Involvement of root ABA and hydraulic conductivity in the control of water relations in wheat plants exposed to increased evaporative demand

We studied the possible involvement of ABA in the control of water relations under conditions of increased evaporative demand. Warming the air by 3°C increased stomatal conductance and raised transpiration rates of hydroponically grown Triticum durum plants while bringing about a temporary loss of relative water content (RWC) and immediate cessation of leaf extension. However, both RWC and extension growth recovered within 30 min although transpiration remained high. The restoration of leaf hydration and growth were enabled by increased root hydraulic conductivity after increasing the air temperature. The use of mercuric chloride (an inhibitor of water channels) to interfere with the rise on root hydraulic conductivity hindered the restoration of extension growth. Air warming increased ABA content in roots and decreased it in shoots. We propose this redistribution of ABA in favour of the roots which increased the root hydraulic conductivity sufficiently to permit rapid recovery of shoot hydration and leaf elongation rates without the involvement of stomatal closure. This proposal is based on known ability of ABA to increase hydraulic conductivity confirmed in these experiments by measuring the effect of exogenous ABA on osmotically driven flow of xylem sap from the roots. Accumulation of root ABA was mainly the outcome of increased export from the shoots. When phloem transport in air-warmed plants was inhibited by cooling the shoot base this prevented ABA enrichment of the roots and favoured an accumulation of ABA in the shoot. As a consequence, stomata closed.     

Elevated pCO(2 )favours nitrate reduction in the roots of wild-type tobacco (Nicotiana tabacum cv. Gat.) and significantly alters N-metabolism in transformants lacking functional nitrate reductase in the roots

The impact of elevated pCO(2 )on N-metabolism of hydroponically grown wild-type and transformed tobacco plants lacking root nitrate reduction was studied in order to elucidate the effects on (i) nitrate uptake, (ii) long-distance transport of N, (iii) nitrate reduction with emphasis on root-NR, and (iv) the allocation of N between the root and shoot. The findings were related to alterations of growth rates. At elevated pCO(2 )the wild type exhibited higher growth rates, which were accompanied by an increase of NO(3)(-)-uptake per plant, due to a higher root:shoot ratio. Furthermore, elevated pCO(2 )enhanced nitrate reduction in the roots of the wild type, resulting in enhanced xylem-loading of organic N (amino-N) to supply the shoot with sufficient nitrogen, and decreased phloem-transport of organic N in a basipetal direction. Transformed tobacco plants lacking root nitrate reduction were smaller than the wild type and exhibited lower growth rates. Nitrate uptake per plant was decreased in transformed plants as a consequence of an impeded root growth and, thus, a significantly decreased root:shoot ratio. Surprisingly, transformed plants showed an altered allocation of amino-N between the root and the shoot, with an increase of amino-N in the root and a substantial decrease of amino-N in the shoot. In transformed plants, xylem-loading of nitrate was increased and the roots were supplied with organic N via phloem transport. Elevated pCO(2 )increased shoot-NR, but only slightly affected the growth rates of transformed plants, whereas carbohydrates accumulated at elevated pCO(2 )as indicated by a significant increase of the C/N ratio in the leaves of transformed plants. Unexpectedly, the C/N balance and the functional equilibrium between root and shoot growth was disturbed dramatically by the loss of nitrate reduction in the root.     

