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a Dep. of Soil and Crop Sci., Texas A&M Univ., College Station, TX 77843-2474
b Dep. of Biochem. and Biophysics, Texas A&M Univ., College Station, TX 77843-2128
* Corresponding author (p-morgan{at}tamu.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONFLOWERINGSHADE AVOIDANCE SYNDROMEAPICAL DOMINANCE AND SHOOT...ROOT DEVELOPMENTOPTIONSREFERENCES |
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Abbreviations: LD, long day • PHYA, PHYB, phytochrome wild-type genes • phyA, phyB, mutant genes • PHY, phytochrome apoprotein • phy, holoprotein • QTL, quantitative trait locus • SD, short day
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONFLOWERINGSHADE AVOIDANCE SYNDROMEAPICAL DOMINANCE AND SHOOT...ROOT DEVELOPMENTOPTIONSREFERENCES |
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0.5% per year (Mann, 1997; Vasil, 1998). Secondly, affluence is adjusting diets to include more meat, which in turn increases the volume of grain needed several fold (Mann, 1997; Vasil, 1998). In fact, India and China, which became net food exporters due to the Green Revolution, have again become net food importers due to rising demand for poultry and meat (Vasil, 1998). Options for increasing agricultural productivity are limited. Expansion of agriculture into areas not presently cultivated will, according to Tilman et al. (2001), result in global environmental change characterized by ecosystem simplification and species extinctions. Scarcity of water will impact a large proportion of the world's population during the next 50 years and limit agricultural uses (Vörösmarty et al., 2000). Production capacity is deteriorating due to soil depletion/erosion and other factors (Mann, 1997; Vasil, 1998). The need for more food and limitations in both new land and conventional technology all point to the need for genetic crop improvement as a major means to significantly increase world food production (Mann, 1997; Vasil, 1998; Moffat, 2000).
Opportunities for plant scientists to help make additional improvements in monocotyledonous crop species are increasing rapidly through the development of high through-put approaches to gene discovery and genomics-based understanding of plant biology. The combination of classical trait locus mapping, candidate gene sequencing, and gene validation by cosuppression or antisense approaches is providing a deeper understanding of plant gene function. Moreover, genome sequencing and genome-wide gene expression profiling are revealing the total collection of plant genes and their regulation. Of special importance for grass species is the high degree of synteny among grass genomes (Devos and Gale, 1997). According to Phillips and Freeling (1998), "This concept has led the plant genetics community to view the grass family as a single genetic system". However, detailed knowledge of each grass genome will be useful because inversions, translocations, duplications, and mutations also differentiate the different grass genomes as well (Bennetzen, 2000). A full understanding of grass gene function will require detailed investigation of genes in several species as well as their diverse germplasm collections. The combined knowledge of general plant gene function and the importance of allelic variation in natural plant populations provide the framework for improvements in plants adapted to production agriculture.
The obvious targets for genetic crop improvement have been increased resistance to insects and diseases. Stress resistance is also an important target because of the large impact that drought stress has on crop yields (Boyer, 1982). However, in order to increase basic productivity, changes will have to be made in development and metabolism. Several aspects of the development of grain crop species have major impacts on adaptability, stress tolerance, and yield (Morgan and Finlayson, 2000). These processes include photoperiodism, shoot elongation, apical dominance, shade avoidance, and root development.
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FLOWERING |
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TOPNOTESABSTRACTINTRODUCTIONFLOWERINGSHADE AVOIDANCE SYNDROMEAPICAL DOMINANCE AND SHOOT...ROOT DEVELOPMENTOPTIONSREFERENCES |
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Beyond the relatively simple question of when a crop will flower, there are a number of problems in reproductive development, including pollen incompatibility in both self and cross species situations, apomixis, parthenocarpic fruit set, embryo abortion, viviparity, dormancy, and quality of seed. The larger issues of polyploidy and epistasis are related and of great importance. Many of these systems impact on monocotyledonous crops and, in some cases, achieving solutions may be aided by knowing more about how flowering is controlled.
