Abstract
Phenylalanine ammonia-lyase (PAL) catalyzes the first step of the phenylpropanoid pathway, which produces precursors to a variety of important secondary metabolites. Arabidopsis (Arabidopsis thaliana) contains four PAL genes (PAL1-PAL4), but there has been no genetic analysis to assess the biological functions of the entire gene family. Here, we report the generation and analysis of combined mutations for the four Arabidopsis PAL genes. Contrary to a previous report, we found that three independent pal1 pal2 double mutants were fertile and generated yellow seeds due to the lack of condensed tannin pigments in the seed coat. The pal1 pal2 double mutants were also deficient in anthocyanin pigments in various plant tissues, which accumulate in wild-type plants under stress conditions. Thus, PAL1 and PAL2 have a redundant role in flavonoid biosynthesis. Furthermore, the pal1 pal2 double mutants were more sensitive to ultraviolet-B light but more tolerant to drought than wild-type plants. We have also generated two independent pal1 pal2 pal3 pal4 quadruple knockout mutants, which are stunted and sterile. The quadruple knockout mutants still contained about 10% of the wild-type PAL activity, which might result from one or more leaky pal mutant genes or from other unknown PAL genes. The quadruple mutants also accumulated substantially reduced levels of salicylic acid and displayed increased susceptibility to a virulent strain of the bacterial pathogen Pseudomonas syringae. These results provide further evidence for both distinct and overlapping roles of the Arabidopsis PAL genes in plant growth, development, and responses to environmental stresses.
Highlights
Phenylalanine ammonia-lyase (PAL) catalyzes the first step of the phenylpropanoid pathway, which produces precursors to a variety of important secondary metabolites
The double mutant was male sterile and could not set seeds (Rohde et al, 2004). Since these two reported pal mutants were in the Arabidopsis ecotype C24 (Rohde et al, 2004), we identified two new T-DNA insertion mutants each for PAL1 and PAL2 in the Arabidopsis ecotype Columbia (Col-0), so their genetic background is identical to that of later isolated pal3 and pal4 mutants
Attempts have been made to address the biological functions of the PAL genes through silencing of PAL genes in tobacco and disruption of the PAL1 and PAL2 genes in Arabidopsis
Summary
Phenylalanine ammonia-lyase (PAL) catalyzes the first step of the phenylpropanoid pathway, which produces precursors to a variety of important secondary metabolites. The quadruple mutants accumulated substantially reduced levels of salicylic acid and displayed increased susceptibility to a virulent strain of the bacterial pathogen Pseudomonas syringae. These results provide further evidence for both distinct and overlapping roles of the Arabidopsis PAL genes in plant growth, development, and responses to environmental stresses. Arabidopsis PAL Gene Family exhibited unusual phenotypes such as localized fluorescent lesions, altered leaf shape and texture, reduced lignification in xylem, stunted growth, reduced pollen viability, and altered flower morphology and pigmentation (Elkind et al, 1990) These plants did not develop systemic acquired resistance in response to infection by Tobacco mosaic virus (Pallas et al, 1996). If a vast majority of SA is synthesized from the ICS pathway, it is unclear how silencing or inhibition of PAL leads to a substantial reduction in SA accumulation and enhanced pathogen susceptibility
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