The Role of Microtubule Associated Proteins in Plant Hypocotyl Elongation

Abstract

IntroductionHypocotyl is the part of a plant embryo or seedling plant that is between the cotyledons and the radicle or root. The regulation of hypocotyls elongation is crucial in the process of plant growth and development, because after the seeds germination, hypocotyls fully elongate to search of the soil surface and percept light then significantly inhibits hypocotyls elongation, the cotyledons unfold and the photosynthetic growth process could successfully begin. The Arabidopsis hypocotyls is widely used as a model for studies of cell elongation as hypocotyls cells elongate quickly without division during the postembryonic growth process [1,2]. Hypocotyl cells growth in the darkness start their elongation at the base and proceed in the acropetal direction reach their maximum length. The elongation of hypocotyls cells is regulated by both external and internal cues such as light, phytohormones, transcription factors, and the cytoskeleton [3][4][5].Microtubule (MT) is one of the main members of cytoskeleton and plays important role in regulating cell expansion, division, and plant cell morphogenesis. Cortical microtubules regulate cell elongation by orientating cellulose fibrils and cellulose fibril arrays, thereby influencing the mechanical properties of the cell wall [6][7][8][9]. The orientations of cortical microtubules are associated with the growth status of etiolated hypocotyls [10,11]. The parallel array of cortical microtubules is dominantly transversely oriented to the hypocotyls longitudinal growth axis in elongating hypocotyls cells and longitudinally oriented when cell elongation stops [10], suggesting the regulation of the organization and dynamics of cortical microtubules is crucial for hypocotyls cell growth. The organization and dynamics of cortical microtubules is regulated by microtubule associated proteins (MAPs) [12]. In this article, plants MAPs involve in hypocotyls elongation are classified, on the basis of their function, into two categories: one is positive regulator and the other is negative regulator in the elongation of hypocotyls. In addition, we also discuss how these proteins mediate the synergistic and antagonistic effects in hypocotyls elongation. MAPs Play Positive Roles In Hypocotyl ElongationUntil now, several MAPs play positive roles in regulation of Hypocotyl elongation have been identified. Arabidopsis SPIRAL1 (SPR1) encodes a plant-specific MT-localizing protein [13]. Plants over expressing SPR1 shows enhanced resistance to an MTdisrupting drug and increased Hypocotyl elongation. In addition, the site of high SPR1 expression moved up wards along darkgrown hypocotyls. The temporal and spatial distribution of SPR1-expressing cells in hypocotyls generally matches the zone of rapid cell elongation [13,14]. Moreover, microtubule destabilizing protein 40 (MDP40) as a positive regulator involves in Brassinosteroid (BR) mediated Hypocotyl elongation by destabilizing cortical microtubules [15]. MDP40 is the target of Brassinazole-resistant1 (BZR1), a key transcription factor in...