Progress 07/01/99 to 06/30/04
Outputs
The goal of this project was to examine the role of ethylene in plant responses to edaphic stress. We examined a variety of root responses to low phosphorus availability and investigated the role of ethylene in each of these. In the model plant Arabidopsis, low phosphorus increases root hair density and length. Using mutants with altered ethylene responsiveness and chemical inhibition or promotion of ethylene action, we found that ethylene action is required for low phosphorus enhancement of root hair length. Ethylene does not participate in alterations of root anatomy leading to increased numbers of potential root-hair forming cells, but does contribute to increasing root hair density by increasing the proportion of such cells that actually form hairs and by reducing their length. Ethylene is similarly involved in maintaining primary root elongation in the presence of low phosphorus, which would otherwise greatly inhibit root growth. In common bean (Phaseolus
vulgaris), ethylene treatment increases the shallowness of basal roots, and ethylene inhibitors result in deeper roots. There is a significant interaction with phosphorus, indicating that ethylene sensitivity may be increased by low phosphorus, allowing shallower root growth. Likewise, genotypes with shallower roots are more ethylene sensitive, indicating that the regulation of root angle may be via ethylene sensitivity. We also noted genetic variation in the number of whorls giving rise to basal roots, which determines the number of potential basal roots an individual plant could form. Roots forming from the whorl closest to the shoot showed the largest variation in angle with genotype, phosphorus and ethylene treatment, and therefore provide potential for increasing topsoil exploration. We used ethylene-insensitive mutants of petunia and tomato to study the role of ethylene in whole-plant responses to low phosphorus stress. In tomato, we found that ethylene was important for the
formation of adventitious roots in response to low phosphorus. In petunia, we found that ethylene was important for the recovery from transplant stress, especially with low phosphorus. Petunias lacking ethylene sensitivity were highly susceptible to black root rot (Theilaviopsis basicola). We studied the ethylene production and sensitivity of Regal pelargonium (Pelargonium X domesticum) flowers from genotypes with varying postharvest longevity. Genotypes developed by Richard Craig at Penn State had slightly reduced ethylene production but greatly reduced ethylene sensitivity. Ethylene sensitivity was highly correlated with floret longevity. Whole plant postharvest performance was related to floret longevity, but also to other characteristics such as the total number of buds formed, which was also very high in the Penn State genotypes. Treatment with 1-methylcyclopropene (MCP) reduced abscission of petals from older florets during shipping, but failed to protect younger florets, and
had no lasting impact on postharvest performance of the plants.
Impacts
Plants have several adaptations that improve their productivity in soils with low phosphorus availability. Some of these adaptations are plastic, meaning the characteristic changes when conditions warrant it. One important adaptation to low phosphorus is the increase in root development in the topsoil, where phosphorus availability tends to be higher. Shallower basal root growth angle and increased development of adventitious roots are two genetically controlled traits that improve topsoil exploration. These traits are also plastic, i.e. they increase under low phosphorus availability, and plasticity itself is genetically controlled. Increased root hair density and length also contributes to phosphorus acquisition, and this trait is also genetically controlled and plastic. Genetic determinants of ethylene sensitivity may play a part in regulation of these traits, particularly of their plasticity. Undoubtedly other genetic determinants exist that are independent of
ethylene synthesis and action, including root growth angles under sufficient phosphorus availability and the number of basal root whorls. Identification of traits important for improving performance under low phosphorus conditions and their genetic determinants will help plant breeders develop crops with improved performance in low-input systems. Genetically enhanced Regal pelargoniums were superior to plants treated with MCP to inhibit ethylene action, since they demonstrated reduced ethylene response throughout their lifetime, and developed more flowers to provide an outstanding postproduction display.
Publications
- He, Z. X., Ma, Z., Brown, K. M. and Lynch, J. P. 2004. The assessment of inequality of root hair density in Arabidopsis thaliana using the Gini coefficient: a close look at the effect of phosphorus and its interaction with ethylene. Annals of Botany, Accepted for Publication.
- Kim, H. J. 2004. Ethylene involvement in stress responses of horticultural crops. Ph.D. Thesis, The Pennsylvania State University, University Park, PA. 139 pp.
