By Shi Yeung
Aquaporins, also known as water channels, are proteins embedded in the cell membrane that facilitate water transport across the membrane in various organisms including microorganisms, plants, and animals. Plant aquaporins are classified into subfamilies, which includes plasma membrane intrinsic proteins (PIP), tonoplast intrinsic proteins (TIP), Nod26-like intrinsic proteins (NIP), small and basic intrinsic proteins (SIP), and identified uncharacterised intrinsic proteins (XIP) (Liu et al., 2013). At least 33 aquaporin genes have been identified in the rice genome (Sakurai et al., 2005). In this article, we will be looking at the roles of specific aquaporins in seed germination and how they contribute to the drought resistance of rice.
Aquaporin PIPs (OsPIP) can be divided into PIP1 and PIP2, which PIP2 shows high water channel activity in contrast to PIP1 in Xenopus Oocyte (Liu et al., 2007). Liu et al. (2007) examined the OsPIPs expression pattern in dry seeds and during germination. Most OsPIPs expression started to increase within 24 hours, peaked before radicle emergence and declined immediately afterwards, except for OsPIP2;2 and OsPIP2;6 whose expressions were maintained during the post-germination phase. This suggests PIP expression is developmentally regulated and each type of aquaporin has its role at different stages of seed germination and plant growth.
To clarify the function of aquaporins in seed germination, the group genetically modified rice seeds to overexpress and silence OsPIP1;3. The germination rate and water uptake of OsPIP1;3 sense (overexpressed), antisense (silenced) and wild-type (non-treated) rice seeds was studied in the experiment. Sense-transgenic seeds had similar OsPIP1;3 expression compared to wild-type seeds under normal conditions and a significantly increased OsPIP1;3 expression upon water deficit. Antisense-transgenic seeds had the lowest OsPIP1;3 expression for both conditions and is lower under water-deficit condition. The germination rate and water uptake of seeds correlates to OsPIP1;3 expressions, suggesting OsPIP1;3 is required for seed germination in rice (Liu et al., 2007)
The study also hypothesised that OsPIP1;1 is required for seed germination due to OsPIP1;1 expression being partially reduced in the OsPIP1;3 antisense-transgenic seeds. Since OsPIP1 generally has low, or no water channel activity alone in Xenopus Oocytes, the group suggests that PIP1-members function as water channels via PIP interactions. The hypothesis is verified by another group of researchers who published their paper 6 years later. They found that OsPIP1;1 water channel activity was significantly increased while co-expressed with OsPIP2;1. Furthermore, they compared the germination rate of seeds with different levels of OsPIP1;1 overexpression. Interestingly, they found that a high level of overexpression of OsPIP1;1 made rice sterile, but low expression increased seed yield. The germination rate was 50% higher in the low level and middle level of OsPIP1;1 overexpression compared to the wild type at 72 hours of germination (Liu et al., 2013).
OsPIP2;3 is not expressed until radicle emergence; thus, it is suggested that OsPIP2;3 does not contribute to seed germination but plant growth (Liu et al., 2007). Further experiments regarding OsPIP2;3 was done and the hypothesis was verified (Sun et al., 2021). Similar to OsPIP1;3, OsPIP2;3 was also upregulated upon water deficit and salt stress. Polyethylene glycol (PEG) is a polymer that is commonly used to induce and control plant water deficit in experimental hydroponics culture. Under 10% PEG, the root OsPIP2;3 was found to be upregulated thirtyfold compared to the control. To study the effect of OsPIP2;3, again an overexpressed OsPIP2;3 (OE) line and silenced OsPIP2;3 (SE) line was introduced in comparison to the WT. The OE line had better rice growth, increased survival rates and reduced water loss compared to the wild type under water deficit condition. However, there was no significant difference in seed germination between the transgenic lines and wild type, which provided evidence to support the hypothesis that OsPIP2;3 does not contribute to seed germination. Another interesting finding from this paper was that even OsPIP2;3 was upregulated by salt stress, and no salt stress resistance was introduced in the overexpressed plant. There was no difference in growth and salt stress phenotype for OE and SE lines (Sun et al., 2021).
In addition to OsPIP2;3, OsPIP1;3 is also involved in drought tolerance of the plant after germination stages. Drought avoidance is one of the mechanisms that plants employ to tolerant drought stress. It is the ability to maintain relative higher tissue water content despite decreased water content in soil (Basu et al., 2016). Research studying the response of upland rice (drought tolerant) and lowland rice (flood tolerant) upon water deficit found there were different physiological responses and OsPIP1;3 expression patterns between the two types of rice. Young leaf rolling was observed only in upland rice species but not in lowland rice. And by looking at the OsPIP1;3 mRNA and protein levels, there was an increase in OsPIP1;3 expression in upland rice and a decrease in OsPIP1;3 expression in lowland rice upon water deficit, showing upland rice and lowland rice adopt different mechanisms of drought tolerance (Lian et al., 2004).
The group then introduced the SWPA2 promoter-OsPIP1;3 into lowland rice to obtain a transgenic variety of rice. The SWPA2 promoter is a stress-inducible promoter taken from sweet potatoes. The transgenic variety will thus have induced OsPIP1;3 expression upon water deficit (PEG treatment). Despite there being no phenotype difference between transgenic and wild-type rice before PEG treatment, transgenic lowland rice had higher leaf water potential than wild-type plants at 10 hours after initiation of the PEG treatment, indicating a higher capacity of water uptake in the transgenic plants caused by OsPIP1;3 overexpression (Lian et al., 2004).
To conclude, aquaporins are well studied for their role in maintaining plant water balance. However, we should take note of many factors other than aquaporins activity that contribute to specific characteristics, such as the drought tolerance we discussed in this article. Understanding aquaporins has revealed the drought tolerance mechanism behind selectively bred and transgenic rice varieties and is especially important due to the global challenge of water security.
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Sun, J. Y., Liu, X. S., Khan, I. U., Wu, X. C. & Yang, Z. M. (2021) OsPIP2;3 as an aquaporin contributes to rice resistance to water deficit but not to salt stress. Environmental and Experimental Botany. 183 (104342). Available from: doi: 10.1016/j.envexpbot.2020.104342