Casparian strips prevent apoplastic diffusion of boric acid into root steles for excess B tolerance.

Casparian strips are ring-like structures consisting of lignin, sealing the apoplastic space between endodermal cells. They are thought to have important functions in controlling radial transport of nutrients and toxic elements in roots. However, Arabidopsis mutants with a defective Casparian strip structure have been found to maintain nutrient homeostasis in ranges supportive of growth under standard laboratory conditions. In this study, we investigated the function of Casparian strips under excess boron (B) conditions using sgn3 and sgn4 mutants with defective Casparian strip development but which do not exhibit excessive deposition of suberin, another endodermal diffusion barrier. The growth of sgn3 and sgn4 mutants did not differ significantly from that of wild-type (WT) plants under different B conditions in plate cultures; however, they were highly sensitive to B excess in hydroponic culture, where transpiration drives the translocation of boric acid toward the shoot. In hydroponic culture with sufficient to excess boric acid, B accumulation in shoots of the sgn3 and sgn4 mutants was higher than that in the WT. A time-course tracer study using 10B-enriched boric acid at a sufficient or slightly excessive concentration showed higher translocation of B into shoots of the sgn3 and sgn4 mutants. Furthermore, a genetically encoded biosensor for boric acid expressed under a stele-specific promoter (proCIF2:NIP5;1 5'UTR : Eluc-PEST) visualized faster boric acid flux into the mutant steles. Collectively, our results demonstrate the importance of Casparian strips in preventing apoplastic diffusion of boric acid into the stele under excess supply.

ANTH domain-containing proteins are required for the pollen tube plasma membrane integrity via recycling ANXUR kinases.

During plant reproduction, sperm cells are delivered to ovules through growing pollen tubes. This process involves tip-localized receptor kinases regulating integrity and/or guidance of pollen tubes, whose localizations must be strictly regulated. However, the molecular basis for tip-localization of these molecules remains largely elusive. Here we show that a pair of AP180 N-terminal homology domain-containing proteins, PICALM5a and PICALM5b, is responsible for the tip-localization of ANXUR receptor kinases acting in an autocrine signaling pathway required for pollen tube integrity in Arabidopsis thaliana. The picalm5a picalm5b double mutant exhibits reduced fertility, and the double mutant pollen is defective in pollen tube integrity with premature bursts. The tip localization of ANXUR proteins is severely impaired in picalm5a picalm5b pollen tubes, whereas another receptor kinase PRK6 acting in pollen tube guidance is not affected. Based on these results, we propose that PICALM5 proteins serve as specific loading adaptors to recycle ANXUR proteins.