Abstract
Rubber tree (Hevea brasiliensis) latex, the source of natural rubber, is synthesised in the cytoplasm of laticifers. Efficient water inflow into laticifers is crucial for latex flow and production since it is the determinant of the total solid content of latex and its fluidity after tapping. As the mature laticifer vessel rings are devoid of plasmodesmata, water exchange between laticifers and surrounding cells is believed to be governed by plasma membrane intrinsic proteins (PIPs). To identify the most important PIP aquaporin in the water balance of laticifers, the transcriptional profiles of ten-latex-expressed PIPs were analysed. One of the most abundant transcripts, designated HbPIP2;3, was characterised in this study. When tested in Xenopus laevis oocytes HbPIP2;3 showed a high efficiency in increasing plasmalemma water conductance. Expression analysis indicated that the HbPIP2;3 gene was preferentially expressed in latex, and the transcripts were up-regulated by both wounding and exogenously applied Ethrel (a commonly-used ethylene releaser). Although regular tapping up-regulated the expression of HbPIP2;3 during the first few tappings of the virginal rubber trees, the transcriptional kinetics of HbPIP2;3 to Ethrel stimulation in the regularly tapped tree exhibited a similar pattern to that of the previously reported HbPIP2;1 in the virginal rubber trees. Furthermore, the mRNA level of HbPIP2;3 was associated with clonal yield potential and the Ethrel stimulation response. Together, these results have revealed the central regulatory role of HbPIP2;3 in laticifer water balance and ethylene stimulation of latex production in Hevea.
Highlights
Natural rubber, an indispensable industrial raw material for producing high performance rubber products, is formed from the latex harvested through tapping the perennial tropical rubber tree
In order to identify the transcripts expressed in laticifers and determine the most important plasma membrane intrinsic proteins (PIPs) in water balance of laticifers, the latex transcriptome of rubber tree clone RRIM928 was analysed
By comparing wounding alone with Ethrel stimulation alone on the virginal rubber trees where there was no tapping induced sink effect superposition, we found that HbPIP2;3 was up-regulated by both wounding and Ethrel stimulation alone
Summary
An indispensable industrial raw material for producing high performance rubber products, is formed from the latex harvested through tapping the perennial tropical rubber tree Any conditions favouring water supply of a rubber tree, especially water movement into phloem, could contribute to latex flow and production. The transmembrane water movement into cells was previously explained only by simple diffusion [5]. Since it could not explain many phenomena, such as the high velocity of water exchange, the remarkable variations between different cell types and the transient change of water permeability subjected to the stimulation of some reagents [5], the existence of hydrophilic pores within biological membranes was proposed [6]. Following the identification and characterisation of the channel forming integral protein (CHIP28), an aquaporin from human erythrocytes [7, 8], our knowledge on molecular basis of transmembrane water flow has significantly increased [9]. Numerous aquaporin genes have been cloned and functionally confirmed to play pivotal roles in plant transmembrane water transport [9, 10] and whole plant water balance [11,12,13,14]
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