- Kim, H. J., Holcomb, E. J. and Brown, K. M. 2004. Lanthanum effects on gravitropic response of cut tulip flowers. Acta Horticulturae, Accepted for Publication.
- Kim, H. J., Craig, R. and Brown, K. M. 2004. Genetically enhanced postproduction quality of Regal pelargonium. Acta Horticulturae, Accepted for Publication.
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Progress 01/01/03 to 12/31/03
Outputs
The hypothesis that ethylene participates in the regulation of root hair development by phosphorus availability in Arabidopsis thaliana was tested by chemically manipulating ethylene synthesis and response and with ethylene-insensitive mutants. Low phosphorus-induced root hair development could be mimicked by adding the ethylene precursor, 1-aminocyclopropane-1-carboxylate (ACC), to high phosphorus media, and inhibited by adding ethylene inhibitors to low phosphorus media. Ethylene insensitive mutants showed a reduced response to low phosphorus, indicating ethylene involvement in root hair responses to phosphorus deficiency. To dissect the nature of this involvement, we investigated the morphological and anatomical changes associated with increased root hair density. Growth in low phosphorus resulted in smaller, more numerous cortical cells, resulting in a larger number of root hair-bearing epidermal cell files. Cortical cell number was not affected by ethylene
inhibitors, ACC, or mutations reducing ethylene sensitivity in roots grown with low phosphorus, indicating that ethylene does not participate in this response. The exception was the eir1 mutation, which strongly reduced this change in radial anatomy, supporting a role for polar auxin transport in this process. Trichoblast cell length was reduced by low phosphorus availability in all genotypes, but even more so for ein2-1 and ein4. The proportion of epidermal cells forming hairs and root hair length were reduced in ethylene-insensitive mutants, especially in the presence of low phosphorus. These results demonstrate multiple effects of low phosphorus from the earliest stages of root hair development, and cross-talk between ethylene and phosphorus in control of a subset of the low phosphorus effects, concentrating on those later in development. In a separate project, we investigated the importance of ethylene for responses to low phosphorus stress using genetic ethylene-insensitivity
conferred by the Nr mutant of tomato and petunia containing the dominant ethylene insensitivity gene from Arabidopsis, etr1. Reduced ethylene sensitivity in petunia had significant effects on shoot weight, root weight, and total biomass at 4 weeks after transplanting, but there was no difference between the genotypes at 7 weeks. Low phosphorus reduced growth similarly in both genotypes. Adventitious root formation in tomato plants was increased by low P in wild-type, but not in Nr. The results suggest that ethylene perception plays an important role in carbon reallocation to root development. In tomato, Fv/Fm, an indicator of photo-oxidative stress, was affected by phosphorus level but not by ethylene-insensitivity. Fv/Fm of etr1-1petunia, which is more ethylene insensitive, decreased 2 weeks after transplanting, and recovered to the level of the wild-type by 5 weeks. Shoot dry weight of the wild-type was greater than that of etr1-1 petunia at 4 weeks after transplanting, but not
different at 7 weeks, indicating that the extent of ethylene sensitivity is important for plant recovery from transplant stress.
Impacts
The ability of plants to adapt to stress is crucial for agricultural productivity. We have investigated the role of ethylene in plant responses to low phosphorus stress. Ethylene is often referred to as a stress hormone, but it also plays a role in many aspects of normal development and senescence. Our work with root hair development in Arabidopsis shows that ethylene is important for increased root hair density and root hair length under low phosphorus stress, but that it does not participate in all aspects of these responses. Increased length and density of root hairs greatly increases the volume of soil that can be exploited by a root for phosphorus acquisition, and therefore improves productivity when phosphorus is limiting. In petunia, we used a transgenic plant with a gene conferring ethylene insensitivity to show that ethylene is important for the ability of the plant to recover from transplant stress. This was less obvious in a tomato mutant with a weaker form
of ethylene insensitivity. Transplant stress is a significant barrier to plant establishment for vegetable crops like tomatoes and peppers, many floriculture crops, and woody nursery crops, all of which are transplanted at least once in their production cycles. Ethylene is also important for adventitious rooting, which is crucial for both cutting production and for continued development of the root system when seedling roots are lost.
