LeProT1, a transporter for proline, glycine betaine, and gamma-amino butyric acid in tomato pollen

During maturation, pollen undergoes a period of dehydration accompanied by the accumulation of compatible solutes. Solute import across the pollen pla sma membrane, which occurs via proteinaceous transporters, is required to s upport pollen development and also for subsequent germination and pollen tu be growth. Analysis of the free amino acid composition of various tissues i n tomato revealed that the proline content in flowers was 60 times higher t han in any other organ analyzed. Within the floral organs, proline was conf ined predominantly to pollen, where it represented >70% of total free amino acids. Uptake experiments demonstrated that mature as well as germinated p ollen rapidly take up proline. To identify proline transporters in tomato p ollen, we isolated genes homologous to Arabidopsis proline transporters. Le ProT1 was specifically expressed both in mature and germinating pollen, as demonstrated by RNA in situ hybridization. Expression in a yeast mutant dem onstrated that LeProT1 transports proline and gamma-amino butyric acid with low affinity and glycine betaine with high affinity. Direct uptake and com petition studies demonstrate that LeProT1 constitutes a general transporter for compatible solutes.