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

拟南芥ACAULIS 5（ACL 5）基因的破坏导致严重的矮化表型。为了阐明ACL 5介导的调节途径的茎生长，我们分离出四个抑制无茎（囊）突变体，逆转矮化表型。结果表明，sac 51-d突变破坏了SAC 51的一个短的上游开放阅读框（uORF），该uORF编码一个转录因子，表明ACL 5在SAC 51的翻译激活中起作用。为了检验ACL 5是否编码精胺合酶或热精胺合酶，我们试图检测植物提取物中的这些多胺。结果表明，热精胺在野生型幼苗中检测到，但在ac 15 -1突变体中检测不到。我们进一步研究了外源施用这些异构体对ac 15 -1生长的影响。在茎尖每天施用热精胺可部分挽救ac 15 -1的矮化表型，而精胺对突变体的形态没有影响。ac 15 -1幼苗中的ac 15 -1转录水平远高于野生型幼苗中的ACL 5转录水平，被发现被热精胺降低。因此，热精胺是通过ACL 5的作用产生的，是拟南芥茎伸长所必需的。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下具有保护作用。这些结果对了解多胺在植物生长发育中的作用具有重要意义。

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.