Peroxidase-Induced Wilting in Transgenic Tobacco Plants

Peroxidases are a family of isoenzymes found in all higher plants. However, little is known concerning their role in growth, development, or response to stress. Plant peroxidases are heme-containing monomeric glycoproteins that utilize either H2O2 or O2 to oxidize a wide variety of molecules. To obtain more information on possible in planta functions of peroxidases, we have used a cDNA clone for the primary isoenzyme form of peroxidase to synthesize high levels of this enzyme in transgenic plants. We were able to obtain Nicotiana tabacum and N. sylvestris transformed plants with peroxidase activity that is 10-fold higher than in wild-type plants by introducing a chimeric gene composed of the cauliflower mosaic virus 35S promoter and the tobacco anionic peroxidase cDNA. The elevated peroxidase activity was a result of increased levels of two anionic peroxidases in N. tabacum, which apparently differ in post-translational modification. Transformed plants of both species have the unique phenotype of chronic severe wilting through loss of turgor in leaves, which was initiated at the time of flowering. The peroxidase-induced wilting was shown not to be an effect of diminished water uptake through the roots, decreased conductance of water through the xylem, or increased water loss through the leaf surface or stomata. Possible explanations for the loss of turgor, and the significance of these types of experiments in studying isoenzyme families, are discussed.     

Root and stem hydraulic conductivity as determinants of growth potential in grafted trees of apple (Malus pumila Mill.)

The anatomy of the graft tissue between a rootstock and its shoot (scion) can provide a mechanistic explanation of the way dwarfing Malus rootstocks reduce shoot growth. Considerable xylem tissue disorganization may result in graft tissue having a low hydraulic conductivity (k(h)), relative to the scion stem. The graft may influence the movement of substances in the xylem such as ions, water and plant-growth-regulating hormones. Measurements were made on 3-year-old apple trees with a low-pressure flow system to determine k(h) of root and scion stem sections incorporating the graft tissue. A range of rootstocks was examined, with different abilities of dwarfing; both ungrafted and grafted with the same scion shoot cultivar. The results showed that the hydraulic conductivity (k(hroot)) of roots from dwarfing rootstocks was lower compared with semi-vigorous rootstocks, at least for the size class of root measured (1.5 mm diameter). Scion hydraulic conductivity (k(hs)) was linked to leaf area and also to the rootstock on to which it was grafted, i.e. hydraulic conductivity was greater for the scion stem on the semi-vigorous rootstock. Expressing conductivities relative to xylem cross-sectional areas (k(s)) did not remove these differences suggesting that there were anatomical changes induced by the rootstock. The calculated hydraulic conductivity of the graft tissue was found to be lower for grafted trees on dwarfing rootstocks compared to invigorating rootstocks. These observations are discussed in relation to the mechanism(s) by which rootstock influences shoot growth in grafted trees.     

Shoot Apex Demand Determines Assimilate and Nutrients Partitioning and Nutrient-uptake Rate in Tobacco Plants

Our previous experiment revealed that apex-removed plants have larger root systems but a lower K＋-uptake rates than intact tobacco plants. Since the apex is not only a center of growth and metabolism, but also an important place of auxin synthesis and export, the aims of this study were to distinguish whether the apex demand or auxin synthesized in the apex regulates assimilate and nutrients partitioning within plant, and to explain the reason for the lower K＋-uptake rate of the apex-removed plant. In comparison with the control plant, covering the shoot apex with a black transparent plastic bag reduced net increases in dry matter and nutrients; however, the distribution of the dry matter and nutrients between shoot and roots and nutrient-uptake rates were not changed. Removal of the shoot apex shifted the dry mass and nutrients distributions to roots, and reduced the rate of nutrient uptake. Application of 1-naphthylacetic acid （NAA） could partly replace the role of the removed apex, stimulated assimilate and nutrient deposition into the treated tissue, and enhanced the reduced plasma membrane ATPase activity of roots to the control level. However, treatment of the apex-removed plants with NAA could not rescue the reduced nutrient uptake rate and the shifted assimilates and nutrients partitioning caused by excision of the apex. Higher nutrient uptake rate of the intact plants could not be explained by root growth parameters, such as total root surface area and number of root tips. The results from the present study indicate that strong apex demand determined assimilates and nutrients partitioning and nutrient-uptake rate in tobacco （Nicotiana tabacum） plants.     