Evidence from several species indicates that the progression to flowering is the failsafe condition (Sussex, 1989; Sung et al., 1992; Araki and Komeda, 1993; Koornneef et al,. 1998). Photoperiodism delays the genetic tendency to flower by forcing the plant to wait until a specific signal (night length) is sensed (Sung et al., 1992; Weigel, 1995; Koornneef et al., 1998). Studies of natural and induced mutations of Arabidopsis thaliana (L.) Heynh. have identified >48 genes which influence flowering time. These genes, along with >33 additional quantitative trait loci (QTLs) for flowering time, have been located on the A. thaliana genetic map (Koornneef et al., 1998) and appear in its genome sequence. These genes have been examined in epistasis studies and placed in a model (Fig. 1) which predicts that plants flower when the vegetative gene repression of an autonomous or "automatic" flowering program is turned off (Koornneef et al., 1998). The vegetative gene pathway appears to be regulated by three groups (subpathways) of genes: (i) the daylength group, (ii) the constitutive group, and (iii) the vernalization group. Because (i) there are so many genes involved, (ii) there are several examples of redundancy in the subpathways, and (iii) a connection of the genes to both long day (LD) and short day (SD) behavior exists, there are many opportunities to modify flowering genetically, but modification may not be simple nor entirely predictable (Koornneef et al., 1998).
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170 to 190 d for plants dominant at both loci. The study of these six Sorghum maturity genes has progressed at varying rates. Ma3 encodes the phytochrome B apoprotein (PHYB), and maR3 contains a single base pair deletion (Childs et al., 1997). Because PHYB protein has not been detected in a cultivar homozygous for maR3, this allele is considered to be a null mutation and has been redesignated as phyB-1 (Childs et al., 1997). The mapping of two other Sorghum maturity genes, Ma1 and Ma4, is ongoing, but current data indicate that neither is located on the same linkage group as the three Sorghum phytochrome genes (Lin et al., 1995; Childs et al., 1997; Peng et al., 1999; Ulanch et al., 1999; Childs and Mullet, 2001, unpublished data). Ma5 and Ma6 are hugely powerful when both are dominant (Rooney and Aydin, 1999). Mapping studies are in progress (J. Brady, P.E. Klein, W.L. Rooney, and J.E. Mullet, 2001, personal communication), but it is already evident that they have potential value in grass improvement. These genes do not have obvious pleiotropic effects. They have been used to produce forage hybrids that are extremely photoperiod-sensitive and do not flower until very late in the growing season (F.R. Miller, 2001, personal communication). They may be useful in producing photoperiod-sensitive hybrids for regions of the world where photoperiod-sensitive landraces are grown.
Interest in Ma1 is increased by the fact that its general map location is similar to the location of the major photoperiod gene in maize, rice, wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), and sugarcane (Lin et al., 1995; Paterson et al., 1995). Substitution of recessive alleles for Ma1 has been the basis of the Sorghum conversion program (to temperate latitude adaption) (Stephens et al., 1967), and similar conversions have probably occurred in many of the temperate adaptions of other grain crops. Recently, Yano et al. (2000) isolated Hd1, a major photoperiod sensitivity QTL in rice, and sequence analysis indicated it has very high homology with the Arabidopsis flowering time gene CONSTANS. Hd1 is allelic to the rice flowering gene Se1. Photoperiod did not change the amount of expressed Hd1 mRNA, but the gene promotes flowering under SD and inhibits it under LD. Hd1 contains a zinc-finger region and appears to be a transcription factor (Yano et al., 2000). Efforts are underway to determine whether or not the Sorghum Ma1 gene also codes a CONSTANS-like zinc-finger protein.