Publications
- Zhang, Y.J., Lynch, J.P. and Brown, K.M. 2003. Ethylene and phosphorus availability have interacting yet distinct effects on root hair development. Journal of Experimental Botany 54:2351-2361.
- Fan, M., Zhu, J., Richards, C., Brown, K.M. and Lynch, J.P. 2003. Physiological roles for aerenchyma in phosphorus stressed roots. Functional Plant Biology 30:493-506.
- Ma, Z., Baskin, T.I., Brown, K.M. and Lynch, J.P. 2003. Regulation of root elongation under phosphorus stress involves changes in ethylene responsiveness. Plant Physiology 131:1381-1390.
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Progress 01/01/02 to 12/31/02
Outputs
We characterized the growth of the primary root of Arabidopsis thaliana under phosphorus sufficiency (1 mM phosphate) and deficiency (1 microM phosphate), focusing on the role of ethylene. We quantified the spatial profile of relative elongation with a novel method based on image processing, as well as the production rates of cortical cells, trichoblasts, and atrichoblasts. Phosphorus deficiency moderately decreased the maximal rate of relative elongation, shortened the growth zone, and decreased the production rate of both epidermal cell types but not of cortical cells. Inhibiting ethylene production (with aminoethoxyvinyglycine, AVG) or action (with 1-methylcyclopropene, MCP) increased elongation in high phosphorus and decreased it in low phosphorus. That these effects were specific to ethylene was confirmed by negating the effect of inhibited ethylene production with simultaneous treatment with an ethylene precursor (1-aminocyclopropane-1-carboxylic acid, ACC).
Under both phosphorus regimes, ethylene regulated the maximal rate of relative elongation, rather than the size of the growth zone. Additionally, inhibiting ethylene action in high versus low phosphorus elicited opposite responses for the position of root hair initiation and for the production rates of cortex cells and atrichoblasts. We conclude that the root system acclimates to phosphorus deficiency by changing the signal transduction pathway connecting ethylene levels to growth and division.
Impacts
This research defines the role of ethylene on root elongation under low phosphorus conditions. Low phosphorus is a major limitation to crop productivity world-wide, especially in the tropics where food security is an important issue. Low phosphorus reduces overall plant growth, but some plants are able to adjust their growth habit in ways that improve their productivity in low phosphorus soils. It is particularly important for root growth to continue so that roots may encounter and exploit additional sources of phosphorus to support plant growth. This work demonstrates the importance of ethylene for root elongation under low phosphorus conditions and defines the components of growth affected.
Publications
- Zhang, Y. J. 2002. Ethylene and phosphorus responses in plants. Ph.D. Thesis. The Pennsylvania State University, University Park, PA. 126 pp.
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Progress 01/01/01 to 12/31/01
Outputs
Formation of aerenchyma, or cortical gas spaces in plant roots, is well known to occur during flooding or waterlogging of soils. The presence of these gas spaces enhances diffusion of oxygen and other gases so that roots can continue to grow under these conditions. Aerenchyma also forms when roots are subject to nutrient deficiency. In bean (Phaseolus vulgaris) and maize (Zea mays), low phosphorus (P) induced the formation of aerenchyma beginning in the oldest part of the root. Aerenchyma forms in the maize primary root after 3-4 days regardless of P treatment, and in seminal roots (non-primary main roots) after 10-12 days of exposure to low P. Aerenchyma forms starting in the oldest part of the root and progresses toward the root tip, but does not form in the growing region of the root. In bean, aerenchyma forms in the basal roots after several weeks of growth with low P. Aerenchyma forms by collapse of cortical cells in the root in a regular radial pattern, but
connecting tissues remain in a spoke-like pattern even after aerenchyma formation is extensive. Aerenchyma fails to form in the region of the cortex where lateral roots emerge. Treatment of roots with ethylene or its precursor induces aerenchyma formation even in the absence of P deficiency, and use of ethylene synthesis or action inhibitors prevents aerenchyma formation in response to low P. Aerenchyma formation requires biochemical alterations in the cell walls of specific cells which are sensitive to the inducing signals. We tested the hypothesis that expansins might be associated with the formation of aerenchyma in maize, since expansins have been shown to be involved in several types of cell wall degradation events. We have identified two members of the beta-expansin gene family whose expression is correlated with the development of aerenchyma in the maize primary root. Expression of these genes could serve as a molecular marker for the development of aerenchyma, since their
expression increases before aerenchyma becomes visible by microscopy. When aerenchyma forms in response to P deficiency rather than waterlogging, it is less obvious what function it might serve in the plant. We tested the hypothesis that aerenchyma reduces the phosphorus and carbon requirements of the root, freeing these resources for production of new plant tissue. Respiration of maize roots was inversely correlated with the extent of aerenchyma when aerenchyma was varied by P treatment, genotype, and manipulation of ethylene. Aerenchyma formation also significantly reduced the P content of the root, supporting the hypothesis that aerenchyma formation reduces the respiratory and P costs of the root in which it forms.
