Despite the fact that roots are the organs most subject to microbial interactions, very little is known about the response of roots to microbe-associated molecular patterns (MAMPs). By monitoring transcriptional activation of b-glucuronidase reporters and MAMP-elicited callose deposition, we show that three MAMPs, the flagellar peptide Flg22, peptidoglycan, and chitin, trigger a strong tissue-specific response in Arabidopsis thaliana roots, either at the elongation zone for Flg22 and peptidoglycan or in the mature parts of the roots for chitin. Ethylene signaling, the 4-methoxy-indole-3-ylmethylglucosinolate biosynthetic pathway, and the PEN2 myrosinase, but not salicylic acid or jasmonic acid signaling, play major roles in this MAMP response. We also show that Flg22 induces the cytochrome P450 CYP71A12-dependent exudation of the phytoalexin camalexin by Arabidopsis roots. The phytotoxin coronatine, an Ile-jasmonic acid mimic produced by Pseudomonas syringae pathovars, suppresses MAMP-activated responses in the roots. This suppression requires the E3 ubiquitin ligase COI1 as well as the transcription factor JIN1/MYC2 but does not rely on salicylic acid-jasmonic acid antagonism. These experiments demonstrate the presence of highly orchestrated and tissue-specific MAMP responses in roots and potential pathogen-encoded mechanisms to block these MAMP-elicited signaling pathways.

In an attempt to understand the process mediating K+transport into roots, we examined the contribution of the NH4  +-sensitive and NH4  +-insensitive components of Rb+transport to the uptake of Rb+ in barley (Hordeum vulgare L.) plants grown in different ionic environments. We found that at low external Rb+ concentrations, an NH4  +-sensitive component dominates Rb+ uptake in plants grown in the absence of NH4  +, while Rb+ uptake preferentially occurs through an NH4  +-insensitive pathway in plants grown at high external NH4  + concentrations. A comparison of the Rb+-uptake properties observed in roots with those found in heterologous studies with yeast cells indicated that the recently cloned HvHAK1 K+ transporter may provide a major route for the NH4  +-sensitive component. HvHAK1 failed to complement the growth of a yeast strain defective in NH4  + transport, suggesting that it could not act as an NH4  + transporter. Heterologous studies also showed that the HKT1 K+/Na+-cotransporter may act as a pathway for high-affinity Rb+ transport sensitive to NH4  +. However, we found no evidence of an enhancement of Rb+ uptake into roots due to Na+addition. The possible identity of the systems contributing to the NH4  +-insensitive component in barley plants is discussed.

Detection of microbes by plants relies in part on an array of pattern-recognition receptors that recognize conserved microbial signatures, so-called "microbe-associated molecular patterns." The Arabidopsis thaliana receptor-like kinase FLS2 is the pattern-recognition receptor for bacterial flagellin. Similarly to FLS2, the rice transmembrane protein XA21 is the receptor for the sulfated form of the Xanthomonas oryzae pv. oryzae secreted protein Ax21. Here we show that Ax21-derived peptides activate Arabidopsis immunity, triggering responses similar to those elicited by flagellin, including an oxidative burst, induction of defense-response genes, and enhanced resistance to bacterial pathogens. To identify Arabidopsis Xa21 functional homologs, we used a reverse genetics approach to screen T-DNA insertion mutants corresponding to all 47 of the Arabidopsis genes encoding non-RD kinases belonging to the interleukin-1 receptor-associated kinase (IRAK) family. Surprisingly, among all of these mutant lines, only fls2 mutants exhibited a significant loss of response to Ax21-derived peptides. Ax21 peptides also failed to activate defense-related responses in an fls2-24 mutant that does not bind Flg22. Moreover, a Flg22Δ2 variant of Flg22 that binds to FLS2 but does not activate FLS2-mediated signaling suppressed Ax21-derived peptide signaling, indicating mutually exclusive perception of Flg22 or Ax21 peptides by FLS2. The data indicate that FLS2 functions beyond flagellin perception to detect other microbe-associated molecular patterns.innate immunity | broad spectrum MAMP recognition | non-RD kinases P attern-recognition receptors (PRRs) that recognize conserved microbial signatures, which are referred to as microbe-associated molecular patterns (MAMPs), are a key mechanism by which plants and other organisms detect microbes (1). Among several MAMPs detected by Arabidopsis thaliana, flagellin is the best studied. In Arabidopsis, the leucine-rich repeat (LRR) transmembrane receptor kinase FLAGELLIN SENSITIVE 2 (FLS2) is essential for flagellin perception (2). A 22-aa synthetic peptide (Flg22) corresponding to the recognized domain of flagellin activates FLS2-dependent signaling, triggering the same responses as the native flagellin protein from Pseudomonas syringae pv. tabaci (3). Flg22-triggered responses include activation of MAPK cascades, upregulation of defense genes, transient production of an H 2 O 2 oxidative burst, deposition of callose, and enhanced resistance against pathogens (2, 4, 5).The Arabidopsis FLS2 receptor belongs to the IRAK family of receptor like kinases (RLKs), which includes two other well characterized MAMP receptors, Arabidopsis EFR (TU-elongation factor-receptor 1) and rice XA21 (Xanthomonas resistance protein 21) (6). These RLKs carry the non-RD domain, a motif that is found in many IRAK kinases that function in immune signaling pathways (6). The Arabidopsis genome encodes 47 non-RD IRAK kinases, of which 35 are RLKs and 12 are predicted to be cytoplasmic (6, 7).XA21 recognizes the conserved ...