Although Ma1 regulates SD photoperiod sensitivity in Sorghum, the presumptive Ma1 orthologs in wheat (Ppd1) and barley (Ppd-H1) (Lin et al., 1995; Paterson et al., 1995) regulate LD photoperiod sensitivity. When one compares these Ma1 orthologs (see review in Laurie, 1997), dominant Ppd1 causes early flowering by constitutive induction. The recessive wheat gene and its barley ortholog (Ppd-H1), which is dominant, cause daylength sensitivity and late flowering. Finally, recessive ppd-H1 in barley results in daylength neutrality. Thus, if conclusions from comparative grass genomics are correct (Lin et al., 1995; Paterson et al., 1995), the Ma1 gene orthologs function in floral initiation independently of the LD-SD classification of plants. Additionally, the gene has apparently experienced both gain of function and loss of function mutations (Table 1) , and it obviously represents a great resource for manipulation of flowering time in grasses. This conclusion is supported by evidence for allelic variation in Ma1/ma1 (Quinby, 1967).
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10-fold when the maR3/phyB-1 allele is homozygous. How much the daily ethylene peaks contribute to the phenotype of maR3/phyB-1 plants is not yet known. In addition to ethylene, the levels of biologically active gibberellins (GA20 and GA1) exhibit daily peaks in Sorghum (Foster et al., 1994; Foster and Morgan, 1995; Lee et al. 1998a) and, the timing of the peaks is different in phyB-1 (photoperiod insensitive) than in PHYB (wild-type gene) (photoperiod-sensitive) cultivars. The flowering of phyB-1 mutants is delayed by 18- to 24-h days, and the timing of the GA1 and GA20 peaks shifts to match the wild type under these conditions. Recent evidence reveals that in Lolium temulentum, a LD grass that initiates flowering in response to a single LD, GA5 doubles in concentration in the apical meristem and then declines during the 48 h following initiation of a single LD treatment (King et al., 2001). This response is followed in subsequent days by a many-fold increase in GA1 and GA4. Promotive effects of GA5 on floral initiation as well as the timing of export of GA5 from leaves after LD treatment were consistent with it serving as a florigen-like signal. Changes in GA1 and GA4 suggest that they may be promoting floral development and stem elongation. In addition to gibberellins, brassinosteroids and blue-light pigments called cryptochromes have also been implicated in the control of flowering (Morgan and Finlayson, 2000). It is possible that all of these components act in redundant signal transduction pathways so that their effects are significant but not dominant. The pathways may offer opportunities to fine-tune flowering time and sensitivity at multiple sites.
Phytochromes and Flowering
It is clearly established that the red-light absorbing phytochromes sense daylength, convey that information to the clock, and that the clock, in turn, signals the genome to turn off the vegetative program at appropriate locations in the plant (Smith, 1995, 2000). Nevertheless, more remains to be discovered about how photoperiod length is connected to floral initiation.
Progress is being made on the role of the phytochromes in flowering of grasses. The phyB null mutation in Sorghum causes plants to be very early and to be almost insensitive to photoperiod (Pao and Morgan, 1986a,b; Childs et al., 1992, 1997). Unnaturally long photoperiods (18 to 24 h) delayed initiation in phyB-1 plants, but the delay was not for nearly as many days as would be necessary for Ma3 (PHYB) plants to initiate under the same conditions (Childs et al., 1995; Lee et al., 1998a). Phytochrome B is not required for normal, circadian expression of chlorophyll a/b binding protein (Finlayson et al., 1999), but in its absence the amplitude of rhythmic ethylene production is enhanced, while the period is unaltered. These results suggest that PHYB normally regulates the output of the clock rather than being part of the clock or that there is more than one clock. Since ethylene production is overexpressed in the phyB-1 mutant, phyB (holoprotein) most likely down regulates the genes for ethylene biosynthesis.
Additionally, it is known that Arabidopsis phyA mutants flower late compared with phyB mutants that flower early (Koornneef et al., 1998). Mutants of phyA in rice have recently been isolated (Takano et al., 2001). If both PHYA and PHYB encoding genes regulate flowering, then it might be manipulated by careful selection of their alleles or by transformation with phytochrome trans-genes engineered with tissue-specific and developmental stage-specific promoters. This approach offers the possibility of: (i) modifying a selected hybrid to form multiple lines, each with slightly different flowering dates at a given location, and (ii) increasing the predictability of flowering behavior of hybrids at various latitudes. A recessive mutation in barley, BMDR-1, results in photoperiod and far-red insensitivity and altered phytochrome levels, but the gene is yet to be isolated and characterized (Hanumappa et al., 1999).
