, 2007), and a blue light–inducible

phosphodiesterase PARP inhibitor (PDE) activity, specific for hydrolysis of cyclic di-GMP (c-di-GMP), has been identified in a recombinant protein from Synechococcus elongates (Cao et al., 2010). We have found that some mutant reduction/activation of Xcc growth is related to the intensity of the sensing light, so the degree of reduction/activation of some light sensitivity mutants possibly depends not only on the light wavelength but also on light intensity, which may be why different responses were caused by different sensing light, such as red, far-red, blue and white light, a mixture model of visible light. Thirteen PAS proteins that respond to light signals displayed effects LDK378 on bacterial growth and motility and were thought to be involved in photo-signalling in Xcc. These 13 proteins belong to three broad functional groups, HK, GGDEF-characterized protein and hybrid HK. Four

of these proteins are involved in tricolour (blue, red and far-red) signalling, which contain more than one PAS domain in each protein, and these PAS domains are involved in different clusters of Fig. 1c. It is, therefore, possible that proteins detecting multiple colours do so through the combinatorial action of tandem PAS domains, each responding to a subset of the total protein spectrum. The remaining 20 PAS proteins had no effect on Xcc growth in our assays. The virulence of Xcc mutants was tested by host plant inoculation as described previously (Marie et al., 2004; Lu et al., 2007a, b; Ryan et al., 2007) under light of a defined intensity (strong light of 12 000 lux and weak light of 2000 lux). Some host plants exhibited different levels Miconazole of H2O2, salicylic acid and expression of defence genes such as PR-1, when exposed to changing light conditions (Wang et al., 2010). Previous research has shown that light plays a critical role in the defence response of rice plants (Guo et al., 1993). Increased illumination resulted in thicker leaves and a greater number of palisade cells, but the anticlinal

elongation of those cells is specifically responsive to the flux rate of blue light (Lopez-Juez et al., 2007). Therefore, susceptibility of host plants may vary under different light conditions, and the varying susceptibility of host plants may affect virulence tests, that is, the virulence of mutants of PAS-domain-containing proteins in this research. Because the Xcc strains that showed growth responses to monochromatic light also responded to white light, we concluded that monochromatic light is the primary trigger for PAS proteins as either singly or in conjunction with other colours. Therefore, the light-influencing virulence tests were conducted under white light instead of monochromatic light. A chemotaxis protein, XC_2504, was found to be involved in the virulence of Xcc, according to its significant reduction in LL under strong light.