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EGY1 plays a role in regulation of endodermal plastid size and number that are involved in ethylene-dependent gravitropism of light-grown Arabidopsis hypocotyls

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Abstract

Egy1 was isolated as an ethylene-dependent gravitropism-deficient Arabidopsis mutant. Molecular studies reveal that EGY1 gene encodes a 59-kDa plastid-targeted metalloprotease. It is actively expressed in hypocotyl tissue and targets to endodermal and cortex plastid. Its protein level is up-regulated by both ethylene and light. CAB protein accumulation and chlorophyll level is severely reduced in hypocotyls and endodermal cells, respectively. Sucrose is able to restore the severely reduced starch and lipid contents as well as the deficient endodermal plastid size found in light-grown egy1 hypocotyls yet it fails to rescue the reduced plastid number and chlorophyll level in egy1 endodermal cells. The loss-of-function egy1 mutation results in a smaller size (1.9 ± 0.3 μm in diameter) and less number (5 ± 1) of plastids in endodermal cells, which are nearly 50% of the wild-type. EGY1 is specially required for the development of full-size endodermal plastid in seedlings that are grown on sucrose-free media under light. It plays a direct role in controlling the light-induced chlorophyll production, grana formation and plastid replication in endodermal cell. However, it plays an indirect role in regulation of endodermal plastid size. It is likely that the ethylene-dependent gravitropism-deficient phenotype of egy1 hypocotyls may result from the smaller size and less number of endodermal plastids. Gravicurvature assays performed on ethylene-insensitive mutants, etr1-1, etr2-1, ers2-1, ein4-1 and ein2-5, have clearly demonstrated the necessary role for ethylene in vigorous gravitropism of light-grown hypocotyls. The degree of ethylene-dependent gravicurvature is positively correlated with the combined state of endodermal plastid mass and number. Neither ethylene nor EGY1-regulated full-size endodermal plastid is sufficient for promotion of vigorous hypocotyl gravitropism. Presence of 4 full-size plastids per endodermal cell together with ethylene pretreatment of hypocotyls becomes sufficient to trigger vigorous gravicurvature in light-grown seedlings. A model is therefore proposed to address the role of EGY1 in regulation of endodermal plastid size and number as well as the stimulatory effect of ethylene on hypocotyl gravitropism.

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Acknowledgements

Authors like to express sincere thanks to Drs. Chentao LIN, Joseph Ecker and Elliot Meyerowitz for providing Arabidopsis mutants. The authors also like to extend their gratitude to Drs. Zhenyu CHEN and Ying XIAO for offering the GC columns and the useful instructions in analysis of lipid and many thanks to Ms. Kaying MA for her help in management of GC machine. This work was supported by grants, DAG04/05.SC08, DAG05/06.SC09, HKUST6105/01M, HKUST6276/03M and HKUST6413/06M awarded to Ning LI from the Research Grant Council of Hong Kong.

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Authors and Affiliations

  1. Department of Biology, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong SAR, China

    Di Guo, Xiaorong Gao, Hao Li, Gu Chen, Pingbo Huang & Ning Li

  2. Atmospheric, Marine and Coastal Environment (AMCE) Program, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong SAR, China

    Tao Zhang & Ning Li

  3. Department of Bioscience and Biotechnology, Dalian University of Technology, DaLian, Liao Ning, China, 116024

    Lijia An

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  1. Di Guo
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  2. Xiaorong Gao
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Correspondence to Ning Li.

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Guo, D., Gao, X., Li, H. et al. EGY1 plays a role in regulation of endodermal plastid size and number that are involved in ethylene-dependent gravitropism of light-grown Arabidopsis hypocotyls. Plant Mol Biol 66, 345–360 (2008). https://doi.org/10.1007/s11103-007-9273-5

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