Two cotton isochorismate synthase genes are biased functionally in regulating salicylic acid and phylloquinone biosynthesis

Abstract

Isochorismate synthase (ICS), which catalyzes chorismate (CA) conversion to isochorismate (IC), is a crucial enzyme in the salicylic acid (SA) and phylloquinone (PhQ) biosynthesis. However, the ICS genes’ contribution to the two pathways has not been well studied in crops. Here, we addressed this question by investigating the functional role of ICS genes in plant growth and defense in cotton, an economically important crop. There were two ICS genes, ICS1 and ICS2, with each having homoeologs corresponding to A- and D- subgenomes in tetraploid Gossypium hirsutum (Gh) and G. barbadense (Gb), respectively. ICS2 is mainly expressed in the root and strongly induced by pathogen Verticillium dahliae, while ICS1 is highly expressed in the stems and leaves and weakly responsive to V. dahliae. Sequences alignment showed that ICS1 and ICS2 had obvious N-terminal differences. Subcellular localization indicated that GbICS2A/D were localized in plastid, whereas GbICS1A/D was localized in cell membrane, nucleus and cytoplasm, and GbICS1D was also partially localized in plastid. Silencing ICS1 resulted in greater loss of PhQ and affected the chloroplasts development of plants, while it was indifferent in PhQ content after ICS2 silencing. On the other hand, silencing ICS2 in cotton significantly impaired the resistance to V. dahliae, and overexpression of ICS2 in Arabidopsis increased disease resistance, while silencing ICS1 had a weak effect on the resistance. We verified that differences in disease resistance between ICS1 and ICS2 were largely caused by differences in regulation of SA synthesis. Our findings provide significant insights into functional divergence between ICS1 and ICS2 in cotton, which is different from the previous reports in Arabidopsis. Cotton ICS2 mainly regulates pathogen-induced SA biosynthesis, while ICS1 mainly participates in PhQ biosynthesis. This functional divergence helps effectively plants to modulate the metabolic flow to growth or defense for better plants health.