Long-distance control of the scion by the rootstock under drought stress as revealed by transcriptome sequencing and mobile mRNA identification

Abstract

Grafting with pumpkin rootstock is commonly used not only to improve the quality of cucumber fruits but also to confer biotic or abiotic stress tolerance. However, the molecular mechanism of grafted cucumbers to drought stress and the possible roles of mobile mRNAs to improve stress tolerance have remained obscure. Hence, we conducted transcriptome sequencing and combined it with morpho-physiological experiments to compare the response of homografts (cucumber as scion and rootstock) (C) and heterografts (cucumber as scion and pumpkin as rootstock) (P) to drought stress. After applying drought stress, homografts and heterografts expressed 2960 and 3088 genes in response to drought stress, respectively. The identified DEGs in heterografts under drought stress were categorized into different stress-responsive groups, such as carbohydrate metabolism (involved in osmotic adjustment by sugar accumulation), lipid and cell wall metabolism (involved in cell membrane integrity by a reduction in lipid peroxidation), redox homeostasis (increased antioxidant enzymes activities), phytohormone (increased ABA content), protein kinases and transcription factors (TFs) using MapMan software. Earlier and greater H2O2 accumulation in xylem below the graft union was accompanied by leaf ABA accumulation in heterografts in response to drought stress. Greater leaf ABA helped heterografted cucumbers to sense and respond to drought stress earlier than homografts. The timely response of heterografts to drought stress led to maintain higher water content in the leaves even in the late stage of drought stress. The identified mobile mRNAs (mb-mRNAs) in heterografts were mostly related to photosynthesis which would be the possible reason for improved chlorophyll content and maximum photochemical efficiency of PSII (Fv/Fm). The existence of some stress-responsive pumpkin (rootstock) mRNAs in cucumber (scion), such as heat shock protein (HSP70, a well-known stress-responsive gene), led to the higher proline accumulation than homografts. The expression of the mobile and immobile stress-responsive mRNAs and timely response of heterografts to drought stress could improve drought tolerance in pumpkin-rooted plants.

Genome-Wide Identification and Expression Analysis of Heat Shock Protein 70 (HSP70) Gene Family in Pumpkin (Cucurbita moschata) Rootstock under Drought Stress Suggested the Potential Role of these Chaperones in Stress Tolerance

“…This plant species is also one of the most popular rootstocks for cucurbit grafting, which has the ability to improve fruit quality [37], as well as stress tolerance [20,38,39]. In our previous study [40] we identified HSP70 as the stress-responsive mobile mRNA with a high expression level in response to drought stress in cucumber scion grafted onto pumpkin rootstock. Therefore, we selected this conserved gene family to perform a genome-wide analysis in pumpkin.…”

Grafting enhances plants drought resistance: Current understanding, mechanisms, and future perspectives

“…The PIP1;2 was considered a candidate gene for improving the plant water conductivity in grafted hickory plants ( Kumar et al., 2018 ). Additionally, aquaporins were reported to be transported from pumpkin rootstocks to cucumber scions during drought ( Davoudi et al., 2022 ). Therefore, aquaporins and their related genes need further exploration in grafted plants under drought stress.…”

“…The Rubisco large subunit ( RbcL ), Rubisco small subunit ( RbcS ), chlorophyll a/b binding protein ( Cab ) and photosystem I p700 chlorophyll a apoprotein A2 ( psaB ) genes were up-regulated in chrysanthemum scions of the drought-resistant grafted plants and it helped maintain the photosynthetic performance ( Chen et al., 2018 ; Wilson and Hayer-Hartl, 2018 ). The movement of a series of mobile mRNAs from the pumpkin roots to the scions, also called scion-rootstock communication, can also improve the photosynthetic performance during drought conditions ( Davoudi et al., 2022 ). Specifically, multiple genes, including photosystem II CP47 reaction center protein ( psbB ), photosystem I p700 chlorophyll a apoprotein A2 ( psaB ), photosystem II protein D2 ( psbD ), photosystem I p700 chlorophyll a apoprotein A1 ( psaA ) and photosystem I light-harvesting chlorophyll a/b-binding protein ( LhcI ), functional in different photosynthesis-related processes, were induced to maintain the photosynthetic activity of the grafted plants ( Davoudi et al., 2022 ).…”

“…The movement of a series of mobile mRNAs from the pumpkin roots to the scions, also called scion-rootstock communication, can also improve the photosynthetic performance during drought conditions ( Davoudi et al., 2022 ). Specifically, multiple genes, including photosystem II CP47 reaction center protein ( psbB ), photosystem I p700 chlorophyll a apoprotein A2 ( psaB ), photosystem II protein D2 ( psbD ), photosystem I p700 chlorophyll a apoprotein A1 ( psaA ) and photosystem I light-harvesting chlorophyll a/b-binding protein ( LhcI ), functional in different photosynthesis-related processes, were induced to maintain the photosynthetic activity of the grafted plants ( Davoudi et al., 2022 ). Among them, psaB is the common gene that was not only detected in chrysanthemum scions, but also identified in the molecule movement from pumpkin rootstocks to cucumber scions.…”

“…The raffinose synthase ( SIP1 ) gene was found up-regulated in the grafted peach plants ( Jimenez et al., 2013 ). Furthermore, the genes encoding starch branching enzymes increased the starch content in the cucumber/pumpkin grafted combination ( Davoudi et al., 2022 ). The carbohydrate metabolism-related genes β-fructosidase 3 ( BFRUCT3 ), sucrose phosphate synthase 1F ( SPS1F ), sucrose synthase 3 and 4 ( SUS3 and SUS4 ) were upregulated in grafted drought-resistant rootstocks ( Goncalves et al., 2019 ).…”