Hydrogen Peroxide Is Involved In Hydrogen Sulfide-induced Lateral ...

Lateral root (LR) formation, which entirely originated from pericycle founder cells, is of critical importance for the plant root architecture [1]. Normally, LR formation depends on both genetic determinants and postembryonic developmental processes that are mainly under the influence of plant hormone (usually auxin) and environmental factors, including water and nutrient availability [1, 2]. Genetic and molecular evidence revealed that auxin regulates LR formation by modulating the transcripts of cell cycle regulatory genes, such as cyclins and Cyclin Dependent Kinases (CDK) in the pericycle cells [3,4,5,6]. Previous results showed that nitric oxide (NO) mediated the activation of auxin-dependent cell cycle regulatory genes encoding CYCA2;1, CYCA3;1, CDKA1, and the cell cycle inhibitor Kip-Related Protein KRP2 in tomato seedlings at the beginning of LR primordia formation [6]. On the other hand, auxin response factors (ARFs) appeared to play an essential role in auxin-regulated gene expression during plant development, including LR formation, etc. [7,8,9]. A decade ago, a class of small, non-coding RNAs, called microRNAs (miRNAs), was identified to regulate gene expression [10, 11]. Several miRNAs related to ARFs have been detected via computational approaches [12], such as miR390 targeting ARF2, ARF3 and ARF4 [13], while miR160 targeting ARF10, ARF16 and ARF17 [14].

After NO and carbon monoxide (CO) [15], hydrogen sulfide (H2S) is proposed as the third gaseous messenger to be involved in guard cell signaling [16], root organogenesis [17], and the alleviation of seed germination inhibition caused by heavy metal exposure [18]. In mammalian cells, H2S can be endogenously generated from four enzymes, such as cystathionine-γ-lyase (CSE), cystathionine-β-synthase (CBS), cysteine aminotransferase, and 3-mercaptopyruvate sulfurtransferase (3-MST) [19, 20]. In plants, H2S synthesis is partially catalyzed by L-cysteine desulfhydrase (DES; homolog with CSE in animals) [21, 22]. Related experiments discovered that H2S might be involved in auxin-induced LR formation in tomato seedlings [23]. Importantly, the discovered mechanism of physiological effects achieved by H2S in animals and recently in plants is S-sulfhydration: a posttranslational modification of protein cysteine residues (persulfide R-SSH formation) [24,25,26]. Above modification manner is opposed to S-nitrosylation, another posttranslational modification of protein cysteine residues by NO with the formation of S-nitrosocysteine residues (R-SNO) [27]. However, whether protein S-sulfhydration was involved in H2S-mediated LR formation, is still unknown.

It was well-known that hydrogen peroxide (H2O2) plays various vital roles in signal transduction beside its toxic effects. In fact, H2O2 is an important product of NADPH oxidase, polyamine oxidases (PAO), and diamine oxidases (DAO), etc. [28, 29]. Subsequent results showed that H2O2 mediates plant responses against adversity stresses and takes part in plant development processes, including stomatal closure [30], root gravitropism [31], and cell elongation [32]. Specially, H2O2 is also involved in auxin signaling [31, 33, 34], adventitious rooting [34, 35], and LR formation [36,37,38,39].

Although H2S and H2O2 were respectively suggested to be required for root architecture [17, 32], the cross-talk between H2S and H2O2 in tomato LR development, has not been fully elucidated. In this report, the analysis of H2S-regulated mechanisms leading to LR promotion is expanded. By using pharmacological, anatomic, and molecular approaches, evidence presented here supported the role of RBOH1-mediated H2O2 in the regulation of tomato LR development achieved by H2S. Potential mechanisms, including LR-related ARFs gene expression via miRNAs, are preliminarily elucidated. Additionally, downstream signaling events modulated by H2S might occur in both transcriptional and posttranslational levels (protein S-sulfhydration, etc.). Above results thus provide insights into H2S signaling in plant development.