Abstract
AtKAT1 plays roles as a major channel to uptake K+ in guard cell when stomata open in dicot model plant Arabidopsis. In a recent publication, we isolated 3 KAT-like potassium channels in rice. We expressed them in CHO cell to identify electrophysiological characteristics of the channels. OsKAT2 showed much bigger inwardly rectifying potassium channel activities among them. The histochemical X-glu staining of transgenic rice leaf blades expressing β-glucuronidase fused with OsKAT2 promoter showed that the OsKAT2 is dominantly expressed in rice guard cell. These findings indicate that OsKAT2 may be a functional ortholog of AtKAT1 in rice. Thus this gene will be the prime target for engineering the guard cell movement to improve drought tolerance in monocot plants, including most major crops.
Keywords: OsKAT, shaker like potassium channel, stomatal movement, guard cell, inward rectifier potassium channel
First cloned plant shaker-like potassium channel, AtKAT1 plays a major role in potassium uptake in guard cell and stomata opening.1,2 After then, 9 members of shaker-like potassium channels were identified functionally and classified into 4 groups in Arabidopsis.3 In higher plants, several shaker-like potassium channels were also reported.4-9 In our recent publication, 11 shaker-like potassium channels were predicted in rice by using homology search analysis. Among them, three genes are likely AtKAT1 homologs, and those are named as OsKAT1, OsKAT2, and OsKAT3.10 OsKAT1 was already identified to enhance the salt stress tolerance when it is constitutively overexpressed.5 However, functional analysis of those genes as well as the electrophysiological characteristics of the channels has not been conducted yet. Those genes have very similar amino acid sequences except that OsKAT1 has shorter C-terminal region compared with others.10 Thus, it might not be possible to predict the functional orthologs among them based on the amino acid homology with Arabidopsis atKAT1 and atKAT2. To identify the electrophysiological characteristics of OsKATs we conducted heterologous expressions of OsKAT1, OsKAT2, and OsKAT3 in CHO cell. OsKAT3 did not show any current but OsKAT2 and OsKAT1 showed inward rectifying channel. OsKAT2 has much bigger current than OsKAT1 does in recent.10 In this study we showed that the current of OsKAT2 increased proportionally to K+ concentration of external solution and OsKAT2 are activated voltage-dependently showing typical inward rectifying shaker-like potassium channel activity.10 The potassium channel blocker Ba2+ can block the potassium uptake current of OsKAT2 but cannot completely inhibited.10 Taken together, OsKAT2 showed the typical inward rectifying shaker-like potassium channel characteristics such as AtKAT1.2 Also the expression of those genes were identified that OsKAT2 are dominantly expressed in guard cell.10 The cross-section of the leaf blade histochemically stained showed that OsKAT2 promoter is able to drive mainly β-glucuronidase gene expression in guard cell and a little in vascular bundle (Fig. 1). Taken together, the OsKAT2 might be the prevailing inward rectifier potassium channel that functions in stomata opening in rice. It might be the functional ortholog of AtKAT1 in rice. These studies would give the valuable information to identify the potassium channels regulating the stomata movement in monocot plants, including most major crops.
Figure 1. The OsKAT2 promoters mediate strong reporter GUS expression in guard cell of leaf blade of transgenic rice seedlings. Left panel: 3 µm-thick-cross section of the leaf blade showing strong GUS staining in guard cell and weak staining in vascular tissues of OsKAT2 promoter:: GUS transgenic rice. Right panel: Guard cells exhibited strong GUS staining in guard cell and weak staining in vascular tissues of leaf blade.
Acknowledgments
This work was supported by the grants from Research Program for Agricultural Science and Technology Development (Project No. PJ008598) of National Academy of Agricultural Science and the Next-Generation BioGreen 21 Program (Plant Molecular Breeding Center, PJ00802107) of the Rural Development Administration of Korea.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
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