Seasonal Changes in Auxin Effects on Rooting of Stem Cuttings of Populus nigra and its Relationship with Mobilization of Starch

Stem cuttings of Populus nigra were treated with 10 and 100 mg/1 each of IAA., IBA, 2,4-D and NAA at one month intervals and observations were recorded for the morphophysiological status of the branches, their starch content and their rooting response. — The first phase characterized by delayed, short and scarce roots and the high starch content of cuttings coincided with the onset of winter dormancy in November lasting through February. It was followed by a phase of vigorous rooting and low starch content of cuttings coinciding with the renovation of growth activity in February lasting through October, except in April and May when rooting was more or less completely nullified. — The poor rooting in winter was caused by low activity of hydrolyzing enzymes not mobilizing starch into soluble sugars; and profuse rooting during active growth period by high activity of hydrolyzing enzymes caused by endogenous auxin, resulting in mobilization of reserved food materials necessary for the initiation and development of roots. The low rooting in April and May is ascribed to the fact that bulk of the mobilized food was used up in the growth of sprouted branches leaving very little for rooting when these cuttings were planted. — The seasonal changes in the effectiveness of exogenously applied auxins also appear to be related with the level of endogenous auxin. In June endogenous auxin was high due to high meristematic activity, the exogenously applied auxins raising it to supra-optimal levels that were inhibitory. On the other hand, in October exogenously applied auxins enhanced rooting by raising it to an optimal level as the production of endogenous auxin had been decreasing gradually due to lowering temperatures. — The results demonstrate that auxin effect on differential rooting with season in this plant is determined by the physio-morphological status of the branches that govern the production of endogenous auxin and is mediated primarily through its effect on mobilization of reserve food materials caused by enhanced activity of hydrolytic enzymes.     

Ribonuclease Activity and Auxin Effects in the Lens Root

In Lens root tips, a direct proportionality between RNA and auxin levels and inverse proportionality between RNA content and RNase activity were found. IAA treatment of the Lens seedlings causes, in the root, both an increase of RNA and auxin content and a decrease of RNase activity. Addition of IAA to the excised roots produces an inhibition of both the decrease of the RNA levels and an inhibition of the increase of the RNase activity. The action of IAA on growth, related to the control of the RNA metabolism is discussed.     

Studies on Auxin Protectors XII. Auxin Protectors and Polyphenol Oxidase in Developing Coffee Fruit

The fruit of the coffee plant (Coffea arabica) was analyzed for auxin protector content. Ripe coffee berries were separated into pit and pulp, ground in buffer, and assayed for auxin protectors. The extracts were then subjected to gel filtration in order to determine the molecular weight of the protector(s). In the pit, a single protector was found with a molecular weight approaching 5000 daltons, while the pulp contained several auxin protectors, the largest of which appeared to be about 1000 daltons. Chromato-graphic studies of various gel filtration fractions showed that protector activity was always associated with spots which exhibited a light blue fluorescence under UV. The changing patterns during coffee fruit development were also investigated. Auxin protector production, and polyphenol oxidase (E.C. 1.10.3.1), an enzyme related to protector metabolism, were assayed at weekly intervals. In the unripe berry, an auxin protector was found with a molecular weight exceeding 200,000 daltons; as the berry ripened the amount of this protector gradually decreased until almost none was present in the ripe berry and the pattern changed to the pattern described above. Polyphenol oxidase content decreased as the berry ripened. Commercially roasted pits, i.e., coffee “beans”, contained very high levels of protector activity. However, gel filtration studies showed this activity to be associated entirely with low molecular weight compounds.     