Over-expression of a barley aquaporin increased the shoot/root ratio and raised salt sensitivity in transgenic rice plants

Barley HvPIP2;1 is a plasma membrane aquaporin and its expression was down-regulated after salt stress in barley [Katsuhara et al. (2002) Plant Cell Physiol. 43: 885]. We produced and analyzed transgenic rice plants over-expressing barley HvPIP2;1 in the present study. Over-expression of HvPIP2;1 increased (1) radial hydraulic conductivity of roots (Lp(r)) to 140%, and (2) the mass ratio of shoot to root up to 150%. In these transgenic rice plants under salt stress of 100 mM NaCl, growth reduction was greater than in non-transgenic plants. A decrease in shoot water content (from 79% to 61%) and reduction of root mass or shoot mass (both less than 40% of non-stressed plants) were observed in transgenic plants under salt stress for 2 weeks. These results indicated that over-expression of HvPIP2;1 makes rice plants sensitive to 100 mM NaCl. The possible involvement of aquaporins in salt tolerance is discussed.     

Physiological effects of peracetic acid on hydroponic tomato plants

Peracetic acid (PAA) has potential as a disinfectant of low environmental impact for glasshouse hydroponic systems and other horticultural applications, but can have phytotoxic effects. This study examined the physiological effects of PAA when applied hydroponically to tomato plants. Plants treated with 0.5–5 μg ml^?1 PAA over several weeks exhibited a reduction in size of all vegetative organs. During the first 2 h of PAA treatment, plants also exhibited a transient wilting, with increased stomatal resistance, and reductions in transpiration and CO2 assimilation. The toxicity of PAA to roots was apparent from increased leakage of root electrolytes, reduced oxygen consumption, death of root tips, and collapse of the internal tissues. The shrivelling of PAA‐treated roots resulted from loss of water to the shoot in the transpiration stream, as the effect could be eliminated by removal of the shoot and sealing of the cut stump. HgCl2, a reagent known to reduce the hydraulic conductivity of root systems, caused the same root shrivelling effects as PAA. Long‐term growth of PAA‐treated plants was dependent upon the replacement of taproot systems by adventitious roots, which, initially at least, displayed greater tolerance of PAA. In aqueous solution, PAA exists in equilibrium with H2O2 and acetic acid, both of which were individually toxic, but acetic acid exhibited a syndrome of effects distinct from those of PAA, while the effects of H2O2 paralleled those of PAA more closely, suggesting that oxidative rather than acidic mechanisms were primarily responsible for the phytotoxicity of PAA solutions.     

The effect of root cooling on hormone content, leaf conductance and root hydraulic conductivity of durum wheat seedlings (Triticum durum L.)

Root cooling of 7-day-old wheat seedlings decreased root hydraulic conductivity causing a gradual loss of relative water content during 45 min (RWC). Subsequently (in 60 min), RWC became partially restored due to a decrease in transpiration linked to lower stomatal conductivity. The decrease in stomatal conductivity cannot be attributed to ABA-induced stomatal closure, since no increase in ABA content in the leaves or in the concentration in xylem sap or delivery of ABA from roots was found. However, decreased stomatal conductance was associated with a sharp decline in the content of cytokinins in shoots that was registered shortly after the start of root cooling and linked to increases in the activity of cytokinin-oxidase. This decrease in shoot cytokinin content may have been responsible for closing stomata, since this hormone is known to maintain stomatal opening when applied to plants. In support of this, pre-treatment with synthetic cytokinin benzyladenine was found to increase transpiration of wheat seedlings with cooled roots and bring about visible loss of turgor and wilting.     

Are xylem radial development and hydraulic conductivity in downwardly-growing grapevine shoots influenced by perturbed auxin metabolism?

