receptor in a similar manner as other beta-agonists, and it exhibits potent in vitro antiinflammatory effects as an inhibitor of inflammatory mediator release. Less evidence exists for its in vivo antiinflammatory activity. Salmeterol demonstrates prolonged receptor occupancy, which is thought to contribute to its long duration of action. The recommended dose is 50 micrograms via metered-dose inhaler or dry-powdered inhalation. In the published clinical trials, salmeterol was more effective than albuterol in treating asthma, including exercise and allergen-induced asthma. Salmeterol's major advantage over other inhaled beta-agonists is its long duration of action (12 hours), making it an excellent choice for treatment of nocturnal asthma. A potential disadvantage is delayed onset of action. Tachyphylaxis to salmeterol's bronchodilator effects has not been shown, but tolerance to its protective effects against methacholine-induced bronchoconstriction has occurred. Adverse effects reported have been mild and have included headache, tremor, and palpitations. CONCLUSIONS: Salmeterol is an effective beta 2-agonist in the treatment of asthma. However, several issues require further investigation regarding its long-term effects on disease control, significance of antiinflammatory activity, and role as a rescue medication.

Development of water conducting capacity in the root systems of young plants of corn and some other c4 grasses.

Development of the primary and early nodal roots was studied in Zea mays L., Zea mexicana (Schrad.) Reeves & Mangelsd., Sorghum bicolor (L.) Moench., and Sorghum sudanese (Piper) Stapf. in relation to shoot development. In all the types studied all roots reached lengths of about 30 centimeters before the late metaxylem (LMX) was open, and young plants with total root lengths of around 100 centimeters had almost no open LMX. On average, corn seedlings with up to 36 square centimeters of leaf had no open LMX. The name "immature apices" is suggested for such long but not fully functional roots. In plants up to 50 days old a fairly constant proportion of less than half the total root length had open LMX. A pilot study of stomatal resistance on days of high evaporative demand suggested that young seedlings may show higher resistance than older plants in the afternoon. Estimates of longitudinal permeability of corn roots with only early metaxylem vessels open indicate very steep gradients of water potential would develop under such conditions.