Abstract
The BRASSINOSTEROID INSENSITIVE1 (BRI1) receptor kinase has recently been shown to possess tyrosine kinase activity, and preventing autophosphorylation of the tyrosine-831 regulatory site by site-directed mutagenesis enhances shoot growth. In this study, we characterized the increased leaf growth of Arabidopsis (Arabidopsis thaliana) plants expressing BRI1(Y831F)-Flag compared with BRI1-Flag (both driven by the native promoter and expressed in the bri1-5 weak allele background) and provide insights into the possible mechanisms involved. On average, relative leaf growth rate was increased 16% in the Y831F plants (in the bri1-5 background), and the gain of function of the Y831F-directed mutant was dominant in the wild-type background. Leaves were larger as a result of increased cell numbers and had substantially increased vascularization. Transcriptome analysis indicated that genes associated with brassinolide biosynthesis, secondary cell wall biosynthesis and vascular development, and regulation of growth were altered in expression and may contribute to the observed changes in leaf architecture and whole plant growth. Analysis of gas exchange and chlorophyll fluorescence indicated that Y831F mutant plants had higher rates of photosynthesis, and metabolite analysis documented enhanced accumulation of starch, sucrose, and several amino acids, most prominently glycine and proline. These results demonstrate that mutation of BRI1 can enhance photosynthesis and leaf growth/vascularization and may suggest new approaches to increase whole plant carbon assimilation and growth.
Highlights
The BRASSINOSTEROID INSENSITIVE1 (BRI1) receptor kinase has recently been shown to possess tyrosine kinase activity, and preventing autophosphorylation of the tyrosine-831 regulatory site by site-directed mutagenesis enhances shoot growth
A prominent feature of transgenic plants expressing BRI1(Y831F)-Flag in the bri1-5 background is increased leaf size and shoot biomass relative to plants expressing wild-type BRI1-Flag (Oh et al, 2009a, 2009b), and an important question to address is the basis for the enhanced growth (Fig. 1A)
Because increased vascularization of the Y831F plants is associated with increased leaf size, it appears that cell divisions within the procambium have been increased; the Y831F growth phenotype is distinct from the thickvein mutant, which displays increased vascularization but reduced leaf size as a result of increased outside cell recruitment (Clay and Nelson, 2005)
Summary
The BRASSINOSTEROID INSENSITIVE1 (BRI1) receptor kinase has recently been shown to possess tyrosine kinase activity, and preventing autophosphorylation of the tyrosine-831 regulatory site by site-directed mutagenesis enhances shoot growth. Analysis of gas exchange and chlorophyll fluorescence indicated that Y831F mutant plants had higher rates of photosynthesis, and metabolite analysis documented enhanced accumulation of starch, sucrose, and several amino acids, most prominently glycine and proline These results demonstrate that mutation of BRI1 can enhance photosynthesis and leaf growth/vascularization and may suggest new approaches to increase whole plant carbon assimilation and growth. To further understand the role of Tyr phosphorylation in BR signaling, we compared transgenic Arabidopsis plants expressing BRI1(Y831F)-Flag with plants expressing wild-type BRI1-Flag, both in the bri weak allele background We characterized these plants in terms of leaf morphology and relative growth rate (RGR), photosynthetic parameters, and gene expression in an effort to understand the basis for the increased growth. The results obtained confirm the importance of Tyr phosphorylation in plant receptor kinase signaling and indicate the potential for improving plant performance by engineering receptor kinase function
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