Genome-wide identification of rubber tree (Hevea brasiliensis Muell. Arg.) aquaporin genes and their response to ethephon stimulation in the laticifer, a rubber-producing tissue.

Zhi Zou,Jikun Wang,Feng An,Jun Gong,Yeyong Mo,Lifu Yang,Guishui Xie

doi:10.1186/s12864-015-2152-6

Abstract

BackgroundNatural rubber, an important industrial raw material, is specifically synthesized in laticifers located inside the rubber tree (Hevea brasiliensis Muell. Arg.) trunk. Due to the absence of plasmodesmata, the laticifer water balance is mediated by aquaporins (AQPs). However, to date, the characterization of H. brasiliensis AQPs (HbAQPs) is still in its infancy.ResultsIn this study, 51 full-length AQP genes were identified from the rubber tree genome. The phylogenetic analysis assigned these AQPs to five subfamilies, including 15 plasma membrane intrinsic proteins (PIPs), 17 tonoplast intrinsic proteins (TIPs), 9 NOD26-like intrinsic proteins (NIPs), 4 small basic intrinsic proteins (SIPs) and 6 X intrinsic proteins (XIPs). Functional prediction based on the analysis of the aromatic/arginine (ar/R) selectivity filter, Froger’s positions and specificity-determining positions (SDPs) showed a remarkable difference in substrate specificity among subfamilies. Homology analysis supported the expression of 44 HbAQP genes in at least one of the examined tissues. Furthermore, deep sequencing of the laticifer transcriptome in the form of latex revealed a key role of several PIP subfamily members in the laticifer water balance, and qRT-PCR analysis showed diverse expression patterns of laticifer-expressed HbAQP genes upon ethephon treatment, a widely-used practice for the stimulation of latex yield.ConclusionsThis study provides an important genetic resource of HbAQP genes, which will be useful to improve the water use efficiency and latex yield of Hevea.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-2152-6) contains supplementary material, which is available to authorized users.

Highlights

Natural rubber, an important industrial raw material, is synthesized in laticifers located inside the rubber tree
According to the phylogenetic analysis, 51 H. brasiliensis AQPs (HbAQPs) were grouped into five subfamilies, i.e. plasma membrane intrinsic protein (PIP) (15), tonoplast intrinsic protein (TIP) (17), NOD26-like intrinsic protein (NIP) (9), small basic intrinsic protein (SIP) (4) and X intrinsic protein (XIP) (6) (Table 1; Fig. 1)
Similar effects of ethephon on latex yield and latex total solid content (TSC) of the PB217 clone were observed by Tungngoen et al, they used mature virgin trees as materials [21]. To our knowledge, this is the first genome-wide study of the rubber tree AQP gene family and using systematic nomenclature assigned 51 HbAQPs into five subfamilies based on the sequence similarity and phylogenetic relationship with their Arabidopsis and poplar counterparts. Their structural and functional properties were investigated based on the analysis of the ar/R filter, Froger’s positions and SPDs, which suggested the potentially key role of HbPIPs and HbTIPs in the laticifer water balance

Summary

Introduction

An important industrial raw material, is synthesized in laticifers located inside the rubber tree Aquaporins (AQPs), known as major intrinsic proteins (MIPs), are a class of integral membrane proteins that facilitate the passive transport of water and other small solutes across biological membranes [1]. Since their first discovery in 1990s, AQPs have been found in almost all living organisms [2]. LB and LE from opposite sides dip into the membrane and form two half helices (HB and HE), at the N-termini of which, two highly conserved NPA (Asn-Pro-Ala) motifs form one selectivity region Another region that determines the substrate specificity is known as the aromatic/arginine (ar/R) selectivity filter (H2 in TM2, H5 in TM5, LE1 and LE2 in LE) [13]. Nine specificity-determining positions (SDPs) for non-aqua substrates, i.e. urea, boric acid, silicic acid, ammonia (NH3), carbon dioxide (CO2) and hydrogen peroxide (H2O2) were proposed for each group via a comprehensive analysis of functionally characterized AQPs [15]

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Discussion

Conclusion