Plants have evolved morphological,physiological,and molecular adaptations to respond to and overcome environmental fluctuations.While these fluctuations are sensed locally,coordinated responses occur at the whole plant level,necessitating specific and tightly controlled long-distance signaling mechanisms.These involve sugars,RNAs,secreted peptides,and plant hormones as signaling molecules.Small RNAs have emerged as ma jor long-distance signaling molecules in mediating stress response,in particular to nutrient deficiency.For example,anumber of microRNAs (miRNAs) that are induced in response to nutrient deficiency and transported through the phloem are known,suggesting that they might coordinate responses between the shoot and the root (Pant et al.,2009).Qualities that make small RNAs to be good signaling molecules include their high degree of specificity,rapid and direct mode of action,and the ability to exert a gradient response.Small RNAs are a chemically and structurally distinct class of molecules with distinct mechanisms for movement and regulation,thereby enabling another mode of action for long-distance signaling.However,little is known about what makes an miRNA mobile.
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