MOLECULAR GENETIC ANALYSIS OF THE REGULATORY MECHANISM OF POLYAMINE-DEPENDENT STEM ELONGATION IN HIGHER PLANTS

拟南芥ACAULIS5(ACL5)基因的缺失导致了严重的矮秆表型。为了阐明ACL5介导的茎生长调控途径，我们分离了四个逆转矮秆表型的无茎抑制因子(Sac)突变体。我们的结果表明，ac51-d突变破坏了SAC51的一个短上游开放阅读框(UORF)，该uORF编码一个可能的转录因子，并提示ACL5在SAC51的翻译激活中起作用。我们还发现了一个负责sa52-d的基因编码核糖体蛋白RPL10A。我们还在扁叶苔藓中发现了三个与ACL5同源的基因。此外，为了检测ACL5编码的是精胺合成酶还是温敏合酶，我们试图在植物提取物中检测这些多胺。结果表明，在野生型幼苗中检测到温敏素，但在ac15-1突变体中未检测到。我们进一步研究了外源应用这些异构体对ac15-1生长的影响。每天在顶端喷施温度敏能部分挽救ac15-1的矮秆表型，而精胺对突变体的形态没有影响。在ac15-1幼苗中的ac15-1转录水平远高于野生型幼苗中的ac15-1转录水平。另一方面，对编码精胺合成酶的SPMS突变体的生理研究表明，精胺在干旱和高盐胁迫反应中具有保护作用。这些结果应该为理解多胺在植物生长和发育中的作用提供重要的依据。

Disruption of the Arabidopsis thaliana ACAULIS5 (ACL5) gene results in a severe dwarf phenotype. To elucidate ACL5-mediated regulatory pathways of the stem growth, we isolated four suppressor of acaulis (sac) mutants that reverse the dwarf phenotype. Our results revealed that the sac51-d mutation disrupts a short upstream open reading frame (uORF) of SAC51, which encodes a putative transcription factor and suggested that ACL5 plays a role in translational activation of SAC51.A gene responsible for sac52-d was found to encode a ribosomal protein RPL10A.We also identified three genes in the moss P. patens which are homologous to ACL5.Furthermore, to examine whether ACL5 encodes spermine synthase or thermospermine synthase, we tried to detect these polyamines in plant extracts. The results revealed that thermospermine is detected in wild-type seedlings but is not detectable in the ac15-1 mutant. We further examined the effect of exogenous application of these isomers on the growth of ac15-1. Daily application of thermospermine onto the shoot apex partially rescued the dwarf phenotype of ac15-1, while that of spermine had no effects on the morphology of the mutant. The ac15-1 transcript level in ac15-1 seedlings, which is much higher than the ACL5 transcript level in wild-type seedlings, was found to be reduced by thermospermine. Thus it was concluded that thermospermine is produced through the action of ACL5 and required for stem elongation in Arabidopsis.On the other hand, physiological studies on the mutant of SPMS, which encodes spermine synthase, revealed that spermine plays a protective role in drought and high salt stress responses. These results should provide a important due in understanding the action of polyamines in plant growth and development.