**2. Physiological responses of rice plants to pathogens**

Rice plants have cuticles, silica and cell walls that often serve as the first passive defense to contain pathogens [14–16]. Subsequent failed active resistance leads to visible pathogen damages. For rice blast disease, typically a diamond shaped symptom on the leaf is called leaf blast, and dark brown on the neck as panicle blast, respectively (**Figure 1**). Both leaf and panicle blast can result in significant yield reduction.

When they encounter rice plants, asexual spores of blast fungus, *M. oryzae* germinate and initiate their life cycle by penetrating the cells with infection pegs from tightly adhered swollen hyphal tips with the highest biological turgor pressures known for a biological organism [17]. During penetration *M. oryzae* absorbs nutrients by producing cutinases to degrade cutin in the cuticle and pectolytic enzymes. *M. oryae* secretes heat labile molecules such as endo ß-1,4-D-xylanase to solubilize rice cell wall fragments to kill cells [18, 19]. After penetration *M. oryzae* develops invasive hyphae that are in direct contact with the membrane of the live cells. Soon after that within approximately 48 hrs *M. oryzae* colonizes the host tissues and releases asexual spores at the end of invasion [20, 21]. *M. oryzae* is thus classified as a hemibiotrophic pathogen.

#### **Figure 1.**

*Typical symptoms of rice blast disease. A. Leaf blast disease after an artificial inoculation under greenhouse conditions at USDA ARS Dale Bumpers National Rice Research Center, Arkansas, USA, and B. Panicle blast disease in a rice field under natural infections in Puerto Rico (photograph credit: Miss Adriana Rivera).*

#### **Figure 2.**

*Typical sheath blight disease on sheath and leaves in a rice cultivar at the booting stage in Stuttgart, Arkansas, USA.*

Sheath blight disease can be found on young rice seedlings but usually does not begin vertical development until the plant is at the reproductive growth stage. Typical symptoms are found on the lower leaf sheaths of rice plants at late tillering or the booting stage (**Figure 2**). The sheath blight disease at flag leaf often results in significant yield reduction.

Sheath blight lesions appear as circular, or ellipsoid, green-gray, water-soaked spots at about 1–3 cm long. As a lesion develops it can enlarge to 2 cm in width and 3–10 cm in length and becomes bleached with an irregular purple-brown border at the center [13]. The fungus *R. solani* grows on the surface of the leaf sheath upward and produces new side branches approximately at 45- and 90-degree angles 5–6 mm from the growing tips of mycelia. The continued growing of side branches results in the formation of an infection cushion that is attached to the epidermis often with mucilage like materials. Penetration peg is then formed from the flattened cells at the base of the cushion. The penetration peg then penetrates the inner epidermal cells to obtain nutrients from the inner and later the outer epidermal cells of the leaf sheath. Within approximately, 48 hrs after inoculation new infection hyphae is produced in an epidermal cell lumen [22, 23]. During infection *R. solani* also produces toxin [24]. Such toxins are called host specific toxins and are pathogenicity factors [25]. Toxins are known to be toxic to plants by inhibiting host defense responses.
