Exit control: the role of Arabidopsis hydathodes in auxin storage and nutrient recovery

IF 6.2 1区 生物学 Q1 PLANT SCIENCES The Plant Journal Pub Date : 2024-10-30 DOI:10.1111/tpj.17118
Gwendolyn Kirschner
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Hydathodes were first described by the German botanist Anton de Bary in 1877, and named by the Austrian botanist Gottlieb Haberlandt in 1897, from the Greek ‘hyda’ (water) and ‘hodos’ (way) (Bellenot et al., <span>2022</span>). When Jean-Marc Routaboul, the corresponding author of the highlighted publication, joined Laurent Noël's team at INRAE, France, in 2018, he was surprised to find that hydathodes and the process of guttation were not well understood at the molecular level. Therefore, Routaboul and his colleagues set out to test two long-standing hypotheses about hydathodes: that hydathodes are sites of auxin accumulation, and that they facilitate the withholding of nutrients from guttation fluids (Routaboul et al., <span>2024</span>).</p><p>These hypotheses are based on genes expressed in hydathodes, including those for auxin biosynthesis, transport, and signalling. Moreover, the presence of auxin in hydathodes was detected by antibodies and by using the auxin signalling reporter <i>DR5</i> (Aloni et al., <span>2003</span>). Other hydathode-specific genes encode membrane transporters for amino acids, sugar or ions (Nagai et al., <span>2013</span>), potentially preventing nutrient loss through guttation. For their study, Routaboul <i>et al</i>. combined RNAseq of hydathode-enriched tissue by deep sequencing with a detailed metabolomic analysis of guttation fluids.</p><p>First, the authors compared the transcriptome of macro-dissected leaf margins containing hydathodes with the transcriptome of leaf blade tissue of mature Arabidopsis leaves. They found higher expression of genes related to auxin metabolism, stress, DNA, plant cell wall, transport, RNA and lipids in the hydathode-enriched tissue. Genes related to glucosinolate synthesis and transport, the sulfation pathway, metal handling or photosynthesis were more highly expressed in the leaf blade. Because many genes related to auxin biosynthesis were expressed in hydathodes, the authors measured the accumulation of free auxin in hydathode-enriched tissue and leaf blades with liquid chromatography/mass spectrometry (LC/MS) and found nearly 40% more free auxin in hydathode-enriched tissue than in leaf blades. Reporter gene expression confirmed that genes encoding the key auxin biosynthetic enzymes Tryptophan Aminotransferase of Arabidopsis 1 (TAA1), YUCCA2, YUCCA5, YUCCA8 and YUCCA9 were specifically expressed in hydathodes. This suggests that the auxin concentration in hydathodes was high because of localised auxin biosynthesis. Two GH3 IAA-amido synthetases that contribute to maintaining auxin homeostasis by conjugating excess IAA to amino acid conjugates were also more highly expressed in the hydathode-enriched fraction. Therefore, the authors measured oxindole-3-AIA, a downstream product of that pathway. Higher oxindole-3-AIA in hydathodes suggested that hydathodes have a high auxin storage capability. Auxin signalling and response genes like <i>Auxin Response Factors</i> (<i>ARFs</i>) and <i>AUXIN/INDOLE ACETIC ACIDs</i> (<i>Aux/IAAs</i>) were also more highly expressed in hydathodes, altogether suggesting that hydathodes are active sites of auxin synthesis and signalling.</p><p>As shown before (Krouk et al., <span>2010</span>; Misson et al., <span>2004</span>), the authors found high expression of many genes encoding transporters of water, ions (nitrate, phosphate, sulphate, calcium, zinc, iron, copper, chloride and boron arsenate), hormone transporters (ABA, GA, auxin and cytokinins) and transporters of sugars, peptides, waxes and other organic compounds in hydathodes, suggesting that hydathodes could be sites that actively modify guttation fluid. Therefore, the authors sampled xylem fluid exuding from the petiole and compared its composition to that of pre-hydathode fluid sampled from leaves with excised hydathodes and to guttation fluid collected from hydathodes, using gas chromatography/MS. Leaf tissues captured 91% of the metabolites before they reached the hydathode. From the pre-hydathode fluid, 78% of the remaining metabolites were captured before the fluid got released by the hydathodes. The concentration of 23 metabolites (including amino acids, organic acids, sugars and myo-inositol) was lower in the guttation fluid than in the pre-hydathode fluid. Colorimetric assays showed that inorganic phosphate (Pi) and nitrate were taken up by the hydathode. Additionally, the authors used inductively coupled plasma-optical emission spectrometry (ICP-OES) and found lower concentrations of phosphorus, calcium and magnesium in the guttation fluid than in the pre-hydathode fluid. This suggests that hydathodes capture specific organic and mineral compounds before guttation.</p><p>The authors focused on nitrate transporter NRT2.1 and Pi transporter PHT1;4, which are highly expressed in hydathodes and well-studied in roots (Little et al., <span>2005</span>; Shin et al., <span>2004</span>). Higher nitrate and Pi levels in the guttation fluid of nrt2.1 and pht1;4 mutants suggested that the transporters contribute to the uptake of Pi and nitrate from the guttation fluid. <i>PHT1;4</i> expression matched Pi accumulation sites in leaves, with more Pi in leaf margins than blades (Figure 1c). The margins of <i>pht1;4</i> mutants contained less Pi than WT at the margins, suggesting that Pi is actively captured by PHT1;4 from the guttation fluid. Metabolites and minerals taken up from the guttation fluid can be recycled back into the leaves: an autoradiogram of a barley leaf fed with <sup>32</sup>Pi at hydathodes showed an ingress of Pi from the hydathode after 2 h followed by diffusion to the entire leaf tip after 1 day (Nagai et al., <span>2013</span>). In conclusion, Routaboul <i>et al</i>. speculate that hydathodes act analogously to nephrons in kidneys, filtering the guttation fluid and retrieving valuable nutrients.</p><p>Hydathodes are essential for maintaining plant water status and to get rid of excess water that could lead to mesophyll flooding, lower photosynthesis/respiration and leaf necrosis. Beside their role in metabolite scavenging, hydathodes can also mediate excretion of undesired or toxic compounds such as boron (Sutton et al., <span>2007</span>) or nanoparticles (Zhang et al., <span>2011</span>). They are also the main entry for several plant pathogens (Cerutti et al., <span>2017</span>). 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Abstract

