Protection mechanisms in the resurrection plant Xerophyta viscosa: cloning, expression, characterisation and role of XvINO1, a gene coding for a myo-inositol 1-phosphate synthase.

We have used reverse transcription-PCR coupled with 5'- and 3'-RACE to isolate a full length INO1 cDNA (1692 bp with an ORF of 1530) from the resurrection plant Xerophyta viscosa Baker. XvINO1 encodes 510 amino acids, with a predicted MW of 56.7kD and contains four sequence motifs that are highly conserved in plant myo-inositol-1-phosphate synthases (MIPS, EC5.5.1.4), the enzyme that catalyses the first step in the formation of myo-inositol (Ino). Northern and western analyses show that the transcript and protein are constitutively present in leaves but their expression increases, temporarily, in response to both accumulative salt stress (~300 mM NaCl) and desiccation (to 5% relative water content). Leaf Ino concentration increases 40-fold during the first 6 h of salt stress, and levels of this and other carbohydrates (galactinol, sucrose, raffinose, stachyose and hexoses) remain elevated relative to control leaves for the duration of salt stress treatment. The timing and pattern of accumulation of these carbohydrates differ under desiccation stress and we propose that they perform different functions in the respective stresses. These are elaborated in discussion of our data.