Chen Hao, Xiong Liming
Donald Danforth Plant Science Center, St. Louis, MO 63132, USA.
Planta. 2009 May;229(6):1303-10. doi: 10.1007/s00425-009-0912-8. Epub 2009 Mar 22.
The phytohormone auxin regulates many aspects of plant growth and development. Auxin often acts distantly from the site of its biosynthesis and this long-distance-transported auxin is well known to play a critical role in eliciting physiological responses including regulating root development. Auxin can be produced in roots, yet the function of locally synthesized auxin in root growth is unclear. The major auxin in plants, indole 3-acetic acid (IAA), is mainly synthesized through tryptophan (Trp)-dependent pathways that require pyridoxal phosphate (an active form of vitamin B(6))-dependent enzymes. We previously reported that the Arabidopsis vitamin B(6) biosynthesis mutant pdx1 has stunted root growth although the underlying cause is unknown. Here we showed that the pdx1 root is deficient in auxin biosynthesis. By reciprocal grafting of pdx1 and the wild type, we demonstrated that the stunted root growth in pdx1 is caused by a locally generated signal(s) in roots. To test whether auxin might be one such signal, the auxin responsive DR5::GUS reporter was introduced into the mutant. The DR5::GUS activity in pdx1 root tips was greatly reduced compared with that in the wild type although the auxin response was unaltered. pdx1 also suppresses the root hair growth defects in the auxin overproduction mutant yucca. These data indicate that pdx1 is impaired in Trp-dependent auxin biosynthesis, which may contribute to the short-root phenotype of pdx1. We suggest that locally synthesized auxin may play a critical role in postembryonic root growth.
植物激素生长素调控植物生长发育的诸多方面。生长素通常在远离其生物合成位点的部位发挥作用，这种长距离运输的生长素在引发包括调节根系发育在内的生理反应中起着关键作用，这一点广为人知。生长素可在根中产生，然而局部合成的生长素在根生长中的功能尚不清楚。植物中的主要生长素吲哚 - 3 - 乙酸（IAA）主要通过依赖色氨酸（Trp）的途径合成，这些途径需要磷酸吡哆醛（维生素B6的一种活性形式）依赖的酶。我们之前报道过拟南芥维生素B6生物合成突变体pdx1的根生长受阻，但其根本原因尚不清楚。在此我们表明pdx1根中的生长素生物合成存在缺陷。通过pdx1与野生型的相互嫁接，我们证明pdx1中根生长受阻是由根中局部产生的信号所致。为了测试生长素是否可能是这样一种信号，将生长素响应性DR5::GUS报告基因导入该突变体。尽管生长素响应未改变，但与野生型相比，pdx1根尖中的DR5::GUS活性大幅降低。pdx1还抑制了生长素过量产生突变体yucca中的根毛生长缺陷。这些数据表明pdx1在依赖色氨酸的生长素生物合成中受损，这可能导致了pdx1的短根表型。我们认为局部合成的生长素可能在胚胎后根生长中起关键作用。
