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 the response of grafted cucumbers to drought stress and the possible roles of mobile mRNAs in improving stress tolerance have remained obscure. Hence, we conducted transcriptome sequencing and combined it with morphophysiological experiments to compare the response of homografts (cucumber as scion and rootstock) and heterografts (cucumber as scion and pumpkin as rootstock) to drought stress. After applying drought stress, homografts and heterografts expressed 2960 and 3088 genes, respectively, in response to the stress. The differentially expressed genes identified 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 enzyme activities), phytohormone (increased abscisic acid content), protein kinases, and transcription factors, using MapMan software. Earlier and greater H₂O₂ accumulation in xylem below the graft union was accompanied by leaf abscisic acid accumulation in heterografts in response to drought stress. Greater leaf abscisic acid helped heterografted cucumbers to sense and respond to drought stress earlier than homografts. The timely response of heterografts to drought stress led to the maintenance of higher water content in the leaves even in the late stage of drought stress. The mobile mRNAs identified in heterografts were mostly related to photosynthesis, which would be the possible reason for improved chlorophyll content and maximum photochemical efficiency of photosystem II (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 higher proline accumulation than in homografts. Expression of mobile and immobile stress-responsive mRNAs and timely response of heterografts to drought stress could improve drought tolerance in pumpkin-rooted plants.

中文翻译：

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转录组测序和移动mRNA鉴定揭示干旱胁迫下砧木对接穗的远距离控制

摘要

用南瓜砧木嫁接不仅可以提高黄瓜果实的品质，还可以提高对生物或非生物胁迫的耐受性。然而，嫁接黄瓜对干旱胁迫响应的分子机制以及移动mRNAs在提高胁迫耐受性中的可能作用仍然不清楚。因此，我们进行了转录组测序并将其与形态生理学实验相结合，以比较同种移植物（黄瓜作为接穗和砧木）和异种移植物（黄瓜作为接穗和南瓜作为砧木）对干旱胁迫的响应。在施加干旱胁迫后，同种移植物和异种移植物分别表达了 2960 和 3088 个基因以响应胁迫。在干旱胁迫下异种移植物中鉴定的差异表达基因分为不同的胁迫响应组，例如碳水化合物代谢（通过糖积累参与渗透调节），脂质和细胞壁代谢（通过减少脂质过氧化参与细胞膜完整性），氧化还原稳态（增加抗氧化酶活性），植物激素（增加脱落酸含量），蛋白激酶和转录因子，使用 MapMan 软件。更早和更大的H₂ O ₂在接枝联合下方的木质部积累伴随着响应干旱胁迫的异种移植物中的叶脱落酸积累。较大的叶片脱落酸有助于异种黄瓜比同种黄瓜更早地感知和响应干旱胁迫。异种移植物对干旱胁迫的及时响应导致即使在干旱胁迫后期也能保持叶片中较高的含水量。在异种移植物中鉴定出的可移动 mRNA 主要与光合作用有关，这可能是提高叶绿素含量和光系统 II (Fv/Fm) 最大光化学效率的可能原因。黄瓜（接穗）中存在一些应激反应性南瓜（砧木）mRNA，如热休克蛋白（HSP70，一种众所周知的应激反应基因），导致脯氨酸积累高于同种移植物。移动和固定胁迫响应 mRNA 的表达以及异种移植物对干旱胁迫的及时响应可以提高南瓜根植物的耐旱性。