A historical overview of long-distance signalling in plants

Abstract

Be it a small herb or a large tree, intra- and inter-cellular communication as well as long-distance signalling between distant organs is crucial for every aspect of plant development. Vascular system comprising xylem and phloem acts as a major conduit for transmission of long-distance signals in plants. In addition to our knowledge in vascular development, numerous reports in last two decades revealed that selective populations of RNAs, proteins and phytohormones function as mobile signals. Interestingly, many of these signals are shown to regulate diverse physiological processes like flowering, leaf and root development, nutrient acquisition, crop yield and biotic/abiotic stress responses. In this review, we summarize the significant discoveries made in the last 25 years with special emphasis on key mobile signalling molecules (mRNAs, proteins including RNA-binding proteins and small RNAs) that have revolutionized our understanding of how plants integrate various intrinsic and external cues in orchestrating growth and development. Additionally, we provide detailed insights on the emerging molecular mechanisms that possibly control the selective trafficking and delivery of phloem-mobile RNAs to target tissues. We also highlight the cross-kingdom movement of mobile signals during plant-parasite relationships. Considering the dynamic functions of these signals, their implications in crop improvement are also discussed.