Abstract
Gravity is a critical environmental factor regulating directional growth and morphogenesis in plants, and gravitropism is the process by which plants perceive and respond to the gravity vector. The cytoskeleton is proposed to play important roles in gravitropism, but the underlying mechanisms are obscure. Here we use genetic screening in Physcomitrella patens, to identify a locus GTRC, that when mutated, reverses the direction of protonemal gravitropism. GTRC encodes a processive minus-end-directed KCHb kinesin, and its N-terminal, C-terminal and motor domains are all essential for transducing the gravity signal. Chimeric analysis between GTRC/KCHb and KCHa reveal a unique role for the N-terminus of GTRC in gravitropism. Further study shows that gravity-triggered normal asymmetric distribution of actin filaments in the tip of protonema is dependent on GTRC. Thus, our work identifies a microtubule-based cellular motor that determines the direction of plant gravitropism via mediating the asymmetric distribution of actin filaments.
Highlights
Gravity is a critical environmental factor regulating directional growth and morphogenesis in plants, and gravitropism is the process by which plants perceive and respond to the gravity vector
Our analysis showed that nuclear localization in the gtrC-16 mutant was the same as in wild type (Supplementary Fig. 4), which indicated that the role of GTRC/KCHb in gravitropism was not related to nuclear positioning
The cytoskeleton has long been considered to be involved in gravitropism and most previous studies have focused on the actin filaments, the role of actin being believed to affect the sedimentation of amyloplasts in angiosperms[7,8,9,10]
Summary
Gravity is a critical environmental factor regulating directional growth and morphogenesis in plants, and gravitropism is the process by which plants perceive and respond to the gravity vector. Our work identifies a microtubule-based cellular motor that determines the direction of plant gravitropism via mediating the asymmetric distribution of actin filaments. The microtubular cytoskeleton is an architectural feature of all eukaryotic cells and directly contacts many organelles that may play important roles in gravity signaling. Observations on the transient reversal of gravitropic growth by moss protonemal apical cells during mitosis implicate the possible involvement of microtubules in gravitropism[17,18]. We use several newly screened mutants, as well as a mutant gtrC-5 (gravitropism group C) that was obtained via mutagenesis more than 30 years ago but with the corresponding gene not yet characterized[24,25], to identify the molecular nature of GTRC as a microtubule-based motor and to functionally analyze its roles in gravitropism
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