Submitted to: Plant Signaling and Behavior
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/25/2012
Publication Date: 5/14/2013
Citation: Lawrence, S.D., Novak, N.G., Xu, H., Cooke, J.E. 2013. Discovery of a novel aquaporin ZmPIP2-8 from southern corn rootworm infested maize. Plant Signaling and Behavior. 8:8. Interpretive Summary: Infestation of agricultural crops by pest insects significantly impacts US farmers and the economy. An understanding of how plants defend themselves from attack of pest insects will aid in the design of strategies to circumvent attack. This study examined the corn plant response to the chewing insect, corn rootworm. This insect attacks roots where most work to date has focused on aboveground plant parts. The current paradigm for the plant response to chewing insects is that the plant hormone jasmonic acid is released upon attack by chewing insects. Our work, focusing on the roots, showed that this was not the case as none of the plant root genes tested were responsive to this hormone. Consequently a broader understanding by scientists is needed to fully predict the effect of chewing insects on crops. This work will benefit scientists commercial entities that rely on plant production in understanding plant defenses against pest insects.
Technical Abstract: A common paradigm of infestation by chewing insects is a jasmonic acid (JA) cascade that results in the induction of JA responsive genes. However examination of several maize genes induced by Southern corn rootworm (SCR) infestation, an insect that chews into and significantly damages maize roots, did not result in induction of JA induced genes (Lawrence et al 2012). Perhaps ABA plays a role in induction of SCR infestation regulated genes. Generally maize TIPs and PIPs are repressed by high levels of ABA (Zhu et al 2005). In fact several aquaporins were found to be differentially expressed by SCR infestation of maize. ZmNIP1-1 was induced by SCR infestation while ZmTIP2-2 and a novel ZmPIP2-8 were generally repressed by SCR infestation. Although the infested roots had increased amounts of all 5 SCR inducible genes tested the ABA levels significantly decreased upon infestation. Although the ZmPIP2-8 and ZmTIP2-2 was repressed by exogenous application of 300 µM ABA, clearly ABA levels in infested maize roots does not explain the repression of ZmPIP2-8 and ZmTIP2-2. Therefore, ABA induction does not explain changes in maize root upon SCR infestation. Since the ZmPIP2-8 has not been described previously the amino acid sequence was analyzed in silico and found to contain all the hallmarks of an aquaporin. It is predicted to have 6 trans-membrane domains, 2 loops (B and D) on the cytosolic side and 3 loops (A, C and E) on the extra cytosolic side along with invariant NPA motifs that have been shown to be involved in the pore structure in other aquaporins. Since TIPs and PIPs function in the trans-membrane uptake of water, ZmTIP2-2 and ZmPIP2-8 cRNAs were transformed into Xenopus oocytes and the proteins assayed for their ability to take up water. Both ZmPIP2-8 and ZmTIP2-2 had significant water transport capabilities.