The ER-localized aquaporin SIP2;1 is involved in pollen germination and pollen tube elongation in Arabidopsis thaliana
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The ER membrane localized aquaporin SIP2;1 is involved in adaptation to ER stresses during pollen tube elongation.
Aquaporins play multifaceted roles through selective transport of water and small neutral substrates. Here, we focused on the physiological roles of Arabidopsis thaliana aquaporins, namely SIP1;1, SIP1;2 and SIP2;1, which are localized to the endoplasmic reticulum (ER). While their loss-of-function mutants displayed normal vegetative growth. We identified defects in pollen of sip2;1. Whereas the germination rate of sip2;1 pollen was ~ 60% that of the wild type (WT), in vitro germinated sip2;1 pollen tube length was reduced up to 82% compared to the WT. Importantly, most pollen tubes on pistils from sip2;1 stopped elongation in the mid-region of pistils, and the bottom region of sip2;1 siliques lacked seeds. Consistently, silique of sip2;1 were short, whereby the average seed number per silique was nearly the half of the WT. The above phenotypes recovered in SIP2;1 complementation lines. We detected mRNA of SIP2;1 and protein in pollen, and further revealed that the GFP-linked SIP2;1 localization in the ER of growing pollen tubes. The basal mRNA level of BINDING PROTEIN 3 (BiP3), a key gene induced by ER stress, in pollen was markedly higher than that in roots, suggesting that the pollen underwent ER stress under normal growth conditions. BiP3 mRNA was dramatically increased in sip2;1 pollen. Altogether, our findings suggest that the aquaporin SIP2;1 is probably involved in the alleviation of ER stress and that the lack of SIP2;1 reduces both pollen germination and pollen tube elongation.
KeywordsAquaporin SIP2;1 Arabidopsis ER stress Pollen germination Pollen tube elongation
Confocal laser-scanning microscopy
Green fluorescent protein
Small and basic intrinsic protein
We are grateful to Rie Sakakibara, Kyosuke Miyamoto, and Ayako Tsuchihira (Nagoya University, Japan) for their contribution at the early stage of the study on SIPs, Yoichi Nakanishi, Miki Kawachi, Shoji Segami, Takashi Fujiwara, Sumie Ishiguro and Natsuki Tanaka-Takada (Nagoya University, Japan) for valuable advice and Ali Ferjani for critical reading of the manuscript.
RS and MM designed the experiments and wrote and finalized the manuscript. RS established the knockout mutant lines and the complementation lines, and conducted the phenotyping and construction of SIP2;1-GFP, microscopic observations, quantification of mRNAs, and physiological analyses.
This work was supported by the Japan Society for the Promotion of Science [KAKENHI, grant numbers 26252011 and 26113506 to M.M.].
Compliance with ethical standards
Conflict of interest
The authors have no conflicts of interest to declare.
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