ERECTA is required for protection against heat-stress in the AS1/AS2 pathway to regulate adaxial-abaxial leaf polarity in Arabidopsis

In seed plants, formation of the adaxial-abaxial polarity is of primary importance in leaf patterning. Since Arabidopsis thaliana (L.) Heynh. genes ASYMMETRIC LEAVES1 (AS1) and ASYMMETRIC LEAVES2 (AS2) are key regulators in specifying adaxial leaf identity, and ERECTA is involved in the AS1/AS2 pathway for regulating adaxial-abaxial polarity [L. Xu et al. (2003) Development 130:4097-4107], we studied the physiological functions of the ERECTA protein in plant development. We analyzed the effects of different environmental conditions on a special leaf structure in the as1 and as2 mutants. This structure, called the lotus-leaf, reflects a severe loss of adaxial-abaxial polarity in leaves. Higher concentrations of salt or other osmotic substance and lower temperature severely affected plant growth both in the wild type and the mutants, but did not affect lotus-leaf frequency in the as1 and as2 mutants. as1 and as2 mutants exhibited a very low lotus-leaf frequency at 22degreesC, a temperature that favors Arabidopsis growth. The lotus-leaf frequency rose significantly with an increase in growth temperature, and only in plants that are in the erecta mutation background. These results suggest that ERECTA function is required for reducing plant sensitivity to heat stress during adaxial-abaxial polarity formation in leaves.