Basic Anatomy And Biochemistry Of Roots And Leaves

2.2.1 Rhizoplane

The rhizoplane is the area of the root interfacing with soil. Root tips and root hairs are immersed in mucigel, a substance created by the combination of plant-secreted mucilage (composed of pectins and hemicelluloses) and complex polysaccharides produced by bacteria that also degrade mucilage [23]. Bacteria are exposed to the mucilage when the cuticle covering it on root hairs is punctured or degraded physically or chemically. Depending on the plant, root exudates contain a variety of substances that can act as chemoattract-ants for microorganisms and/or substrates for growth. Sugars, amino acids and other amino compounds, organic acids, fatty acids and sterols, growth factors, nucleotides, and other compounds are produced from the aging epidermal cells [24]. Most natural rhizosphere bacteria attach to specific regions of the root; root hairs are a common site of attachment of the rhizobial bacteria involved in nitrogen fixation [25,26], and, possibly, attachment of human enteric pathogens [27]. Figure 2.1 shows a general schematic of a region of a plant root illustrating an emerging root hair (Figure 2.1A). As noted above, epidermal cells comprise the surface of the root tissue as it develops with specific epidermal cells becoming root hair cells; cortex cells compose a second layer under epidermal cells (Figure 2.1G). After attachment of certain nitrogen-fixing bacteria such as the rhizobia, root hair cells are induced

Root Hair Epidermis

FIGURE 2.1 (Color insert follows page 594) Anatomy of a root hair. Schematic representation of structures that are part of the anatomy of a plant root hair with attached bacteria. (A) Root hair epidermal cell; (B) nucleus; (C) bacteria bound to epidermal cell surface in aggregates and as biofilm; (D) rhizobial bacteria; (E) root hair infection thread initiated by rhizobial bacteria; (F) curling root hair tip; (G) cortex cells; (H) junction between root epidermal cells with attached bacteria; (I) bacteria binding as single cells, then aggregating; (J) magnification of I (not drawn to scale): J-1, single bacterial cell binding by pili/fimbriae; J-2, plant lectins interacting with bacterial carbohydrate (e.g., EPS, LPS, CPS, cellulose fibrils); J-3, bacterial flagellin interacting with plant receptor (e.g., polysaccharide); (K) lesion produced by plant pathogen.

FIGURE 2.1 (Color insert follows page 594) Anatomy of a root hair. Schematic representation of structures that are part of the anatomy of a plant root hair with attached bacteria. (A) Root hair epidermal cell; (B) nucleus; (C) bacteria bound to epidermal cell surface in aggregates and as biofilm; (D) rhizobial bacteria; (E) root hair infection thread initiated by rhizobial bacteria; (F) curling root hair tip; (G) cortex cells; (H) junction between root epidermal cells with attached bacteria; (I) bacteria binding as single cells, then aggregating; (J) magnification of I (not drawn to scale): J-1, single bacterial cell binding by pili/fimbriae; J-2, plant lectins interacting with bacterial carbohydrate (e.g., EPS, LPS, CPS, cellulose fibrils); J-3, bacterial flagellin interacting with plant receptor (e.g., polysaccharide); (K) lesion produced by plant pathogen.

to curl, initiating the complex process of thread formation within the root hair (Figure 2.1E).

2.2.2 Phylloplane

The phylloplane is the interface between the leaf and the environment. Epidermal cells compose the upper and lower surfaces of a leaf (Figure 2.2B), and are covered by the cuticle, which is composed of a polymer matrix (cutin), polysaccharides, and associated waxes. The cuticle acts as a barrier and prevents water loss from the leaf (Figure 2.2A). The cuticular waxes are lipophilic long-chain fatty acids (20 to 40 carbons); some fatty acids are oxygenated forming aldehydes, ketones, sterols, and esters [28,29]. The waxes in leaves are mostly saturated and, therefore, highly resistant to degradation by most microorganisms. However, an important part of the microbial ecology of plants, and of the phyllosphere in particular, are fungi that secrete cutinases that degrade leaf waxes [30]. Lesions in the cuticle can expose potential sites of attachment for other microorganisms (Figure 2.1K and Figure 2.2L). In addition, some bacteria, including epiphytic bacteria, probably adhere to the cuticle and interact with the plant and obtain nutrients

Epiphytic Bacteria

FIGURE 2.2 (Color insert follows page 594) Anatomy of the cross-section of a leaf. Schematic representation of structures that are part of the anatomy of most plant leaves and attached microorganisms. (A) Cuticle layer; (B) upper epidermis; (C) palisade parenchyma; (D) vascular bundle composed of phloem and xylem; (E) biofilm composed of bacteria and other microorganisms; (F) EPS; (G) stomates within upper and lower epidermis; (H) trichome; (I) cuticle; (J) free bacteria and other microorganisms within water droplet; (K) recessed area between epidermal cells; (L) biofilm on underside of leaf forming a lesion into the vascular system.

FIGURE 2.2 (Color insert follows page 594) Anatomy of the cross-section of a leaf. Schematic representation of structures that are part of the anatomy of most plant leaves and attached microorganisms. (A) Cuticle layer; (B) upper epidermis; (C) palisade parenchyma; (D) vascular bundle composed of phloem and xylem; (E) biofilm composed of bacteria and other microorganisms; (F) EPS; (G) stomates within upper and lower epidermis; (H) trichome; (I) cuticle; (J) free bacteria and other microorganisms within water droplet; (K) recessed area between epidermal cells; (L) biofilm on underside of leaf forming a lesion into the vascular system.

without damaging the surface [31]. Depressions formed at junctions of epidermal cells appear to have thinner cuticles, with microorganisms often residing at these sites (Figure 2.2B).

The upper and lower surfaces of plants are not considered favorable to microbes because of the cuticle and the rapid and repetitive fluctuations in the physicochemical conditions (e.g., humidity, temperature, leachates) to which microbes must adapt [32,33]. Many of the nutrients present in root exudates noted above have been detected also in leaf exudates [34]. Some of the chemicals in leaf exudates/leachates are likely chemoattractants, inducing movement of the microbe towards a closer interaction that may involve attachment [35].

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