Auxin autonomy in cultured tobacco teratoma tissues transformed by an auxin-mutant strain ofAgrobacterium tumefaciens

We have studied the mechanism of auxin autonomy in tobacco (Nicotiana tabacum L.) crowngall tissues transformed by the auxin-mutant (tms ^–) A66 strain of Agrobacterium tumefaciens. Normally, tms ^– tobacco tumor tissues require the formation of shoots to exhibit auxin-independent growth in culture. We have isolated from tms ^– tobacco cells several stable variants that are fully hormone-independent and grow rapidly as friable, unorganized tissues, thus mimicking the growth and morphology of tms ⁺ tobacco cells that produce high levels of auxin. However, none of the variants contained the high levels of auxin found in tms ⁺ tumor cells. The variants could be divided into two classes with respect to their response to applied auxin. The first class was highly sensitive to applied auxin: low concentrations (1 M) of -naphthaleneacetic acid (NAA) severely inhibited growth and markedly stimulated the accumulation of the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC). The second class of variants showed a low sensitivity to applied auxin: growth was promoted by concentrations of NAA up to 10 M, and growth inhibition and high ACC levels were observed only at high NAA concentrations (100 M). Unorganized variants with low auxin sensitivity were also isolated from a variant line with high auxin sensitivity. The isolation of tumor cells that exhibited the growth phenotype of tms ⁺ cells while retaining the low auxin content and low auxin sensitivity of tms ^– cells indicates that full hormone autonomy, characteristic of wild-type crown-gall tumors, can be achieved by a mechanism that is independent of changes in the auxin physiology of the cells.Abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - IAA indole-3-acetic acid - MACC N-malonyl ACC - NAA naphthaleneacetic acid - tms tumor morphology shooty, the auxin biosynthesis locus of Agrobacterium Ti plasmids The authors thank Dr. Andrew Binns (University of Pennsylvania, Philadelphia, USA) for providing cell lines TA6-5 and TA66C3-78, and Mr. James Dacey for preparation of the composite photograph used in Fig. 1. Support for this work by the National Science Foundation (DMB84-17087) and the U.S. Department of Agriculture (86-CRCR-1-2150) is gratefully acknowledged.     

Seasonal Changes in the Effects of Auxin on Rooting in Stem Cuttings of Ficus infectoria

Ficus infectoria stem cuttings were treated with 10 and 100 μg/ml each of IAA, IBA, 2,4, -D and NAA at monthly intervals and planted to study their rooting response after recording morphophysiological status and cambial activty of the parent branches. Attempts were also made to surgically expose the cambium before auxin treatment to determine the relationship of seasonal variation in auxin effectivity to cambial activity. The results show that: (1) there are two distinct phases in the sensitivity of Ficus infectoria stem cuttings to auxin-induced rooting; (2) the high rooting phase coincides with renovation of growth and high cambial activity starting in March and lasting through August and the low rooting phase coincides with winter dormancy and low cambial activity; (3) roots emerge in longitudinal rows in slitted auxin-treated cuttings; (4) slitted auxin-treated cuttings root profusely in June when cambial activity is high but not in October when cambial activity is low suggesting a close correspondence of seasonal variation between the rooting activity of auxin and cambial activity.     

Auxin biosynthetic gene TAR2 is involved in low nitrogen‐mediated reprogramming of root architecture in Arabidopsis

In plants, the plasticity of root architecture in response to nitrogen availability largely determines nitrogen acquisition efficiency. One poorly understood root growth response to low nitrogen availability is an observed increase in the number and length of lateral roots (LRs). Here, we show that low nitrogen‐induced Arabidopsis LR growth depends on the function of the auxin biosynthesis gene TAR2 (tryptophan aminotransferase related 2). TAR2 was expressed in the pericycle and the vasculature of the mature root zone near the root tip, and was induced under low nitrogen conditions. In wild type plants, low nitrogen stimulated auxin accumulation in the non‐emerged LR primordia with more than three cell layers and LR emergence. Conversely, these low nitrogen‐mediated auxin accumulation and root growth responses were impaired in the tar2‐c null mutant. Overexpression of TAR2 increased LR numbers under both high and low nitrogen conditions. Our results suggested that TAR2 is required for reprogramming root architecture in response to low nitrogen conditions. This finding suggests a new strategy for improving nitrogen use efficiency through the engineering of TAR2 expression in roots.     

