Root and Excess Water

Excess water in soil determines the full filling of the pore space restricting the diffusion of oxygen by 104-fold than in air (Drew and Armstrong 2002) and causing a stress, named waterlogging, with dramatic impact on plant growth and productivity. Climate change provisions provide that, as a consequence of anomalous weather patterns, waterlogging could be a more exceeding problem for many plant communities.

Under oxygen absence (anoxic soil) or under severe hypoxic conditions, the cytochrome oxidase activity of the plants became oxygen limited with a consequent reduction of ATP and pH of the cytoplasm, carbohydrate starvation, and accumulation of the toxic products due to a switch to a fermentative pathway (Drew 1997; Geigenberger 2003; Bailey-Serres and Chang 2005). Within short-time (few minutes or hours), these toxic effects caused severe damages to the plant growth and ultimately leads to death of many plant species. However, the supply of small amount of oxygen (hypoxic soil) stimulated several acclimative mechanisms which allowed the plants to survive to the transient waterlogging.

In waterlogging soils, root system represented the first and more sensitive target of plants which could be seriously damaged in its form and function. Significant inhibition of the root growth, exposed to waterlogging stress were observed in Arabidopsis (van Dongen et al. 2009), Trifolium glomeratum (Gibberd et al. 1999), wheat (Malik et al. 2002) , maize (Wei and Li 2000; Qiu et al. 2007) , woody species (Poot and Lambers 2003; Nicoll and Ray 1996; Nicoll and Coutts 1998; Coutts and Philipson 1978). At the same time, the root system was also able to engender several adaptative responses to waterlogging that could enhance the plant survival in flooded soils. The root adaptation mechanisms were addressed to improve the cellular energy status and reduce the accumulation of toxic end products that acidify the cytosol or damage membrane integrity.

To provide sufficient oxygen for maintaining the root respiration and, consequently, the ATP production it was essential to improve the root energy status and, ultimately, the root growth in anaerobic and chemically reduced soils. Aerenchyma, a plant tissue containing enlarged gas spaces, is an important trait for the root growth and function which provides a low-resistance pathway to obtain the oxygen from the atmosphere to the flooded below-ground organ. The development of aerenchyma has been associated with the tolerance to waterlogging in many plant species (Colmer et al. 1998). Evans (2003) distinguished two types of aerenchyma: (1) the schizogeneous derived from a differential cell expansion and specific pattern of cell separation with subsequent creation of cell spaces and (2) the lysigenous produced by the death and dissolution of the root cortical cells. While the former was common in various wetland species as Rumex (Laan et al. 1989) , the lysigenous was typical of many crop species as soybean (Thomas et al. 2005), rice (Kaway et al. 1998) , maize (Drew et al. 1979; Gunawardena et al. 2001) , wheat (Huang et al. 1994) , and pasture species (Gibberd et al. 2001; Ashi-Smiti et al. 2003, , However, there was a third type of aerenchyma defined secondary aer-enchyma, a white spongy tissue filled with gas spaces, which was found in stem, hypocotyls, tap roots, and root nodules of Glycine max (Shimamura et al. 2003), Lotus uliginosus (James and Sprent 1999), and Sesbania rostrata (Shiba and Daimon 2003) . Generally, the aerenchyma was constitutively expressed in rice and wetland species. Recently, Seago et al. (2005) described the pattern of aerenchyma formation in 85 species representing 41 families of wetland plants. On the other hand, the aerenchyma development was also induced by flooding and other stresses, e.g., nutritional and drought in many field crops. For example, soybean, very sensitive species to flooding stress during the vegetative stage developed a secondary aerenchyma in stems, roots, and root nodules within few weeks of stress (Shimamura et al. 2003). Complex physiological and molecular mechanisms were involved in the development of aerenchyma in plants subjected to the flooding stress (Colmer 2003a; Evans 2003). In maize roots, the hypoxia conditions (3-12% oxygen) promoted the ethylene biosynthesis which triggered a signal transduction cascade involving Ca2+ and protein kinases, inducing a programmed cell death in target cells of the root cortex (Drew et al. 2000) .

The genetic variability of the plant species in the differing tolerance to waterlogging was associated with the aerenchyma formation which determined higher root porosity. Indeed, the superior tolerance to waterlogging of Trifolium tomentosum, T. fragiferum and T. repense than T. subterraneum var. subterraneum and T. glom-eratum, more sensitive species were due to the development of aerenchyma in adventitious roots

(Gibberd et al. 1999). The different ability to form aerenchyma and, hence, to exhibit a greater tolerance to the waterlogging stress were observed among the genotypes of soybean (Bacanamwo and Purcell 1999), maize (Zaidi et al. 2004), wheat (Boru et al. 2003) , and woody species (Aguilar et al. 1999). Further, the wetland Rumex species tolerant produced a higher root porosity than sensitive ones (Laan et al. 1989).

Beside the aerenchyma, other root mechanisms which improved the presence of oxygen in plant tissues subjected to the flooding stress, such as the aerotropic roots and extensive lateral roots (Gibberd et al. 2001), the herringbone-type root architecture (Bouma et al. 2001), the emergence of adventitious roots (Mergemann and Sauter 2000) and the anatomical barriers such as exoderism (Colmer 2003b) were suggested to be part of the basis of tolerance to waterlogging. The recovery of the root cellular energy status in oxygen deficiency conditions was obtained through different metabolic adaptative mechanisms: the switch to a fermentative pathway (Sachs et al. 1996; Chang et al. 2000), the global depression of ATP-consuming processes (van Dongen et al. 2009), the death of metabolically intensive tissues as root tip (Subbaiah and Sachs 2000; Subbaiah and Sachs 2001) and the induction of anaerobic protein synthesis (Lal et al. 1998; Mujer et al. 1993; Chang et al. 2000) . During waterlogging, the roots were more prone to oxidative stress which caused the generation of "reactive oxygen species" (ROS), including superoxide anion radicals (O2-), hydroxyl radicals (OH), hydrogen peroxide (H2O2), alkoxy radicals (RO), and singlet oxygen (O1/2) (Munne-Bosch and Penuelas 2003). The production of ROS led to enhanced peroxidation of membrane lipids and degradation of nucleic acids, and both structural and functional proteins. In this respect, the induction of free radical scavenging enzymes observed in wheat roots subjected to hypoxia determining tolerance to the waterlogging stress (Biemelt et al. 1998).

Proteomic and genomic analysis supported the morphological and physiological mechanisms that made up the root acclimative responses to the waterlogging stress. Indeed, the roots stressed by water shortage exhibited a large-scale repro-

gramming of gene expression and metabolism: (1) up- and downregulation of genes in Arabidopsis that mainly encoded proteins involved in fermentative process and energy-consuming processes (transport, lipid and secondary metabolism), respectively (van Dongen et al. 2009), (2) a quantitative trait locus containing the ethylene response factor-like genes was regulated by submergence in rice (Fukao et al. 2006), and (3) several quantitative trait loci associated with waterlogging tolerance were detected in maize plants (Qiu et al. 2007).

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