A homeobox gene with potential developmental control function in the meristem of the conifer Picea abies

Many homeobox genes control essential developmental processes in animals and plants. In this report, we describe the first cDNA corresponding to a homeobox gene isolated from a gymnosperm, the HBK1 gene from the conifer Picea abies (L.) Karst (Norway spruce). The sequence shows distinct similarities specifically to the KNOX (knotted-like homeobox) class of homeobox genes known from different angiosperm plants. The deduced amino acid sequence of HBK1 is strikingly similar within the homeodomain (84% identical) to the maize gene Knotted1 (Kn1), which acts to regulate cell differentiation in the shoot meristem. This similarity suggested that the phylogenetic association of HBK1 with the KNOX genes might be coupled to a conservation of gene function. In support of this suggestion, we have found HBK1 to be expressed in the apical meristem in the central population of nondifferentiated stem cells, but not in organ primordia developing at the flanks of the meristem. This pattern of expression is similar to that of Kn1 in the maize meristem. We show further that HBK1, when expressed ectopically in transgenic Arabidopsis plants, causes aberrations in leaf development that are similar to the effects of ectopic expression of angiosperm KNOX genes on Arabidopsis development. Taken together, these data suggest that HBK1 has a role, similar to the KNOX genes in angiosperms, in the control of cellular differentiation in the apical meristem of spruce. The data also indicate that KNOX-gene regulation of vegetative development is an ancient feature of seed plants that was present in the last common ancestor of conifers and angiosperms.

同源框（homeobox）基因广泛调控动植物体内的关键发育进程。本研究首次报道了从裸子植物中分离得到的首个同源框基因的互补DNA（cDNA）序列，该序列来自针叶植物欧洲云杉（Picea abies (L.) Karst，挪威云杉）的HBK1基因。该序列尤其与已在多种被子植物中报道的KNOX（类knotted同源框，knotted-like homeobox）类同源框基因具有显著的序列相似性。HBK1的推定氨基酸序列在同源结构域（homeodomain）区域与玉米Knotted1（Kn1）基因具有极高的相似性，同源结构域一致性达84%；而Kn1基因的功能是调控茎顶端分生组织的细胞分化。这种序列相似性提示，HBK1与KNOX基因的系统发育关联，可能伴随着基因功能的保守性。为验证这一推测，我们发现HBK1仅在顶端分生组织中央区域的未分化干细胞群中表达，而不在分生组织侧翼发育的器官原基中表达。这种表达模式与玉米Kn1基因在分生组织中的表达模式一致。进一步研究表明，HBK1在转基因拟南芥（Arabidopsis）中异位表达时，会引发叶片发育异常，这与被子植物KNOX基因在拟南芥中异位表达所产生的发育异常表型相似。综合以上结果，HBK1与被子植物的KNOX基因功能类似，均参与调控云杉顶端分生组织的细胞分化过程。本研究数据同时表明，KNOX基因对营养发育的调控功能是种子植物的古老特征，这一特征早在针叶植物与被子植物的最近共同祖先中就已存在。