Maturity and Senscence
Perhaps as much as 71% of the reductions in yields of crops may be due to drought (Boyer, 1982). The search for the physiological mechanisms and specific genes involved in drought resistance has been difficult (Rosenow et al., 1983); however, postanthesis drought resistance has been linked to resistance to premature senscence. This postflowering drought resistance trait, commonly termed stay-green, has been mapped to identify seven QTLs in Sorghum (Crasta et al., 1999). In addition, two maturity QTLs were mapped. Interestingly, one of the maturity QTLs mapped near a stay-green QTL, and the major, independent maturity QTL's markers were significantly correlated with stay-green ratings. Thus, there is an undefined relationship between two of the maturity QTLs and several of the stay-green QTLs. This work is continuing (Xu et al., 2000; Subudhi et al., 2000). Another study compared pairs of near isogenic lines of Sorghum differing in specific QTLs associated with drought tolerance, and two QTLs were identified which appear to condition the expression of stay-green (Tuinstra et al., 1998). Thus, the genetic disposition to earliness may work against maintenance of assimilatory capacity during postanthesis drought stress, which emphasizes the need to understand the relationship. Stay-green may also be viewed as a perennial trait introduced into an annual plant background (Thomas et al., 2000) which relates to the control of programmed cell death.
Temperature and Flowering
The biological clock keeps time regardless of the temperature, within physiological limits. Nevertheless, temperature can serve as an entraining signal in circadian phenomena (Sweeney, 1987), and there is mounting evidence that temperature signals are involved in floral initiation. For example, placing thermoperiods (day and night temperature differences) out of phase with photoperiods markedly hastens floral initiation in some Sorghum maturity genotypes (Morgan et al., 1987). Ellis et al. (1997) confirmed these findings and discovered that some thermoperiod and photoperiod combinations can delay flowering. Using circadian ethylene synthesis as a reporter system, Finlayson et al. (1998)(1999) further characterized the temperature signaling system. They found that: (i) both a photoperiod signal and a day and night temperature signal are necessary to entrain circadian ethylene rhythms, (ii) to achieve entrainment, temperature must exceed a threshold and the difference between day and night temperature must exceed a threshold, (iii) reversal of the thermoperiod to give cool days and warm nights altered the ethylene rhythm so that the peak occurred at night and a minimum during the light period, (iv) reversal of the thermoperiod also reversed the normal diurnal growth pattern so that higher growth rates occurred at night. These findings suggest that Sorghum plants recognize day and night thermoperiods, and that thermoperiods have a major effect on circadian ethylene production, circadian growth rhythms, and possibly flowering.
There are some very practical opportunities associated with thermoperiod effects on flowering. Hybrids planted in various international locations often do not flower after the same number of days under equal photoperiods. It is likely that some aspect of the temperature pattern overrides the photoperiod effect. Indeed, when temperate Sorghum cultivars were planted at various latitudes in the USA, they flowered at the northern locations under photoperiods long enough to cause lengthy delays if applied in growth chambers, prompting the investigators to surmise that Sorghum may not be photoperiodic (Sorrells and Myers, 1982). On the basis of the variety of evidence for photoperiodism (Quinby and Karper, 1945, 1947; Lane, 1963; Pao and Morgan, 1986a; Foster and Morgan, 1995) it is more likely that the behavior was due to a strong temperature effect.
How temperature effects are detected by plants is unknown. However, the data on flowering responses (Morgan et al., 1987; Ellis et al., 1997) and those on circadian ethylene production (Finlayson et al., 1998) both suggest that the biological clock is involved, either in perception or in signal transduction. Major et al. (1990) concluded that Ma2 is involved in a temperature x photoperiod interaction. The two genotypes that are dominant, Ma1 and Ma2, are the most sensitive to photoperiod and thermoperiod asynchrony treatments (Morgan et al., 1987); however, the effect of Ma1 Ma2 was negated by the phytochrome B null mutation maR3. Indeed, the three maR3 genotypes were the most insensitive to the experimental alteration of the synchrony of the thermo- and photoperiods. If the nature of the thermoperiod effect on flowering can be understood in detail, that might open up new options for genetically altering flowering behavior of plants in the field.