Auxin level and regeneration of Begonia leaves

Summary As previously found, both the level of ether-extractable auxin (presumably indole-3-acetic acid) and the root-forming ability of B.xcheimantha leaves are increased under long-day conditions by high temperature, whereas the capacity for adventitious bud formation is reduced. However, this relation is present under relatively high light intensity only. Under the low light intensities in late fall neither auxin level nor regeneration ability were significantly affected by temperature.Dark treatment of detached leaves for 2 to 16 days greatly counteracted the inhibitory effect of high temperature on bud formation and reduced both the auxin level and the root-forming ability of the leaves.The great seasonal changes in the regeneration ability of Begonia leaves seem to be the result of a complex interaction of temperature, day-length, and daily light energy on the level of endogenous auxin and other growth regulators.     

Auxin Stimulation of Cambial Activity in Pinus silvestris II. Dependence upon Basipetal Transport

Natural auxin content has been determined in the cambial region of large Pinus silvestris L. trees at various dates during the year. The tissue was collected from the stem of intact or ring-barked trees and from stumps remaining after the trees were cut down at breast height in early summer or late autumn. No seasonal decrease of concentration of the extractable auxin in the cambial region could be detected. Decapitation or ring-barking produced severe reduction in auxin content and arrested cambial division. In the next season the auxin level and the cambial activity remained completely depressed. It is concluded that without tissue continuity in the region external to xylem and without basipetal supply of substances, no mechanism operated by roots or remaining stem tissue near the tree base can ensure a high level of auxin in the cambial region or activate and maintain the cambial division. The activity of extracted pine auxin was found not to be identical with the stimulatory potential of authentic IAA determined by standard bioassays. The possibility of interaction with other extracted substances is discussed.     

The Relationship between Flower Abortion and Endogenous Auxin Content of Rose Shoots

The amount of endogenous growth substances in stem, flowers and leaves of rose plants grown under different temperature and light conditions has been determined. It appeared to be two main growth promoting factors in the acidic fraction of the ether extract. One of them is assumed to be an auxin, probably indol-3yl-acetic acid (IAA); the other is not identified. The level of auxin was much higher in extracts from shoots grown at high temperature than in shoots grown at low temperature. Increasing light intensity also seemed to increase the auxin content of the shoots. Shoots which developed after a high cut back of the rose stem had a higher auxin content than shoots which developed after a low cut back. These findings are discussed in relation to the effect of temperature, light intensity and cut back practise on blind shoot formation in roses. The result of these investigations strongly indicate that abortion in roses is promoted by a low auxin level in the shoots.     

The Interrelation of RNA, Auxin and Auxin-oxidases in Lentil Roots

The correlation between auxin and RNA metabolism was investigated in lentil roots. IAA and NAA both cause a considerable rise in the RNA level of germinating lentil roots, though no effect of IAA was found on the DNA level. In untreated germinating roots various sections were isolated and a direct relation found between RNA and auxin content, and an indirect relation between RNA content and auxin oxidase activity. In excised roots, incubated for 24 hours, the loss of RNA is paralleled by a loss of endogenous auxin. Excised roots treated with 10^?4M IAA or M 10^?4 NAA loose little RNA. The findings suggest that in lentil roots the RNA levels may be controlled by auxin levels, which in turn may be controlled by the levels of auxin oxidase.     

Auxin metabolism

Auxin metabolism encompasses transport, conjugation, deconjugation, conversion, and catabolism. The balance between auxin metabolism and biosynthesis determines the actual level of the hormone in a given cell and consequently plays an important role in many developmental processes from seed germination to fruit ripening. Mass spectrometry used in conjunction with stable isotope labeling studies has enabled comprehensive examination of auxin biosynthesis and turnover along with the identification of many auxin conjugate. It appears that the conjugate moiety may signal the metabolic fate (e.g. storage and eventual hydrolysis to free hormone, or catabolism). Recently identified auxin-metabolizing enzymes are encoded by gene families which vary in specificity for auxin metabolites. The expression patterns of these genes will reveal a great deal about the mechanics of auxin metabolism.     

Protein Turnover in Response to Transient Exposure to Exogenous Auxin is Necessary for Restoring Auxin Autotrophy in a Stressed Arachis hypogea Cell Culture

An auxin autotrophic Arachis hypogea cell culture was sensitive to stress treatments leading to water loss whereas the growth of its auxin-supplemented counterpart was unaffected under similar conditions. Here we show that an hour of transient auxin treatment in the post stress period was sufficient for restoring the auxin autotrophic growth potential of the stress driven quiescent Arachis cells. Qualitative proteome analysis revealed protein turnover to have a role in mediating auxin-originated signals in these cells. In consonance, MG132 a cell permeable inhibitor of the ubiquitin mediated protein turnover completely inhibited the auxin dependent growth restoration of the stressed Arachis cells. Thus protein turnover is a necessary downstream event in exogenous auxin mediated stress tolerance in Arachis cells.     