Hydathodes are organs on the leaves of all vascular plants. They regulate the secretion of fluids derived from the xylem sap (Bellenot et al., 2022; Cerutti et al., 2019). When stomata are closed at night and the humidity level levels are too high, the xylem delivers excess water from the roots, which is secreted at the hydathodes in a process called guttation (Figure 1a) (Singh, 2020). Hydathodes are composed of an epidermal surface layer with water pores, and an inner parenchyma, called the epithem, which is highly vascularized and constitutes a direct connection between leaf surface and xylem vessels (Figure 1b) (Bellenot et al., 2022). Hydathodes were first described by the German botanist Anton de Bary in 1877, and named by the Austrian botanist Gottlieb Haberlandt in 1897, from the Greek ‘hyda’ (water) and ‘hodos’ (way) (Bellenot et al., 2022). When Jean-Marc Routaboul, the corresponding author of the highlighted publication, joined Laurent Noël's team at INRAE, France, in 2018, he was surprised to find that hydathodes and the process of guttation were not well understood at the molecular level. Therefore, Routaboul and his colleagues set out to test two long-standing hypotheses about hydathodes: that hydathodes are sites of auxin accumulation, and that they facilitate the withholding of nutrients from guttation fluids (Routaboul et al., 2024).

These hypotheses are based on genes expressed in hydathodes, including those for auxin biosynthesis, transport, and signalling. Moreover, the presence of auxin in hydathodes was detected by antibodies and by using the auxin signalling reporter DR5 (Aloni et al., 2003). Other hydathode-specific genes encode membrane transporters for amino acids, sugar or ions (Nagai et al., 2013), potentially preventing nutrient loss through guttation. For their study, Routaboul et al. combined RNAseq of hydathode-enriched tissue by deep sequencing with a detailed metabolomic analysis of guttation fluids.