Impacts
Plants undergo a variety of changes when exposed to stress, some of which may assist its adaptation, i.e. survival and reproduction, and some which may just be negative effects of the stress. We have identified a number of root responses to phosphorus deficiency which may help the plant to prosper under low phosphorus conditions. One such response is formation of aerenchyma, which are gas-filled cavities in the roots, stems or leaves of plants. We have shown that maize plants which form aerenchyma in their seminal roots spend less carbon (in respiration) and phosphorus on the aerenchyma filled roots than they would without aerenchyma. Thus, carbon, which is fixed as sugar by photosynthesis of the shoot, and phosphorus, which is scarce and must be acquired by the roots, can be reallocated for other forms of growth which may assist plant survival when phosphorus is deficient. If these physiological data can be confirmed by improved agronomic performance of plants which
form aerenchyma, then this would be a trait which plant breeders may chose to select when breeding phosphorus-efficient plants. Phosphorus efficient plants are important for crops in developing countries, where low phosphorus availability frequently limits yield, and in developed countries, where overuse of fertilizer causes pollution of ground and surface waters.
Publications
- Umantha, S. K. Nuwan. 2001. Expression of expansins in relation to the development of aerenchyma in the primary root of Zea mays. M.S. Thesis. The Pennsylvania State University, University Park, PA. 63 pp.
- Ma, Z., Bielenberg, D., Brown, K. M. and Lynch, J. P. 2001. Effect of phosphorus availability on root hair density in Arabidopsis thaliana. Plant Cell Environ. 24:459-467.
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Progress 01/01/00 to 12/31/00
Outputs
Plants respond to stress conditions in the soil by a variety of mechanisms, including changes in root development. When phosphorus (P) availability is limiting, one important adaptation is continued development of roots in the upper soil horizons, where P availability is typically higher than in the subsoil. Bean plants can do this in several ways. One is to have shallower basal roots, which can grow at different angles with respect to gravity. The genotype G19833 displays this strategy. Another is to develop more adventitious roots from the lower (underground) portion of the stem. These roots are very shallow and tend to grow horizontally, at least early in development. DOR364 makes abundant adventitious roots. We examined whether these changes are associated with changes in ethylene production. We had previously shown that increasing ethylene production made deep basal roots more shallow, and inhibiting ethylene made shallow roots more deep, indicating that ethylene
might be involved in this response. We also showed that ethylene production rates of 5-week old DOR364 bean plants were higher when the plants were grown with low phosphorus availability. Recently, we studied the ethylene production rates of seedlings of DOR364 and another, contrasting genotype, G19833, at intervals corresponding with the development of major parts of the root system. At 4 days after germination, when the seedling has only a tap root, and the basal roots have not yet emerged, there were no significant differences in ethylene production between P treatments in the radicle or hypocotyl for either genotype. At day 8, when the basal root system is well developed and all main roots bear lateral roots, ethylene production was higher for basal roots and tap roots of DOR364 plants grown under low P availability than for the same tissues grown with high P. Roots of G19833 did not show significant differences between P treatments and rates of ethylene production were similar to
high-P DOR364 roots. G19833, but not DOR364, shows significant changes in basal root angle under low phosphorus availability, producing shallower roots. There was no apparent association of this response with ethylene production rates. This could mean that (1) ethylene production varies only in a particular part of the root, and differences could not be detected with current methods; or (2) ethylene sensitivity changes are responsible for this effect, and ethylene production does not change. Ethylene production from lower stems was very low for both genotypes, but the highest rate of production was from the lower stems of DOR364, which begins to show adventitious roots at about 10 days after germination. By day 12, when adventitious roots had emerged, the ethylene production rate of the lower stems of DOR364 had increased for both P treatments, but was about 3 times higher for low P stems than for high P stems. Ethylene production by G19833 stems remained very low and was slightly
higher at high P. Therefore, ethylene production by the lower stem was correlated with increased adventitious rooting in DOR364 plants, particularly under P stress.
