Overexpression of the peanut CLAVATA1-like leucine-rich repeat receptor-like kinase AhRLK1 confers increased resistance to bacterial wilt in tobacco.

Bacterial wilt caused by Ralstonia solanacearum is a devastating disease affecting hundreds of plant species, yet the host factors remain poorly characterized. The leucine-rich repeat receptor-like kinase gene AhRLK1, characterized as CLAVATA1, was found to be up-regulated in peanut upon inoculation with R. solanacearum. The AhRLK1 protein was localized in the plasma membrane and cell wall. qPCR results showed AhRLK1 was induced in a susceptible variety but little changed in a resistant cultivar after inoculated with R. solanacearum. Hormones such as salicylic acid, abscisic acid, methyl jasmonate, and ethephon induced AhRLK1 expression. In contrast, AhRLK1 expression was down-regulated under cold and drought treatments. Transient overexpression of AhRLK1 led to a hypersensitive response (HR) in Nicotiana benthamiana. Furthermore, AhRLK1 overexpression in tobacco significantly increased the resistance to R. solanacearum. Besides, the transcripts of most representative defense responsive genes in HR and hormone signal pathways were significantly increased in the transgenic lines. EDS1 and PAD4 in the R gene signaling pathway were also up-regulated, but NDR1 was down-regulated. Accordingly, AhRLK1 may increase the defense response to R. solanacearum via HR and hormone defense signaling, in particular through the EDS1 pathway of R gene signaling. These results provide a new understanding of the CLAVATA1 function and will contribute to genetic enhancement of peanut.

Identification of low Ca(2+) stress-induced embryo apoptosis response genes in Arachis hypogaea by SSH-associated library lift (SSHaLL).

Calcium is a universal signal in the regulation of wide aspects in biology, but few are known about the function of calcium in the control of early embryo development. Ca(2+) deficiency in soil induces early embryo abortion in peanut, producing empty pods, which is a general problem; however, the underlying mechanism remains unclear. In this study, embryo abortion was characterized to be caused by apoptosis marked with cell wall degradation. Using a method of SSH cDNA libraries associated with library lift (SSHaLL), 62 differentially expressed genes were isolated from young peanut embryos. These genes were classified to be stress responses, catabolic process, carbohydrate and lipid metabolism, embryo morphogenesis, regulation, etc. The cell retardation with cell wall degradation was caused by up-regulated cell wall hydrolases and down-regulated cellular synthases genes. HsfA4a, which was characterized to be important to embryo development, was significantly down-regulated under Ca(2+) -deficient conditions from 15 days after pegging (DAP) to 30 DAP. Two AhCYP707A4 genes, encoding abscisic acid (ABA) 8'-hydroxylases, key enzymes for ABA catabolism, were up-regulated by 21-fold under Ca(2+) -deficient conditions upstream of HsfA4a, reducing the ABA level in early embryos. Over-expression of AhCYP707A4 in Nicotiana benthamiana showed a phenotype of low ABA content with high numbers of aborted embryos, small pods and less seeds, which confirms that AhCYP707A4 is a key player in regulation of Ca(2+) deficiency-induced embryo abortion via ABA-mediated apoptosis. The results elucidated the mechanism of low Ca(2+) -induced embryo abortion and described the method for other fields of study.