First, the authors compared the transcriptome of macro-dissected leaf margins containing hydathodes with the transcriptome of leaf blade tissue of mature Arabidopsis leaves. They found higher expression of genes related to auxin metabolism, stress, DNA, plant cell wall, transport, RNA and lipids in the hydathode-enriched tissue. Genes related to glucosinolate synthesis and transport, the sulfation pathway, metal handling or photosynthesis were more highly expressed in the leaf blade. Because many genes related to auxin biosynthesis were expressed in hydathodes, the authors measured the accumulation of free auxin in hydathode-enriched tissue and leaf blades with liquid chromatography/mass spectrometry (LC/MS) and found nearly 40% more free auxin in hydathode-enriched tissue than in leaf blades. Reporter gene expression confirmed that genes encoding the key auxin biosynthetic enzymes Tryptophan Aminotransferase of Arabidopsis 1 (TAA1), YUCCA2, YUCCA5, YUCCA8 and YUCCA9 were specifically expressed in hydathodes. This suggests that the auxin concentration in hydathodes was high because of localised auxin biosynthesis. Two GH3 IAA-amido synthetases that contribute to maintaining auxin homeostasis by conjugating excess IAA to amino acid conjugates were also more highly expressed in the hydathode-enriched fraction. Therefore, the authors measured oxindole-3-AIA, a downstream product of that pathway. Higher oxindole-3-AIA in hydathodes suggested that hydathodes have a high auxin storage capability. Auxin signalling and response genes like Auxin Response Factors (ARFs) and AUXIN/INDOLE ACETIC ACIDs (Aux/IAAs) were also more highly expressed in hydathodes, altogether suggesting that hydathodes are active sites of auxin synthesis and signalling.

As shown before (Krouk et al., 2010; Misson et al., 2004), the authors found high expression of many genes encoding transporters of water, ions (nitrate, phosphate, sulphate, calcium, zinc, iron, copper, chloride and boron arsenate), hormone transporters (ABA, GA, auxin and cytokinins) and transporters of sugars, peptides, waxes and other organic compounds in hydathodes, suggesting that hydathodes could be sites that actively modify guttation fluid. Therefore, the authors sampled xylem fluid exuding from the petiole and compared its composition to that of pre-hydathode fluid sampled from leaves with excised hydathodes and to guttation fluid collected from hydathodes, using gas chromatography/MS. Leaf tissues captured 91% of the metabolites before they reached the hydathode. From the pre-hydathode fluid, 78% of the remaining metabolites were captured before the fluid got released by the hydathodes. The concentration of 23 metabolites (including amino acids, organic acids, sugars and myo-inositol) was lower in the guttation fluid than in the pre-hydathode fluid. Colorimetric assays showed that inorganic phosphate (Pi) and nitrate were taken up by the hydathode. Additionally, the authors used inductively coupled plasma-optical emission spectrometry (ICP-OES) and found lower concentrations of phosphorus, calcium and magnesium in the guttation fluid than in the pre-hydathode fluid. This suggests that hydathodes capture specific organic and mineral compounds before guttation.

The authors focused on nitrate transporter NRT2.1 and Pi transporter PHT1;4, which are highly expressed in hydathodes and well-studied in roots (Little et al., 2005; Shin et al., 2004). Higher nitrate and Pi levels in the guttation fluid of nrt2.1 and pht1;4 mutants suggested that the transporters contribute to the uptake of Pi and nitrate from the guttation fluid. PHT1;4 expression matched Pi accumulation sites in leaves, with more Pi in leaf margins than blades (Figure 1c). The margins of pht1;4 mutants contained less Pi than WT at the margins, suggesting that Pi is actively captured by PHT1;4 from the guttation fluid. Metabolites and minerals taken up from the guttation fluid can be recycled back into the leaves: an autoradiogram of a barley leaf fed with 32Pi at hydathodes showed an ingress of Pi from the hydathode after 2 h followed by diffusion to the entire leaf tip after 1 day (Nagai et al., 2013). In conclusion, Routaboul et al. speculate that hydathodes act analogously to nephrons in kidneys, filtering the guttation fluid and retrieving valuable nutrients.

Hydathodes are essential for maintaining plant water status and to get rid of excess water that could lead to mesophyll flooding, lower photosynthesis/respiration and leaf necrosis. Beside their role in metabolite scavenging, hydathodes can also mediate excretion of undesired or toxic compounds such as boron (Sutton et al., 2007) or nanoparticles (Zhang et al., 2011). They are also the main entry for several plant pathogens (Cerutti et al., 2017). Understanding hydathode physiology could thus have implications for adapting plant performance in stressful conditions such as flooding, drought, immunity or phytoremediation.