Auxin Relations of the Woody Shoot: The Distribution of Diffusible Auxin in Shoots of Apple and Plum Rootstock Varieties

Surveys have been made of diffusible auxin in the stem tissuesof growing shoots of apple and plum rootstock varieties. Usingagar plates as carriers auxin was collected from the lower surfaceof isolated stem sections and assayed by the Avena curvaturemethod. The stool and layer shoots studied grew for severalmonths producing many leaves and reaching considerable lengths.The data provide information on selected internodes and showthe auxin status of the shoot at various times during growth,and the auxin gradients down the stem at these various times.Free auxin content of the shoot apex was consistently less thanthat of the internodes below. In 1946 auxin content declinedthroughout growth with a steady positioning of the auxin peakin the upper shoot. In 1947, following a period of drought,when growth almost ceased, a secondary auxin peak occurred positionedin lower internodes distant from the apex. This seasonal contrastwas reflected in the auxin relations of the individual internode,and was observed both in apple and plum. The nature of the auxindecline below the peak region, and the total disappearance offree auxin from the shoot as growth subsides, is discussed.The reappearance of free auxin in mature internodes, which hasnot been transmitted from the stem apex, implies either derivationfrom a stored state or the ability of the internode to produceits own auxin.     

Auxin Conjugation by Tobacco Mesophyll Protoplasts : Correlations between Auxin Cytotoxicity under Low Density Growth Conditions and Induction of Conjugation Processes at High Density

Auxin induction of the proliferation of Nicotiana tabacum (cv Xanthi) mesophyll protoplasts and of protoplast-derived cells was studied. The growth-promoting properties and cytotoxicities at high concentrations of IAA and naphthaleneacetic acid were strongly affected by cell density. The induction of growth by 2,4-dichlorophenoxyacetic acid and picloram was not affected by cell density. The comparison of catabolism of these [¹⁴C]-labeled auxins by protoplasts showed that IAA and naphthalene-acetic acid were rapidly accumulated and conjugated unlike 2,4-dichlorophenoxyacetic acid and picloram. The major catabolite derived from naphthaleneacetic acid was identified as naphthaleneacetyl-l-aspartate. The biosynthesis of this conjugate in protoplasts was inducible by naphthaleneacetic acid concentrations found to be cytotoxic under low density growth conditions. However, although it was taken up by cells, the conjugate was not cytotoxic at concentrations as high as 0.2 mm under low density growth conditions. The relationship between conjugation processes and auxin cytotoxicity is discussed.     

Auxin Effects on the Aggregation and Heat Coagulability of Cytoplasmic Proteins and Lipoproteins

The kinetics of induction of heat stability of cytoplasmic proteins and lipoproteins by auxin (2,4,-D) were determined for basal sections of soybean hypocotyl. Maximum heat stabilization occurred after 4 h of tissue incubation with 10^-5M 2,4-D. The effect was less pronounced or absent with longer incubations. Membrane fractions sedimenting between 10,000 and 100,000 g and proteins of the 100,000 g supernatant were most affected. The auxin-induced protein aggregation response varied among experiments. With many tissue lots, the response was small or absent even though the tissue responded to the auxin uniformly by increased growth. The magnitude of response was proportional to the logarithm of auxin concentration but with low 2,4-D the portion of the homogenate protein coagulated by heat was increased and with supraoptimal concentrations it was decreased relative to the control. The smallest auxin-induced change in heat coagulability was observed at the auxin concentration nearest the optimum for growth. No direct correlation was found between the auxin-induced protein and lipo-protein aggregation phenomenon and total protein, chloroform-extractable lipid, residual lipid, growth or tissue deformability. Total sulfhydryl equivalent of the homogenates, however, did correlate with auxin effects on aggregation. This result, plus experiments where homogenates were exposed to oxidizing or reducing conditions, suggests that heat stabilization and associated protein aggregation phenomena are related to conversion of protein sulfhydryl to intramolecular disulfide bonds. No significance is attached to heat stabilization of cytoplasmic proteins as a requisite of auxin-induced growth.     

