ENVIRONMENTAL CONTROL . . . . . . . . . . . . . . . . .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Genetics of Sensitivity to Environmental Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Control by Day Length . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Control of floral evocation and morphogenesis

Adventitious rooting (AR) is a multifactorial response leading to new roots at the base of stem cuttings, and the establishment of a complete and autonomous plant. AR has two main phases: a) induction, with a requirement for higher auxin concentration; b) formation, inhibited by high auxin and in which anatomical changes take place. The first stages of this process in severed organs necessarily include wounding and water stress responses which may trigger hormonal changes that contribute to reprogram target cells that are competent to respond to rooting stimuli. At severance, the roles of jasmonate and abscisic acid are critical for wound response and perhaps sink strength establishment, although their negative roles on the cell cycle may inhibit root induction. Strigolactones may also inhibit AR. A reduced cytokinin concentration in cuttings results from the separation of the root system, whose tips are a relevant source of these root induction inhibitors. The combined increased accumulation of basipetally transported auxins from the shoot apex at the cutting base is often sufficient for AR in easy-to-root species. The role of peroxidases and phenolic compounds in auxin catabolism may be critical at these early stages right after wounding. The events leading to AR strongly depend on mother plant nutritional status, both in terms of minerals and carbohydrates, as well as on sink establishment at cutting bases. Auxins play a central role in AR. Auxin transporters control auxin canalization to target cells. There, auxins act primarily through selective proteolysis and cell wall loosening, via their receptor proteins TIR1 and ABP1. A complex microRNA circuitry is involved in the control of auxin response factors essential for gene expression in AR. After root establishment, new hormonal controls take place, with auxins being required at lower concentrations for root meristem maintenance and cytokinins needed for root tissue differentiation.

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When stress and development go hand in hand: main hormonal controls of adventitious rooting in cuttings

A few years ago it was demonstrated that nitric oxide (NO) and cGMP are involved in the auxin response during adventitious root (AR) formation in cucumber (Cucumis sativus). More recently, a mitogen-activated protein kinase cascade was shown to be induced by IAA in a NO-dependent, but cGMP-independent, pathway. In the present study, the involvement of Ca2+ and the regulation of Ca2+-dependent protein kinase (CDPK) activity during IAA- and NO-induced AR formation was evaluated in cucumber explants. The effectiveness of several broad-spectrum Ca2+ channel inhibitors and Ca2+ chelators in affecting AR formation induced by IAA or NO was also examined. Results indicate that the explants response to IAA and NO depends on the availability of both intracellular and extracellular Ca2+ pools. Protein extracts from cucumber hypocotyls were assayed for CDPK activity by using histone IIIS or syntide 2 as substrates for in-gel or in vitro assays, respectively. The activity of a 50 kDa CDPK was detected after 1 d of either NO or IAA treatments and it extended up to the third day of treatment. This CDPK activity was affected in both extracts from NO- and IAA-treated explants in the presence of the specific NO-scavenger cPTIO, suggesting that NO is required for its maximal and sustained activity. The in-gel and the in vitro CDPK activity, as well as the NO- or IAA-induced AR formation, were inhibited by calmodulin antagonists. Furthermore, the induction of CDPK activity by NO and IAA was shown to be reliant on the activity of the enzyme guanylate cyclase.

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Calcium and calcium-dependent protein kinases are involved in nitric oxide- and auxin-induced adventitious root formation in cucumber

Adventitious roots are a post-embryonic root which arise from the stem and leaves and from non-pericycle tissues in old roots and it is one of the most important ways of vegetative propagation in plants. Many exogenous and endogenous factors regulate the formation of adventitious roots, such as Ca2+, sugars, auxin, polyamines, ethylene, nitric oxide, hydrogen peroxide, carbon monoxide, cGMP, MAPKs and peroxidase, etc. These mediators are thought to function as signaling and mediate auxin signal transduction during the formation of adventitious roots. To date, only a few genes have been identified that are associated with the general process of adventitious rooting, such as ARL1, VvPRP1, VvPRP2, HRGPnt3, LRP1 and RML, etc. Auxin has been shown to be intimately involved in the process of adventitious rooting and function as crucial role in adventitious rooting. Great progress has been made in elucidating the auxin-induced genes and auxin signaling pathway, especially in auxin response Aux/IAA and ARF genes family and the auxin receptor TIR1. Although, some of important aspects of adventitious rooting signaling have been revealed, the intricate signaling network remains poorly understood.