Impacts
Study of the means by which plants adapt to soil stresses will lead to breeding of improved plant cultivars with multiple adaptations for improved stress tolerance. In this work, we have investigated the interaction between phosphorus, an important plant nutrient that is limiting in natural soils throughout much of the world, and ethylene, an important hormonal mediator of stress responses. We have shown that ethylene seems to participate in several, but not all, root changes associated with responses to low P. These studies will contribute to molecular marker development for traits associated with important changes in root architecture for use by breeders, particularly of common bean, the most important food legume in the world. They will also advance our understanding of how plants respond to stresses in their environment, which has importance for all crops.
Publications
- Hilioti, Z., Richards, C., Iversen, S. L., and Brown, K. M. 2000. Regulation and role of pollination-induced ethylene on petal abscission in Pelargonium Xhortorum. Physiol. Plant. 109:322-332.
- Krupnik, G.A., Avila, G., Brown, K. M., and Stephenson, A. G. 2000. Effects of herbivory on internal ethylene production and sex expression in Cucurbita texana. Func. Ecology 14:215-225.
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Progress 01/01/99 to 12/31/99
Outputs
Plant roots respond to low phosphorus (P) availability in several ways which may permit them to acquire more P, one of which may be to distribute more roots in upper, P -rich soil horizons. Two common bean (Phaseolus vulgaris L.) genotypes ( G19833 and DOR364) were used to investigate root gravitropic responses under P deficiency. G19833, a P efficient genotype, had shallow basal roots which were more shallow with P deficiency, while DOR364, a P inefficient genotype, had deep basal roots which were unaffected by P limitation. Low P induced more ethylene production by roots of G19833 but did not change ethylene production by DOR364 roots. AVG, an inhibitor at ethylene synthesis, reduced ethylene production for both genotypes. The basal roots of G19833 treated with AVG failed to grow more shallow under low P, while DOR364 had no response to AVG. These results suggest that ethylene might mediate the gravitropic response of G19833 roots to P availability. Another aspect
of root morphology affected by low P availability is root hair development. We used several ethylene-insensitive genotypes of Arabidopsis thaliana to test the hypothesis that ethylene is required for this response. All of the genotypes tested responded to low P availability with greater root hair elongation and higher root hair density. This indicates that the effects of ethylene and low P on root hair development may be independent. However, other work using only wildtype plants showed that plants grown with high P were more sensitive to root hair inhibition by AVG, an inhibitor of ethylene synthesis. We investigated the development of aerenchyma (air spaces) in bean roots grown in high and low P. When roots of young plants grown in moist, rolled germination paper were examined, we found aerenchyma present even in young roots (as young as 3 days old), but no difference between P treatments. As roots grow older, the frequency and size of aerenchyma increases to a larger extent in low
P plants, so that by 4 weeks, basal roots from low P plants had about 4x greater frequency of aerenchyma than high P plants. We have observed differences in development of aerenchyma in plants grown in sand culture and in hydroponic solution. We have developed the porosity method to quantify the amount of air space in the root tissues.
Impacts
Hormones integrate information from a variety of environmental signals. In this work, we are looking specifically at the role of ethylene in mediating root growth responses to low phosphorus. This will show us how plants can change several aspects of their growth and development to increase the probability of acquiring phosphorus efficiently and in adequate quantity for high quality and yield.
Publications
- Borch, K., Bouma, T, Lynch, J.P., and Brown, K.M. 1999. Interactions of ethylene and phosphorus nutrition on root growth. Plant Cell Environ. 22:425-431.
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