Downwardly-growing grapevine shoots have smaller and more frequent vessels than upwardly-growing ones and, as a consequence, a lower hydraulic conductivity. Here, grapevine (Vitis vinifera L.) shoot growth orientation was manipulated to test whether downward shoot orientation negatively affects vessel growth in the apex via a shortage of water and nutrients. The orientation of the central vine shoot portion was inverted by two consecutive 135 degrees bends, resulting in double-bent N-shaped vines; the central downward shoot portion was of different lengths in the experimental treatments to induce increasing reductions of shoot conductivity. These treatments reduced shoot conductivity and water flow, but had no effects on vessel development and frequency in the apex. In a second experiment, auxin concentration was assessed in shoots of upwardly- and downwardly-growing plants. IAA concentration at the apical internodes was higher in downwardly-oriented shoots than in shoots growing upwards. In addition, a higher density and a lower vessel diameter were observed in the lower, than the upper side, of the downwardly-oriented shoot, suggesting increased accumulation of auxin in the lower side. These results suggest that the downward orientation induces accumulation of auxin in the apex, which in turn affects the density and the size of the xylem vessels, causing reduction of hydraulic conductivity.     

Growth kinetics, polyamine pattern and biosynthesis in hairy root lines of Nicotiana tabacum

The possibility of a relation between the expression of root inducing (Ri) T-DNA genes of Agrobacterium rhizogenes and changes in polyamine metabolism has been explored in fast-growing tobacco hairy roots. Transformed root cultures have been established on hormone-fee medium; they came from transgenic plants of Nicotiana tabacum L. cv. Xanthi with different altered phenotypes, designated transformed (T) and supertransformed (T'). T and especially T' roots developed more rapidly both by elongation and lateral branching, and showed a higher growth rate than the untransformed control. After 3 weeks in culture, normal roots showed a very reduced meristematic zone, and flow cytometric analysis indicated that 2C nuclei were predominant in the apical parts in contrast to T and T' roots, in which endopolyploidisation also appeared. Putrescine, spermidine and traces of spermine were present in all the samples, both in free and in conjugated forms. Putrescine was the major polyamine detected in controls and in transformed roots. At the time of excision, the polyamine levels were similar in normal, T and T' roots. Significant differences were found during the progression of growth, particularly in the TCA-insoluble fraction in which polyamines varied differently according to the type of roots, increasing considerably in T roots on day 8, then decreasing. The lower polyamine contents found in growing transformed roots were concomitant to low arginine (EC 4.1.1.19) and ornithine (EC 4.1.1.17) decarboxylase activities. It is suggested that polyamine levels and related enzyme activities are linked to growth kinetics rather than being a consequence of foreign gene expression.     

Enhancement of Naphthaleneacetic Acid-Induced Rhizogenesis in T(L)-DNA-Transformed Brassica napus without Significant Modification of Auxin Levels and Auxin Sensitivity

Determination of the abscisic acid and indoleacetic acid (IAA) contents of floral stem segments of nontransformed and pRi A₄ T_L-DNA-transformed rape (Brassica napus L. var oleifera, cv Brutor) using a high performance liquid chromatography-enzyme-linked immunosorbent assay procedure and mass spectrometry controls showed that IAA levels were not modified. The regeneration abilities of the in vitro cultured explants were compared on media supplemented with several plant growth regulator combinations. No regeneration occurred on hormone-free media, and shoot production was similar in both genotypes when supplemented with benzyladenine. In the presence of naphthaleneacetic acid (NAA), transformed explants were characterized by faster root regeneration and reduced shoot organogenesis. The optimum for root formation was the same in nontransformed and transformed plants, but the sensitivity threshold was slightly lower in the latter. The NAA inductive period was shorter (14 versus 22 h) with transformed tissue. Root neoformation occurred about 72 h earlier on transformed explants. Our results suggest mainly that there is an acceleration of the auxinic signal transduction and/or that the events preliminary to the formation of roots occur faster in the transformed tissues than in the normal ones.     

