Investigating factors that affect grapevine fruit set during abiotic stress

Abstract

In angiosperms pollen tubes play a crucial role in sexual reproduction in delivering male gametes to female tissue for fertilization. Any type of impairment in pollen tube growth (PTG) might lead to the poor fruit set. Poor fruit set leads to partially developed berries and poor yields in grapevine. Salinity is a major environmental factor that constrains optimal fruit set. However, whether PTG is restricted in the style during saline conditions is still unknown. PTG relies upon many co- ordinated processes including cytoskeletal rearrangements, vesicle trafficking, signal transduction pathways and pollen–pistil interactions. Various chemical factors are known to affect PTG, including gamma-aminobutyric acid (GABA) which at low concentrations guides the pollen tube to the ovary but at high concentrations inhibit pollen tube growth. GABA concentration also increases in plant tissues under stress, including salinized conditions. The transport of ions across various pollen tube membranes is crucial for PTG; the proteins responsible for the ion movement across grapevine pollen are unknown. ALMTs/QUAC (Aluminum activated malate transporters/Quick activating anion channels) were found in Arabidopsis pollen tubes and are candidates for the movement of Cl⁻. Recently it was discovered that anion currents through ALMT are gated by GABA. Here, the link between ALMTs and GABA and its role in controlling PTG under stress is explored. Pollen performance and its potential role in poor fruit set under saline conditions was explored using Shiraz (BVRC17) vines. Pollen tube length and growth rate for salt treated vines was found to be significantly less in first 4 hours as compared to untreated pollen grains when grown in in vitro conditions on pollen germination media (PGM). Pollen grains were treated with exogenously-applied GABA (1-100 mM) using the in vitro pollen assay; length increased after 1-5 mM GABA and then decreased after 20-100 mM GABA treatment. An analogue of GABA (Muscimol) was inhibitory to PTG and an antagonist of GABA binding in mammalian GABAᴀ receptors (Bicuculline) was stimulatory adding more evidence for the role of GABA in regulating PTG. In order to check the GABA levels GABase assay was done and GABA levels were found to be nearly 2 fold increase in salt stressed flowers as compared to control flowers which may be contributing to the reduced fruit set through stunted pollen tube growth. GABA concentration in tissue was examined using a GABase assay and found to be nearly 2 fold increased in salt stressed flowers as compared to control flowers which was hypothesized to be contributing to the reduced fruit set through stunted pollen tube growth. Gene expression levels were examined for GABA shunt enzymes in control and salt treated flowers but were not significantly different. ALMT expression in flowers, pollen grains and pollen tubes was examined. Five ALMT were found in flowers (Vv ALMT 7, 9-like, 9-like_2, 10 and 13), two in pollen tubes (Vv ALMT 9- like and 10) and one in pollen grains (Vv ALMT 10). Ion transport by Vv ALMT 9-like was found to be GABA sensitive and therefore is a prime candidate for transducing GABA signals in pollen tubes which regulate PTG under standard and salinized conditions.