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SHADE AVOIDANCE SYNDROME |
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TOPNOTESABSTRACTINTRODUCTIONFLOWERINGSHADE AVOIDANCE SYNDROMEAPICAL DOMINANCE AND SHOOT...ROOT DEVELOPMENTOPTIONSREFERENCES |
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The phenotype of maR3 (phyB-1) containing plants is that of the shade avoidance syndrome (K.L. Childs, 1995, personal communication). Plants exhibit tall shoots, lack of tillers and early flowering (Pao and Morgan, 1986a), and significantly increased shoot:root ratios (Morgan et al., 1996). An investigation of which phytochrome regulates shade avoidance and circadian ethylene synthesis revealed that: (i) phyB-1 plants are tall not only because of the absence of PHYB protein, but also due to the overexpression of the PHYA gene; (ii) in the absence of phyB, both PHYA mRNA and PHYA protein are more abundant than in the presence of phyB (wild type); and (iii) response to shading during the day and to a skeletal photoperiod also implicate PHYA as the agent that regulates circadian ethylene production (Finlayson, Mullet, and Morgan, 2001, unpublished data). Thus, plant breeders should be able to reduce shade avoidance behavior in Sorghum by overexpressing PHYB or reducing the expression of PHYA. Such work would be aided by knowing the detailed sequence of the promoters and the structural genes for several alleles of both PHYA and PHYB.
There is evidence that decreases in shade avoidance behavior can increase yields. Maize yields in the USA increased an average of 92 kg ha-1 yr-1 from 1950 to 1980 (Duvick, 1984), and experiments which tested commercial hybrids released from 1934 to 1978 revealed that most of the genetically based yield increase was associated with increased tolerance to high density populations. The greatest yield increases occurred at the highest population density tested. Tolerance to high population density is likely to represent a repression of the shade avoidance syndrome. This conclusion is supported by the fact that parallel to the increase in yield at high population densities has been a decrease in root lodging and, to a lesser extent, a decrease in stem lodging. This conclusion is further supported by recent evidence linking lodging susceptibility to shading intolerance and an increase in the shoot:root ratio in shade along with a marked variation in this set of characters between different maize cultivars (Hébert et al., 2001). Since a major effect of shade and shade-avoiding behavior is to increase the shoot:root ratio and to promote shoot elongation (Smith, 1992; Hébert et al., 2001), it is likely that repression of shade avoidance in high density populations would reduce root and shoot lodging. Many other agronomic characters did not change in parallel to yield increases (Duvick, 1984). These relationships suggest that many grain crops may exhibit significant yield increases if perception of and response to shade is minimized genetically.
It is now apparent that phytochromes can achieve their effects through multiple-step, signal transduction pathways (Bowler et al., 1994; Huq et al., 2000; Martinez-Garcia et al., 2000; Soh et al., 2000). Allelic or mutation-based variability in these signal transduction component genes would also allow manipulation of the tendency of plants to exhibit shade avoidance in dense stands. Work on the plant transcription factor PIF3 indicates that it binds to phytochrome B (Martinez-Garcia et al., 2000). Suppressing this signal could make plants less sensitive to dense populations. Occasionally, treatment effects on flowering behavior have not paralleled those on shoot growth or tillering (Foster et al., 1997; Lee et al., 1998b). Thus, there is some disconnect between the component symptoms of shade avoidance, and it might be possible to manipulate flowering date independently of height and tillering. This is of interest since even in cultivars and hybrids which flower at an optimum time, often tillering or stem stiffness are less than ideal.