Root-shoot Hormone Relations I. The Importance of an Aerated Root System in the Regulation of Growth Hormone Levels in the Shoot of Helianthus annuus

Using mature, vegetative, sunflower (Helianthus annuus) plants,a study was made of the interactions of flooding of the rootsystem with several other simultaneous treatments. Leaf epinastywas induced in intact plants by flooding, but this effect ofwaterlogging of the root system was removed when the shoot wasdecapitated. The addition of indole-3-acetic acid (IAA) in lanolinto the cut surface of decapitated plants restored the epinasty-inducingeffect of flooding. This is taken as evidence that floodingbrings about an increased auxin content of the shoot. In unfloodedplants decapitation of the shoot resulted in a hyponastic movementby the leaves, and applications of IAA to the stem tip antagonizedthis. Girdling of flooded plants resulted in a great reduction ofthe tendency to form adventitious roots in the region belowthe girdle. Thickening and various morphological changes wereproduced in the hypocotyl as a result of waterlogging the rootsystem. The effects of flooding could be simulated in the upperregion of the shoot of unflooded plants by girdling the morebasal region of the stem. These various results are interpreted as indicating that theroot system may normally serve as a centre for the oxidativeinactivation of excess shoot-synthesized auxin, so regulatingshoot auxin level. Certain of the results obtained, however,show that an adequately aerated root system supplies some growthpromoting,and hyponasty-inducing, principle to the shoot. The effectsof this hypothetical principle were found to be similar to thoseof gibberellic acid. It is envisaged, therefore, that an earlyeffect of flooding on plants is to raise shoot auxin levels(either by preventing entry of auxin from the stem, or by inhibitingthe oxidative catabolism of shoot auxin in the roots), and alsoto stop the synthesis of a non-auxin shoot growth-hormone inthe roots. The induction of wilting of the shoot by floodingwas found to be correlated with low root-temperatures.     

Shoot‐derived abscisic acid promotes root growth

The phytohormone abscisic acid (ABA) plays a major role in regulating root growth. Most work to date has investigated the influence of root‐sourced ABA on root growth during water stress. Here, we tested whether foliage‐derived ABA could be transported to the roots, and whether this foliage‐derived ABA had an influence on root growth under well‐watered conditions. Using both application studies of deuterium‐labelled ABA and reciprocal grafting between wild‐type and ABA‐biosynthetic mutant plants, we show that both ABA levels in the roots and root growth in representative angiosperms are controlled by ABA synthesized in the leaves rather than sourced from the roots. Foliage‐derived ABA was found to promote root growth relative to shoot growth but to inhibit the development of lateral roots. Increased root auxin (IAA) levels in plants with ABA‐deficient scions suggest that foliage‐derived ABA inhibits root growth through the root growth‐inhibitor IAA. These results highlight the physiological and morphological importance, beyond the control of stomata, of foliage‐derived ABA. The use of foliar ABA as a signal for root growth has important implications for regulating root to shoot growth under normal conditions and suggests that leaf rather than root hydration is the main signal for regulating plant responses to moisture.     

Role of auxins and cytokinins in the development of lateral roots in wheat plants with several roots removed

The removal of four of five roots of 7–8-day-old wheat plants resulted in the activation of lateral root growth and the initiation of lateral root primordia on the remained root as compared to the main root of intact plants. The extent of this growth response depended on placing cut surface above or beneath the surface of the nutrient solution. The measurement of the IAA and cytokinin contents showed accumulation of these hormones in the root of experimental plants as compared to the main root of intact plants. IAA accumulation was correlated with the number of lateral roots and their primordia. The analysis of hormonal balance and their transport from the shoot to the root permits discussing the involvement of these hormones and their interaction in the control of root growth at the stages of both primordium initiation and development and lateral root elongation.     

