Kim Jae-Yean, Yuan Zhuang, Jackson David
Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724, USA.
Development. 2003 Sep;130(18):4351-62. doi: 10.1242/dev.00618.
Intercellular communication delivers critical information for position-dependent specification of cell fate. In plants, a novel mechanism for cell-to-cell communication involves the intercellular trafficking of regulatory proteins and mRNAs. The maize KNOTTED1 (KN1) gene acts non cell-autonomously in the maize leaf, and KN1 was the first plant protein shown to traffic cell-to-cell, presumably through plasmodesmata. We have compared the intercellular trafficking of green fluorescent protein (GFP) fusions of KN1 and Arabidopsis KN1-related homeobox proteins to that of the viral movement protein from turnip vein clearing tobamovirus. We show that there is specific developmental regulation of GFP approximately KN1 trafficking. GFP -- KN1 was able to traffic from the inner layers of the leaf to the epidermis, but not in the opposite direction, from epidermis to mesophyll. However, GFP or the GFP -- movement protein fusion moved readily out of the epidermis. GFP -- KN1 was however able to traffic out of the epidermal (L1) layer in the shoot apical meristem, indicating that KN1 movement out of the L1 was developmentally regulated. GFP -- KNAT1/BREVIPEDICELLUS and GFP -- SHOOTMERISTEMLESS fusions could also traffic from the L1 to the L2/L3 layers of the meristem. In a test for the functional significance of trafficking, we showed that L1-specific expression of KN1 or of KNAT1 was able to partially complement the strong shootmeristemless-11 (stm-11) mutant. However, a cell-autonomous GUS fusion to KN1 showed neither trafficking ability nor complementation of stm-11 when expressed in the L1. These results suggest that the activity of KN1 and related homeobox proteins is maintained following intercellular trafficking, and that trafficking may be required for their normal developmental function.
细胞间通讯传递对于细胞命运的位置依赖性特化至关重要的信息。在植物中，一种新的细胞间通讯机制涉及调节蛋白和mRNA的细胞间运输。玉米的KNOTTED1（KN1）基因在玉米叶片中具有非细胞自主性作用，并且KN1是首个被证明能在细胞间运输的植物蛋白，推测是通过胞间连丝进行运输。我们已将KN1和拟南芥KN1相关同源框蛋白的绿色荧光蛋白（GFP）融合体的细胞间运输与来自芜菁脉明烟草花叶病毒的病毒运动蛋白的细胞间运输进行了比较。我们发现GFP-KN1的运输存在特定的发育调控。GFP-KN1能够从叶片内层运输到表皮，但不能从表皮运输到叶肉，即不能反向运输。然而，GFP或GFP-运动蛋白融合体能够轻易地从表皮运出。不过，GFP-KN1能够从茎尖分生组织的表皮（L1）层运出，这表明KN1从L1层运出受到发育调控。GFP-KNAT1/短柄（BREVIPEDICELLUS）和GFP-无茎分生组织（SHOOTMERISTEMLESS）融合体也能够从分生组织的L1层运输到L2/L3层。在一项关于运输功能意义的测试中，我们表明KN1或KNAT1在L1层的特异性表达能够部分互补强无茎分生组织-11（stm-11）突变体。然而，当在L1层中表达时，与KN1融合的细胞自主性GUS既没有运输能力也不能互补stm-11。这些结果表明，KN1和相关同源框蛋白在细胞间运输后仍能保持活性，并且运输可能是它们正常发育功能所必需的。