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APICAL DOMINANCE AND SHOOT ELONGATION |
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TOPNOTESABSTRACTINTRODUCTIONFLOWERINGSHADE AVOIDANCE SYNDROMEAPICAL DOMINANCE AND SHOOT...ROOT DEVELOPMENTOPTIONSREFERENCES |
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As of yet, there is no comprehensive understanding of either the genetics or physiology of apical dominance in grasses, and understanding the regulation of height is far from complete. The first Sorghum species introduced into the USA were both very late in flowering and very tall (Karper and Quinby, 1946). These tall plants were not practical for mechanized harvesting, but when mutations were found which flowered earlier and were dwarfed in height, planting of Sorghum increased (Quinby, 1967, 1973). Eventually, at least four genes were recognized which caused dwarfing without modifying leaf size, panicle size, or root development significantly (Quinby and Karper, 1954). These genes were combined to allow production of 3- and 4-dwarf hybrids which could be harvested with a combine. Other dwarfing genes in Sorghum are pleiotropic and have such detrimental effects as to be useless in plant breeding (see review in Morgan and Finlayson, 2000).
Dwarfism in maize has been linked to several genes which encode enzymes in the gibberellin biosynthesis pathway (Phinney and Spray, 1982; Phinney, 1984). Dwarf plants are severely affected, displaying dwarfism, reduced leaf size, sexual abnormalities, and much reduced yield. These genes are recessive, but there are a number of dominant dwarfing genes which are presumed to either code for GA receptors or components of signal transduction pathways (Phinney and Spray, 1982). The modern trend in maize breeding in the USA has been to produce shorter, single-eared plants with smaller tassels and stiffer stalks. It seems possible that this model may have unconsciously selected from variability in photoperiodism, thus allowing lower ear placement (earlier flowering); dwarfism, thus allowing shorter internodes and smaller tassels; and apical dominance, thus minimizing the number of side branches (ears). The possibility that the trend toward single-eared maize is the result of selection of stronger apical dominance is favored by the observation that prolificacy (multiple ears) is favored by low planting density and inhibited by high planting densities (Javier Betran, 2001, personal communication). A similar response pattern between number of tillers and plant populations occurs in Sorghum. In addition to Sorghum, dwarfing genes have been studied and used extensively in wheat where there is a strong correlation between the degree of dwarfism, insensitivity to growth in response to applied gibberellins, and accumulation of endogenous gibberellins (Appleford and Lenton, 1991). Various wheat Rht (dwarfism) alleles have been examined for effects on both height and yield, and usually those with moderate effects on height increased yields while those which reduced height the most decreased yields (Flintham et al., 1997).
The morphological evolution of maize, specifically the gene necessary for the shortening of the lateral branches (teosinte branched 1, tb1) to the short stalk of the ear has been studied in detail (Doebley et al., 1995, 1997; Lukens and Doebley, 1999). The sequence of the gene suggests that it encodes a transcription factor, and the authors postulate that allelic variants of this gene are probably responsible for the presence of ears in modern maize. Some features of some of the dwarfing genes in Sorghum suggest that they may encode transcription factors or locally expressed receptors, but these features do not exclude the possibility they encode structural genes (Morgan and Finlayson, 2000). An examination of nucleotide polymorphisms in tb1 revealed that most changes occurred in the gene's regulatory region rather than in the protein coding region (Wang et al., 1999). Synteny of the grass genomes suggests there are probably genes similar to maize tb1 in most grass species and these nonmaize genes are probably not mutated to shorten lateral branches, or they may be expressed to a degree to achieve total apical dominance over aerial lateral buds. Thus, individual species or even cultivars may bear uniquely mutated regulatory genes which could be useful in other species.
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ROOT DEVELOPMENT |
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TOPNOTESABSTRACTINTRODUCTIONFLOWERINGSHADE AVOIDANCE SYNDROMEAPICAL DOMINANCE AND SHOOT...ROOT DEVELOPMENTOPTIONSREFERENCES |
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Low O2 levels (or flooding) are known to prompt plants to form air spaces (aerenchyma) in their roots (Jackson and Drew, 1984). These air spaces form in the actively growing region behind the root tip; therefore, in a root several decimeters long, the aerenchyma might start forming near the tip but be totally immersed in flooded soil. On the other hand, if the aerenchyma had access to O2, they would significantly increase the O2 available to the root tips (Armstrong, 1979).
