Abstract
During early periods of salt stress, reduced stomatal opening can prevent water loss and wilting. Abscisic acid (ABA) signal plays an important role in this process. Here, we show that cucumber grafted onto pumpkin exhibits rapid stomatal closure, which helps plants to adapt to osmotic stress caused by salinity. Increased ABA contents in the roots, xylem sap, and leaves were evaluated in two grafting combinations (self-grafted cucumber and cucumber grafted onto pumpkin rootstock). The expression levels of ABA biosynthetic or signaling related genes NCED2 (9-cis-epoxycarotenoid dioxygenase gene 2), ABCG22 (ATP-binding cassette transporter genes 22), PP2C (type-2C protein phosphatases), and SnRK2.1 (sucrose non-fermenting 1-related protein kinases 2) were investigated. Results showed that a root-sourced ABA signal led to decreased stomatal opening and transpiration in the plants grafted onto pumpkin. Furthermore, plants grafted onto pumpkin had increased sensitivity to ABA, compared with self-grafted cucumbers. The inhibition of ABA biosynthesis with fluridon in roots increased the transpiration rate (Tr) and stomatal conductance (Gs) in the leaves. Our study demonstrated that the roots of pumpkin increases the sensitivity of the scion to ABA delivered from the roots to the shoots, and enhances osmotic tolerance under NaCl stress. Such a mechanism can be greatly exploited to benefit vegetable production particularly in semiarid saline regions.
Highlights
Salinity is one of the major limitations of crop production worldwide
Through Abscisic acid (ABA) biosynthesis inhibitor experiments, we showed that root sourced ABA production confers salt tolerance on grafted cucumber plants by triggering stomatal closure within the first few hours of salt stress and optimizing plant ionic and water balance
Consistent with Photosynthetic rate (Pn), transpiration rate (Tr) and Gs showed a similar decline in the early period of salt treatment, but their responses were faster than the response of Pn
Summary
Salinity is one of the major limitations of crop production worldwide. More than 800 million hectares of agricultural land suffer from soil salinity (Rengasamy, 2010). Plant growth responds to salt stress in two phases, namely, the osmotic phase that inhibits water uptake by roots due to osmotic pressure in soil and subsequent ionic phase, during which the level of accumulated toxic ions in plants exceeds the threshold level and leads to ion toxicity or ion imbalance (Munns and Tester, 2008). Stomatal conductance is a reliable indicator for evaluating plant salt tolerance Using this indicator, Rahnama et al (2010) found a positive correlation between relative stomatal conductance and relative growth rate in durum wheat. This correlation indicates that a plant that can hold its stomata opening has good salt tolerance. Stomatal closure in the early periods of salinity stress has the same function as that in drought conditions (Munns, 2011)
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