Grafting Cucumber Onto Pumpkin Induced Early Stomatal Closure by Increasing ABA Sensitivity Under Salinity Conditions

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.

Keywords: abscisic acid; cucumber; grafting; rootstock; salt tolerance; stomata.

Figure 1

Time course of photosynthetic rate (A), transpiration rate (B) and stomatal conductance (C) in the leaves of pumpkin-grafted cucumber (C/P) and self-grafted cucumber (C/C) plants after 75 mM NaCl treatment. Data are presented as the mean of six biological replicates ( ± SE), points with an asterisk indicate a significant difference from the untreated control (P 0.05) at the same sampling time according to t-test.

Figure 2

Abscisic acid (ABA) accumulation in the leaves (A) and root (B) of the pumpkin-grafted cucumber (C/P) and self-grafted cucumber (C/C) after 75 mM NaCl treatment (NaCl) or non-salinity condition (Control). Data are presented as the mean of three biological replicates. ( ± SE), points with an asterisk indicate a significant difference from the untreated control (P 0.05) at the same sampling time according to t-test.

Figure 3

Effect of abscisic acid (ABA) synthesis inhibitor Flu on photosynthetic rate (A), transpiration rate (B) and stomatal conductance (C) of pumpkin-grafted cucumber (C/P) and self-grafted cucumber (C/C) after 1 h of NaCl treatment. The seedlings were pretreated with Flu on leaves (NaCl+Flu-L) or on roots (NaCl+Flu-R), respectively. Data are presented as the mean of six biological replicates ( ± SE). Different letters indicate significant differences (P 0.05) according to the Duncan’s multiple test.

Figure 4

Effect of abscisic acid (ABA) synthesis inhibitor Flu on ABA contents of the leaves (A), xylem sap (B), and roots (C) of the pumpkin-grafted cucumber (C/P) and self-grafted cucumber (C/C) plants after 3 h of NaCl treatment. Data are presented as the mean of three biological replicates ( ± SE). Different letters indicate significant differences (P 0.05) according to the Duncan’s multiple test.

Figure 5

The effect of abscisic acid (ABA) concentration on stomatal opening in detached abaxial epidermal strips of pumpkin-grafted cucumber (C/P) and self-grafted cucumber (C/C) plants. The fields are provided by a optical microscope (OLYMPUS CX31) under 100× magnification (A) and stomatal aperture are calculated with width/length (B). Data are presented as the mean of six biological replicates ( ± SE). Different letters indicate significant differences (P 0.05) according to the Duncan’s multiple test. Scale bar in Figure 5A = 10 μm.

Figure 6

Transcript abundance of abscisic acid (ABA) biosynthesis related genes [NCED2 (A),PP2C (B),ABCG22 (C) and SnRK2,1 (D)] in the leaves (L) and roots (R) of pumpkin-grafted cucumber (C/P) and self-grafted cucumber (C/C) plants. Data are presented as the mean of three biological replicates ( ± SE), points with an asterisk indicate a significant difference from the untreated control (P 0.05) at 0 h according to t-test.