Studies of maize roots established that ethylene is a signal compound in initiation of aerenchyma in response to low O2 or flooding (Jackson et al., 1985). Low N and P increased sensitivity to ethylene and also caused aerenchyma formation (Drew et al., 1989), and mechanical impedance caused root tip swelling and aerenchyma formation (Sarquis et al., 1991; He et al., 1996a). Aerenchyma are the result of programmed cell death and an extensive signal transduction pathway was shown to exist (He et al., 1996b; C.-J. He, M.C. Drew, and P.W. Morgan, 2002, unpublished data). Root elongation in response to mechanical impedance was favored by inhibiting ethylene synthesis, thus suggesting that not forming aerenchyma might favor root penetration through soil (Sarquis et al., 1992). The opposite conclusion was suggested when low O2 levels and mechanical impedance were found to complement each other to amplify ethylene production and to maximize aerenchyma formation (He et al., 1996a).
Recent ecological observations may have revealed a major developmental role of aerenchyma in grass roots. Workers studying eastern gamagrass [Tripsacum dactyloides (L.) L.] noted considerable variability in drought tolerance of populations of plants on an area of deep soils (Clark et al., 1998). Plants were excavated and those which looked healthy during the severe drought were found to have penetrated a dense soil layer (clay pan) and extended their roots to unexpected depths. Thus, while most bunches of the grass were dying with roots limited to an upper layer of dry soil, the healthy bunches were extracting moisture several feet below the dense soil layer. Careful examination of the drought damaged plants and the prosporous ones revealed that the latter had constitutively expressed aerenchyma, that is, aerenchyma beginning at the root origin points and extending to near their tips (Clark et al., 1998). The interpretation is that the roots could only force their way through the dense soil layer when it was flooded. Under these circumstances, the plants with constitutive aerenchyma formation were able to transport O2 to the root tips which in turn were able to penetrate the dense soil layer. Root development below this layer was unrestricted. Examination of many clones of eastern gamagrass revealed much variability in aerenchyma formation, from freely constitutive to strongly dependent upon stress for initiation (Ray et al., 1998). Work has been initiated to move "constitutive aerenchyma genes" from Tripsacum into maize (Ray et al., 1999).
On the basis of synteny in grass genomes, one would anticipate that genes for constitutive aerenchyma formation are universally present but not expressed. If this assumption is correct, it would be possible to make major improvements in drought tolerance and ability to extract nutrients from both soils with plow pans and those with high clay content and poor aeration. Genes in the ethylene synthesis-perception-signal transduction-response pathway (Jackson et al., 1985; Drew et al., 1989; Sarquis et al., 1991) would be targets for improvement in grass root development.
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OPTIONS |
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TOPNOTESABSTRACTINTRODUCTIONFLOWERINGSHADE AVOIDANCE SYNDROMEAPICAL DOMINANCE AND SHOOT...ROOT DEVELOPMENTOPTIONSREFERENCES |
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Efforts to improve grass crops will be aided if the transformation procedures for monocots can be improved. Most research on transformation in monocots has sought empirical improvements in existing techniques. Incremental progress has been achieved. There is a need for research on the basic mechanisms of transformation in grasses such as that with Arabidopsis (Bent, 2000).
Finally, it is likely that genes as yet unknown may be used in means as yet unknown. As the mechanisms of floral initiation, the biological clock, apical dominance, root development, and other processes become better understood, the chances for improvement will certainly increase.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONFLOWERINGSHADE AVOIDANCE SYNDROMEAPICAL DOMINANCE AND SHOOT...ROOT DEVELOPMENTOPTIONSREFERENCES |
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Received for publication January 30, 2002.
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TOPNOTESABSTRACTINTRODUCTIONFLOWERINGSHADE AVOIDANCE SYNDROMEAPICAL DOMINANCE AND SHOOT...ROOT DEVELOPMENTOPTIONSREFERENCES |
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