La cebada
Es una planta de la familia de las poaceas. Es un cereal, al igual que el arroz, la importancia de la cebada en la agricultura ha sido y sigue siendo enorme. Existen muchas variedades de cebadas, las cuales presentan tallos huecos en forma de caña que nacen de raíces fasciluladas. Al final de cada tallo, se desarrolla una inflorescencia en forma de espiga donde donde se formaran los granos.
Tipos de cebada:
*cebada de dos carreras o cervecera: es aquella en que, después de madurar la espiga, queda solo la espiguilla central, es la mas antigua.
*cebada de seis carreras o caballar: es aquella que se mantienen las tres espiguillas, son las modernas.
*cebada de cuatro carreras: es aquella que se mantiene las dos espiguillas laterales después de desaparecer la central.

*cebada con la semilla protegida: son aquellas en que la semilla esta cubierta por la lema y la palea, se utiliza para la fabricación de la cerveza o en el consumo animal.
*cebada con la semilla desnuda:son aquellas en que la semilla no esta cubierta por la lema y la palea, se utiliza en la fabricación de productos del consumo humano.
Orígenes
Los primeros restos arqueológicos del uso de la cebada silvestre como alimento humano se sitúan hace 21000 años en el poblado neolítico de ohalo II. Los restos de este desplazamiento muestran con claridad que la agricultura y la ganadería se basan en el sustento de la cebada.
Usos:
*alimento o bebida para el hombre: se usa en la fabricación del pan, solo o mezclado con otros cereales, también se pueden fabricar bebidas alcohólicas como la cerveza, la malta, wiski, vino y sustituto del café. También se obtiene el agua cebada.
*alimento para los animales: se utiliza directamente como grano, constituye el cereal principal utilizado para alimentar animales.
*planta medicinal: se usa en el tratamiento del colesterol, diabetes dolor de vientre, diarrea, entre otros.
*planta de jardinería: algunas variedades de cebada se usan en la jardinería. Cross-talk

nature, however, the plant encounters stress combinations concurrently or separated temporally and must present an integrated response to them. In the case of phytochrome signalling, the two pathways leading to red-light-induced CHS and CAB gene expression negatively regulate flux through one another1,2. Seemingly separate abiotic stress signalling pathways are also likely to interact in a similar manner. In addition, several abiotic stress pathways share common elementsthat are potential ‘nodes’ for cross-talk. Cross-talk can also occur between pathways in different organs of the plant when a systemic signal such as hydrogen peroxide moves from a stimulated cell into another tissue to elicit a response3.
Specificity

When stress signalling pathways are examined in the laboratory, they are usually considered in isolation from other stresses to simplify interpretation. In

In spite of considerable overlap between many abiotic stress signalling pathways, there might, in some instances, be a benefit to producing specific, inducible and appropriate responses that result in a specific change suited to the particular stress conditions encountered. One advantage would be to avoid the high energy cost of producing stress-tolerance proteins, exemplified by the dwarf phenotype of plants constitutively overexpressing the frost tolerance protein DREB1A (Ref. 4). In some cases, the signal transduction pathways triggered by different stresses are common to more than one stress type. One possible reason for this is that, under certain conditions, the two stresses cannot be distinguished from one another. Alternatively, each stress might require the same protective action (or at least some common elements). The discovery of separate sensing mechanisms for each stress would invalidate the first suggestion but the second is true in several cases. For example, dehydration protection is required in plants undergoing either freezing or drought and the production of antioxidants and scavenging enzymes (e.g.catalase and peroxidases) that protect against oxidative damage affords protection against a variety of different abiotic (and biological) stresses5. Most abiotic stresses tested have been shown to elicit rises in cytosolic free calcium levels ([Ca2+]cyt) and to involve protein phosphatases and kinases [including mitogen-activated protein kinase (MAPK) cascades]. However, are any of these components truly specific to one stress and which of them are ‘nodes’ at which cross-talk occurs? In the following sections, we consider different classes of signalling component in turn, and examine their potential

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Review

TRENDS in Plant Science Vol.6 No.6 June 2001

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(a)

Stimulus 1 A B C D

Stimulus 2 W X Y Z Response

these molecules themselves have the potential to encode specificity of response. An early event in the response to many different environmental stresses is an elevation in [Ca2+]cyt (Refs 7,8), which is thought to be the primary stimulus-sensing event for several stresses (e.g. cold)9–11. If this is the case then mechanisms could exist for encoding the information that relates to the particular stress through the calcium signature (see below). Alternatively, the stress might be sensed through other components either in parallel to or upstream of Ca2+ in the pathway. It has been postulated that cold is sensed via changes in membrane fluidity12 and cytoskeletal reorganization13affecting calcium channels.
(iii) Stimulus Stimulus A B +

(b) Stimulus
A

(i) Stimulus B

(ii) Stimulus Stimulus A B –

Calcium

(c)

(i) Stimulus 1

(ii) Stimulus 1 Stimulus 2

Signalling component

Response X

Response X

Response Y
TRENDS in Plant Science

Fig. 1. Cross-talk in signalling pathways. (a) Two different stimuli (1 and 2) evoke the same end response via different signalling pathways, using different signalling intermediates (A–D and W–Z, respectively). (b) Positive and negative reciprocal control. Two different stimuli (A and B) activate two signalling pathways (broken arrows), leading to different end responses. (i) Pathways operating totally independently of each other. (ii) Flux through the stimulus-A-mediated pathway negatively regulates the stimulus-B-mediated pathway and inhibits its flux. An example of this is in phytochrome-mediated expression of a chlorophyll a/b binding protein gene (CAB) and a chalcone synthase gene (CHS) by independent pathways, each negatively regulati