Mediators, Genes and Signaling in Adventitious Rooting

Calcium ions are of central importance in cellular physiology, as they carry the signal activating cells to perform their programmed function. Ca2+ is particularly suitable for this role because of its chemical properties and because its free concentration gradient between the extra cellular and the cytosolic concentrations is very high, about four orders of magnitude. The cytosolic concentration of Ca2+ is regulated by binding and chelation by various substances and by transport across plasma and intracellular membranes. Various channels, transport ATPases, uniporters, and antiporters in the plasma mem brane, endoplasmic and sarcoplasmic reticulum, and mitochondria are responsible for the transport of Ca2+ .The regulation of these transport systems is the subject of an increasing number of studies. In this short review, we focus on the mitochondrial transporters, i.e. the calcium uniporter used for Ca2+ uptake, and the antiporters used for the efflux, i.e. the Ca2+/Na+ antiporter in mitochondria and the plasma membrane of excitable cells,and the Ca2+/nH+ antiporter in liver and some other mitochondrial types. Mitochondria are of special interest in that Ca2+ stimulates respiration and oxidative phosphorylation to meet the energy needs of activated cells. The studies on Ca2+ and mitochondria began in the fifties, but interest in mito chondrial Ca2+ handling faded in the late seventies since it had become apparent that mitochondria in resting cells contain very low Ca2+. Interest increased again in the nineties also because it was discovered that mitochondria and Ca2+ had a central role in apoptosis and necrosis. This is of special interest in calcium overload and oxidative stress conditions, when the opening of the mitochondrial permeability transition pore is stimulated.

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A historical review of cellular calcium handling, with emphasis on mitochondria.

Poplar shoots raised in vitro were induced to root by incubation on an auxin (NAA) containing medium for 7 h. After 13 days on an auxin-free medium, 97% of the treated shoots had rooted. The introduction of known antiauxins (PCIB, PBA, POAA) into the rooting expression auxin-free medium, after the 7-h induction by NAA, completely (PCIB and PBA) or severely (POAA) inhibited rooting. The exclusion of calcium from the expression auxin free medium reduced the percentage of rooting by about 42%. The inhibition was still higher in the presence of EGTA, a calcium chelator. Lanthanum chloride, a calcium channel blocker, also completely inhibited rooting, when incorporated into the auxin free medium, with or without calcium. These results support previous hypotheses about the implication of both endogenous auxin and calcium in the late phases of the adventitious rooting process.

Confirmation of the role of auxin and calcium in the late phases of adventitious root formation

Proton pump and IAA sensitivity changes in spinach leaves during the flowering induction

Plasma membrane H+ATPase extracted from leaves of spinach plants induced to flower by gibberellic acid treatments or by a transfer to a photoperiod of 24 h had a lower Kmapp than that from vegetative plants grown in short days. The Kmapp obtained after inhibition by vanadate was decreased in vegetative plants and increased in induced ones showing a differential effect of this inhibitor on the kinetic properties of the enzyme between vegetative and induced plants. The phospholipid fatty acid analysis of the purified plasma membrane showed an increase of C18:1/C18:2 fatty acid ratio upon induction by light or by gibberellic acid treatments, whereas the saturated to unsaturated fatty acid ratio was kept constant. The decrease in the Kmapp observed after induction may be thus interpreted in terms of the observed changes in lipid environment.

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Effect of photoperiod and gibberellic acid on the plasma membrane H+ATPase activity of spinach leaves

La spectroscopic HATR/FTIR a ete utilisee pour analyser la membrane plasmique des feuilles d'epinard Spinacia oleracea avant et apres induction florale. Le coefficient de correlation de Pearson a ete employe dans l'analyse des similitudes entre spectres. La difference principale entre les spectres de membranes de plantes vegetatives et induites se situe entre 1800 et 1500 cm⁻¹. Cela montre que la composition de la membrane plasmique subit une modification qui pourrait etre associee a l'induction florale.

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Characterization and identification of plasma membrane before and after flowering induction by Fourier transform infrared spectroscopy using horizontal attenuated total reflectance

Poplar shoots raised in vitro were induced to root by a 7 h passage on an auxin (1-naphthaleneacetic acid) medium. The percentage of rooting was reduced from ± 97% to ± 47% when vanadate (200 µM) was included in the auxin medium. Introduction of vanadate in the medium without auxin after the 7 h induction on auxin medium, did not inhibit rooting but affected only the development of the roots produced. The Mg2+-dependent ATPase activity of the microsomal vesicles of poplar shoots was increased after 7 h induction on rooting medium and corresponded to an increase in the Vmax of the enzyme. Results from experiments using some inhibitors of the polyamine metabolism suggested that this pathway was not involved in the increase of this activity. The auxin had no effect on the in vitro ATPase activity at any concentration tested except at about 2 mM where it was inhibitory, probably due to a change in the conformation of the enzyme. The transient increase of indole-3-acetic acid during rooting induction could be responsible for the increase in the level of the enzyme. The inhibition of root formation and growth by vanadate indicates strongly that the ATPase activity may be necessary for the induction and expression of rooting.

Jalil Bellamine

Papers

Confirmation of the role of auxin and calcium in the late phases of adventitious root formation

Proton pump and IAA sensitivity changes in spinach leaves during the flowering induction

Effect of photoperiod and gibberellic acid on the plasma membrane H+ATPase activity of spinach leaves

Characterization and identification of plasma membrane before and after flowering induction by Fourier transform infrared spectroscopy using horizontal attenuated total reflectance

The relationship between rooting of poplar shoots and ATPase activity of their crude microsomal vesicles