丛枝菌根真菌对西瓜嫁接苗生长和根系防御性酶活性的影响

在温室盆栽条件下,研究丛枝菌根(AM)真菌地表球囊霉(Glomus versiforme)对连作土壤中西瓜自根苗和嫁接苗生长、根系膜透性、丙二醛(MDA)含量和防御性酶活性的影响.结果表明： 接种AM真菌能显著增加西瓜自根苗和嫁接苗的生物量,提高根系活力,降低根系膜透性和MDA含量.接种AM真菌的自根苗地上部鲜质量、地上部干质量和根系活力分别增加了57.6%、60.0%和142.1%,而接种AM真菌的嫁接苗分别增加了26.7%、28.0%和11.0%;自根苗（C）、嫁接苗（G）、接种AM真菌自根苗（C+M）和接种AM真菌嫁接苗（G+M）的根系细胞膜透性为C>G>C+M>G+M,根系MDA含量为C>G>G+M>C+M.接种AM真菌能提高西瓜自根苗和嫁接苗根系的苯丙氨酸解氨酶(PAL)、过氧化氢酶(CAT)、过氧化物酶(POD)、几丁质酶和β 1,3 葡聚糖酶活性,而且接种AM真菌的西瓜自根苗和嫁接苗根系POD、PAL和β-1,3-葡聚糖酶活性的峰值比不接种的提前2周出现.接种AM真菌能激活西瓜自根苗和嫁接苗与抗逆性有关的防御性酶反应,使根系对逆境产生快速反应,从而提高其抗连作障碍的能力.     

Saturated humidity accelerates lateral root development in rice (Oryza sativa L.) seedlings by increasing phloem-based auxin transport

Auxin transport plays a significant role modifying plant growth and development in response to environmental signals such as light and gravity. However, the effect of humidity on auxin transport is rarely documented. It is shown here that the transport of labelled indole-3-acetic acid (IAA) from the shoot to the root is accelerated in rice (Oryza sativa L. ssp. indica cv. IR8) seedlings grown under saturated humidity (SH-seedlings) compared with plants grown under normal humidity (NH-seedlings). The development of lateral roots in SH-seedlings was greatly enhanced compared with NH-seedlings. Removal of the shoot from SH-seedlings reduced the density of lateral roots, and the application of IAA to the cut stem restored the lateral root density, while the decapitation of NH-seedlings did not alter lateral root development. Phloem-based auxin transport appeared responsible for enhanced lateral root formation in SH-seedlings since (i) the rate of IAA transport from the shoot to the root tip was greater than 3.5 cm h-1 and (ii) naphthylphthalamic acid (NPA)-induced reduction of polar auxin transport in the shoot did not influence the number of lateral roots in SH-seedlings. It is proposed that high humidity conditions accelerate the phloem-based transport of IAA from the leaf to the root, resulting in an increase in the number of lateral roots.     

丛枝菌根真菌对西瓜嫁接苗生长和根系防御性酶活性的影响

在温室盆栽条件下,研究丛枝菌根(AM)真菌地表球囊霉(Glomus versiforme)对连作土壤中西瓜自根苗和嫁接苗生长、根系膜透性、丙二醛(MDA)含量和防御性酶活性的影响.结果表明： 接种AM真菌能显著增加西瓜自根苗和嫁接苗的生物量,提高根系活力,降低根系膜透性和MDA含量.接种AM真菌的自根苗地上部鲜质量、地上部干质量和根系活力分别增加了57.6%、60.0%和142.1%,而接种AM真菌的嫁接苗分别增加了26.7%、28.0%和11.0%;自根苗（C）、嫁接苗（G）、接种AM真菌自根苗（C+M）和接种AM真菌嫁接苗（G+M）的根系细胞膜透性为C>G>C+M>G+M,根系MDA含量为C>G>G+M>C+M.接种AM真菌能提高西瓜自根苗和嫁接苗根系的苯丙氨酸解氨酶(PAL)、过氧化氢酶(CAT)、过氧化物酶(POD)、几丁质酶和β 1,3 葡聚糖酶活性,而且接种AM真菌的西瓜自根苗和嫁接苗根系POD、PAL和β-1,3-葡聚糖酶活性的峰值比不接种的提前2周出现.接种AM真菌能激活西瓜自根苗和嫁接苗与抗逆性有关的防御性酶反应,使根系对逆境产生快速反应,从而提高其抗连作障碍的能力.