Reporter-based screen for Arabidopsis mutants compromised in nonhost resistance

Ethylene Response Factors MbERF4 and MbERF72 Suppress Iron Uptake in Woody Apple Plants by Modulating Rhizosphere pH

Iron (Fe) deficiency limits the yield of fruit trees. When subjected to Fe deficiency, H+ secretion increases in the rhizosphere of dicotyledonous plants and pH decreases. This leads to the acidification of the soil and promotes Fe3+ to Fe2+ conversion, which plants can better uptake. This study investigated the relationship between two inhibitory transcription factors (ethylene response factors MbERF4 and MbERF72) and the H+-ATPase gene MbHA2. Two species of apple woody plants were studied: the Fe-inefficient Malus baccata and the Fe-efficient Malus xiaojinensis. Yeast one-hybrid and electrophoretic mobility shift assays showed that both MbERF4 and MbERF72 bind to the GCC cassette (AGCCGCC) of the MbHA2 promoter. Moreover, yeast two-hybrid and bimolecular fluorescence complementation assays showed that MbERF4 interacts with MbERF72. Furthermore, β-glucuronidase and luciferase reporter assays showed that the MbERF4- and MbERF72-induced repression of MbHA2 expression is synergistic. Virus-induced gene silencing of MbERF4 or MbERF72 increased MbHA2 expression, and thus lowered the rhizosphere pH in M. baccata. Consequently, the high expressions of MbERF4 and MbERF72 induced by Fe deficiency contributed to the Fe sensitivity of M. baccata. Moreover, the low expressions of MxERF4 and MxERF72 contributed to the Fe-deficiency tolerance of M. xiaojinensis via different binding conditions to the HA2 promoter. In summary, this study identified the relationship of two inhibitory transcription factors with the H+-ATPase gene and proposed a model in which ERF4 and ERF72 affect the rhizosphere pH in response to Fe deficiency.

Overexpression of the transcription factor RAP2.6 leads to enhanced callose deposition in syncytia and enhanced resistance against the beet cyst nematode Heterodera schachtii in Arabidopsis roots

BACKGROUND

Cyst nematodes invade the roots of their host plants as second stage juveniles and induce a syncytium which is their source of nutrients throughout their life. A transcriptome analysis of syncytia induced by the beet cyst nematode Heterodera schachtii in Arabidopsis roots has shown that gene expression in the syncytium is different from that of the root with thousands of genes upregulated or downregulated. Among the downregulated genes are many which code for defense-related proteins. One gene which is strongly downregulated codes for the ethylene response transcription factor RAP2.6. The genome of Arabidopsis contains 122 ERF transcription factor genes which are involved in a variety of developmental and stress responses.

RESULTS

Expression of RAP2.6 was studied with RT-PCR and a promoter::GUS line. During normal growth conditions the gene was expressed especially in roots and stems. It was inducible by Pseudomonas syringae but downregulated in syncytia from a very early time point on. Overexpression of the gene enhanced the resistance against H. schachtii which was seen by a lower number of nematodes developing on these plants as well as smaller syncytia and smaller female nematodes. A T-DNA mutant had a reduced RAP2.6 transcript level but this did not further increase the susceptibility against H. schachtii. Neither overexpression lines nor mutants had an effect on P. syringae. Overexpression of RAP2.6 led to an elevated expression of JA-responsive genes during early time points after infection by H. schachtii. Syncytia developing on overexpression lines showed enhanced deposition of callose.

CONCLUSIONS

Our results showed that H. schachtii infection is accompanied by a downregulation of RAP2.6. It seems likely that the nematodes use effectors to actively downregulate the expression of this and other defense-related genes to avoid resistance responses of the host plant. Enhanced resistance of RAP2.6 overexpression lines seemed to be due to enhanced callose deposition at syncytia which might interfere with nutrient import into syncytia.