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出口控制:拟南芥水瘤在辅助素储存和养分恢复中的作用。
水瘤是所有维管植物叶片上的器官。它们调节木质部汁液的分泌(Bellenot 等人,2022 年;Cerutti 等人,2019 年)。当夜间气孔关闭、湿度水平过高时,木质部会从根部输送多余的水分,这些水分在称为 "内渗化 "的过程中分泌到水瘤(图 1a)(Singh,2020 年)。水瘤由带有水孔的表皮表层和称为表皮的内部实质组成,后者血管高度发达,是叶片表面和木质部血管之间的直接连接(图 1b)(Bellenot 等人,2022 年)。水瘤由德国植物学家 Anton de Bary 于 1877 年首次描述,1897 年由奥地利植物学家 Gottlieb Haberlandt 命名,源自希腊语 "hyda"(水)和 "hodos"(路)(Bellenot 等人,2022 年)。该论文的通讯作者让-马克-劳塔布尔(Jean-Marc Routaboul)于 2018 年加入法国国家高等农艺研究所劳伦特-诺埃尔(Laurent Noël)的团队,他惊讶地发现,人们对水合作用和肠化过程在分子水平上并不十分了解。因此,Routaboul和他的同事们开始验证有关水瘤的两个长期存在的假说:水瘤是助长素积累的场所,以及水瘤有助于从内蜕液中扣留养分(Routaboul等人,2024年)。这些假说基于水瘤中表达的基因,包括那些用于助长素生物合成、运输和信号传递的基因。此外,通过抗体和使用辅酶信号报告基因 DR5(Aloni 等人,2003 年),可以检测到水瘤中存在辅酶。其他水瘤特异性基因编码氨基酸、糖或离子的膜转运体(Nagai 等人,2013 年),有可能防止营养物质通过内脏流失。在他们的研究中,Routaboul 等人通过深度测序对富含水瘤的组织进行了 RNAseq 测序,并对开沟液进行了详细的代谢组学分析。首先,作者比较了含有水瘤的大面积解剖叶缘的转录组和成熟拟南芥叶片组织的转录组。他们发现,在富含水瘤的组织中,与辅酶代谢、胁迫、DNA、植物细胞壁、运输、RNA 和脂质有关的基因表达量较高。与葡萄糖苷酸合成和运输、硫酸化途径、金属处理或光合作用有关的基因在叶片中的表达量更高。由于许多与辅酶生物合成有关的基因在水叶中表达,作者用液相色谱/质谱法(LC/MS)测量了水叶富集组织和叶片中游离辅酶的积累,发现水叶富集组织中的游离辅酶比叶片中多近 40%。报告基因表达证实,拟南芥色氨酸氨基转移酶 1 (TAA1)、YUCCA2、YUCCA5、YUCCA8 和 YUCCA9 等关键辅素生物合成酶的编码基因在水瘤中特异性表达。这表明,由于局部辅助素的生物合成,水瘤中的辅助素浓度很高。两种 GH3 IAA-氨基合成酶通过将过量的 IAA 连接成氨基酸共轭物来维持辅素平衡,它们在水合作用富集部分的表达量也更高。因此,作者测量了该途径的下游产物氧化吲哚-3-AIA。水合叶中较高的氧化吲哚-3-AIA 表明水合叶具有较高的辅助素储存能力。辅助素信号和响应基因,如辅助素响应因子(ARFs)和辅助素/吲哚乙酸(Aux/IAAs),在水瘤中也有较高的表达,这共同表明水瘤是辅助素合成和信号传导的活性位点、如前所示(Krouk 等人,2010 年;Misson 等人,2004 年),作者发现许多编码水、离子(硝酸盐、磷酸盐、硫酸盐、钙、锌、铁、铜、氯化物和砷酸硼)转运体、激素转运体(ABA、GA、辅助素和细胞分裂素)以及糖、肽、蜡和其他有机化合物转运体的基因在水瘤中高表达,这表明水瘤可能是积极改变内渗液的部位。因此,作者对叶柄中渗出的木质部液体进行了采样,并利用气相色谱/质谱法将其成分与从切除水瘤的叶片中采样的水瘤前液体和从水瘤中采集的内渗液进行了比较。在代谢物到达水合器之前,叶片组织捕获了 91% 的代谢物。在水合作用前的液体中,78%的剩余代谢物在水合作用释放之前被捕获。肠液中 23 种代谢物(包括氨基酸、有机酸、糖类和肌醇)的浓度低于前阴极液。比色测定显示,水合阳极吸收了无机磷酸盐(Pi)和硝酸盐。
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来源期刊
The Plant Journal
The Plant Journal 生物-植物科学
CiteScore
13.10
自引率
4.20%
发文量
415
审稿时长
2.3 months
期刊介绍: Publishing the best original research papers in all key areas of modern plant biology from the world"s leading laboratories, The Plant Journal provides a dynamic forum for this ever growing international research community. Plant science research is now at the forefront of research in the biological sciences, with breakthroughs in our understanding of fundamental processes in plants matching those in other organisms. The impact of molecular genetics and the availability of model and crop species can be seen in all aspects of plant biology. For publication in The Plant Journal the research must provide a highly significant new contribution to our understanding of plants and be of general interest to the plant science community.
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