Role of the Arabidopsis PIN6 Auxin Transporter in Auxin Homeostasis and Auxin-Mediated Development

Plant-specific PIN-formed (PIN) efflux transporters for the plant hormone auxin are required for tissue-specific directional auxin transport and cellular auxin homeostasis. The Arabidopsis PIN protein family has been shown to play important roles in developmental processes such as embryogenesis, organogenesis, vascular tissue differentiation, root meristem patterning and tropic growth. Here we analyzed roles of the less characterised Arabidopsis PIN6 auxin transporter. PIN6 is auxin-inducible and is expressed during multiple auxin–regulated developmental processes. Loss of pin6 function interfered with primary root growth and lateral root development. Misexpression of PIN6 affected auxin transport and interfered with auxin homeostasis in other growth processes such as shoot apical dominance, lateral root primordia development, adventitious root formation, root hair outgrowth and root waving. These changes in auxin-regulated growth correlated with a reduction in total auxin transport as well as with an altered activity of DR5-GUS auxin response reporter. Overall, the data indicate that PIN6 regulates auxin homeostasis during plant development.     

生长素调控植物重力反应的分子机理研究

重力反应是植物对环境的一种适应现象。生长素参与植物环境适应与发育调控的过程,重力反应过程的核心之一是在重力反应器官形成生长素的浓度梯度,诱导下游基因的差异表达。生长素的合成、代谢、极性运输及信号转导在此过程中发挥了关键作用。该文以拟南芥和水稻的研究为基础,综述了近几年对生长素调控植株重力反应的分子机理的研究进展,并对该领域未来的研究进行展望。     

Endogenous Auxin is Required but Supraoptimal for Rapid Growth of Rice (Oryza sativa L.) Seminal Roots, and Auxin Inhibition of Rice Seminal Root Growth is Not Caused by Ethylene

The dual effects of auxin and ethylene on rice seminal root growth were investigated in this study. Low concentrations of exogenous indole-3-acetic acid (IAA) had no effect on rice seminal root growth, whereas higher concentrations (≥0.003 μM) were inhibitory. In contrast, low concentrations of the auxin action inhibitor p-chlorophenoxyisobutyric acid (PCIB), ranging from 0.5 to 50 μM, promoted rice seminal root growth, whereas high concentrations of PCIB (≥500 μM) and the polar auxin transport inhibitor 2,3,5-triiodobenzoic acid (TIBA) inhibited rice seminal root growth. These results suggest that endogenous auxin is required but supraoptimal for rapid growth of rice seminal roots. In addition, although rice seminal root growth was inhibited by the exogenous ethylene-releasing compound ethephon or the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) as well as exogenous IAA, the 50% inhibition of growth (I₅₀) caused by ethephon or ACC was weakened by certain concentrations of the ethylene action inhibitor Ag⁺ (0.016-0.4 μM). However, the I₅₀ caused by exogenous IAA was strengthened by Ag⁺ or the ethylene biosynthetic inhibitor aminoethoxyvinylglycine (AVG) and weakened by certain concentrations of PCIB (0.5-50 μM). Together, the inhibitory mechanisms of auxin and ethylene on rice seminal root growth should be different, and auxin inhibition of rice seminal root growth should not be caused by ethylene. Furthermore, our results indicated that a certain threshold level of ethylene was required to maintain rice seminal root growth, and that ethylene within the threshold may antagonize auxin inhibition of rice seminal root growth.     

Croissance,teneur en auxine et catabolisme auxinique chez Rhizobium

Summary Rhizobium is cultivated on a synthetic medium in presence of tryptophan. The exponential phase in the bacterial growth is over after three days. During the stationary phase, the IAA content of the medium continues to increase regularly after four days. The auxin degradation by cell-free preparations of 4, 8, 12-day old cultures decreases with time.These correlations between auxin catabolism, IAA content and bacterial growth are compared with those described in tissues of higher plants.